Online reading assignment: electric potential energy

Physics 205B, spring semester 2020
Cuesta College, San Luis Obispo, CA

Students have a bi-weekly online reading assignment (hosted by SurveyMonkey.com), where they answer questions based on reading their textbook, material covered in previous lectures, opinion questions, and/or asking (anonymous) questions or making (anonymous) comments. Full credit is given for completing the online reading assignment before next week's lecture, regardless if whether their answers are correct/incorrect. Selected results/questions/comments are addressed by the instructor at the start of the following lecture.

The following questions were asked on reading textbook chapters and previewing presentations on electric potential energy.


Selected/edited responses are given below.

Describe what you understand from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically demonstrate your level of understanding.

"EPE is equal to the k constant times the source charge q₁ times the test charge q₁divided by a distance denoted as r. Voltage (potential) is equal to the k constant times the source charge Q divided by a distance from the source charge r."

"Electric potential energy can change when moving a source charge and test charge closer together, or moving them farther apart. One direction will give a positive change in EPE when pushing like signs together or pulling two opposite signs apart."

"Electric potential energy can be increased by pushing together like-sign charges or pulling apart opposite-sign charges. This results in a change in electric potential energy because it requires work to be done. Work is done by charges that are allowed to do what they want."

"I understood EPE can be increased by pushing like charges together or pulling apart opposite charges. While, allowing things to act without using 'work' results in a decreased EPE. Also, I believe to calculate ∆EPE, one must calculate the EPE at different locations and subtract."

"EPE utilizes a field of equipotentials, similar to electric fields."

"I get there is a difference between electric potential energy and electric potential, but I don’t really know what it is."

Describe what you found confusing from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically identify the concept(s) that you do not understand.

"Maybe the images on the blog explaining when something was negative rather than positive."

"I did not understand voltage, electric potential energy and what the peaks or wells mean."

"The difference between electric potential and electric potential energy is a bit confusing. I'm also getting lost on how the two-step approach is implemented in finding the potential energy."

"I had a hard time with the end of the presentation because I was confused about the circular equipotentials. Also, are EPE and potential are the same? I don't know why they have different labels in the equations."

"I am a little unclear with how to determine when something is a two-step approach versus a direct approach."

"The textbook does not have good examples of these types of problem. Can you please go over in class?"

"Nothing at this time. Possibly application."

Explain the difference between the units of electric potential V, and electric potential energy, EPE.

"EPE is in joules and potential is in joules per coulombs."

"The electric potential energy is an energy and is measured in joules. The electric potential is an energy per unit charge and is measured in joules per coulomb, or volts."

"The units."

"I don't understand the difference in units for the different energy potentials."

Explain the conceptual difference between the electric potential V, and electric potential energy, EPE.

"EPE is measured in joules because it is an energy. The volt is measured in joules per unit."

"Electric potential energy is an energy while electric potential is an energy per unit charge."

"Electric potential is created by a source charge. EPE is the energy stored in electric potential. I think this is why I'm so confused. I get one is an energy and the other one is an energy per charge but I don’t get how they relate to each other, or if one created the other..."

"Not really sure."

Briefly summarize the difference (if any) between "voltage" and electric potential.

"They are the same."

"A volt is the way to measure electric potential."

"There isn't a difference. Electric potential is measured in volts at a location in space."

"They are interchangeable."

Identify the changes in electric potential energy EPE (if any) for the following test charges (±q):
(Only correct responses shown.)

Positive test charge +q brought closer to a positive source charge +Q: increase [67%]
Negative test charge –q brought closer to a positive source charge +Q: decrease [70%]
Positive test charge +q brought closer to a negative source charge –Q: decrease [70%]
Negative test charge –q brought closer to a negative source charge –Q: increase [73%]

Identify the changes in electric potential V (if any) for the following test charges (±q):
(Only correct responses shown.)

Positive test charge +q brought closer to a positive source charge +Q: increase [48%]
Negative test charge –q brought closer to a positive source charge +Q: increase [48%]
Positive test charge +q brought closer to a negative source charge –Q: decrease [42%]
Negative test charge –q brought closer to a negative source charge –Q: decrease [45%]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.

"Pleeeeease review the difference of potential and EPE."

"Thrilled to see the energy equation back with a new member added to it."

"That was electrifying."

"I am having a hard time with the last question, are you going to go over it in class?"

"Would like to review the last question above."

"You weren't lying when you said this material was THICK."

"Do the concepts in this section relate to how magnets work? (Not yet. Too soon.)

Physics midterm question: comparing net force for afloat vs. submerged sinking block

Physics 205A Midterm 2, fall semester 2019
Cuesta College, San Luis Obispo, CA

A solid object is (a) partially submerged in water as it sinks with increasing speed, then while (b) completely underwater it still sinks with increasing speed. Discuss why the magnitude of the net force on the object is greater for case (a) than for case (b). Ignore friction and drag. Explain your reasoning using the properties of Newton's laws, Archimedes' principle (buoyant forces), and free-body diagrams.

Solution and grading rubric:

• p:
  Correct. Recognizes that:
  1. each block (a) or (b) has two vertical forces acting on it:

     Weight force of Earth on block (downwards, magnitude w = m⋅g, same for both (a) and (b)),
     Buoyant force of water on block (upwards, magnitude F_B = ρ_water⋅g⋅V_sub, less for (a)); and

  2. block (a) has a downwards weight force, and an upwards buoyant force much less than the magnitude of the weight force; and
  3. block (b) has the same downwards weight force as (a), also with an upwards buoyant force less than the magnitude of the weight force, but with a magnitude greater than the magnitude of the buoyant force in (a) (as more volume is submerged); and
  4. from Newton's second law, the downwards net force for (a) has a greater magnitude than the downwards net force for (b), as demonstrated by either explicit comparison of vector lengths and/or comparing terms in ΣF = +F_B − w equations for each case.
  May either draw a free-body diagram, and/or discuss these forces and Newton's laws in words.
• r:
  As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes. Typically does not explicitly demonstrate Newton's second law via vector addition (different up vectors drawn much less than, or a little less than the same down vector for each case; and/or comparing same/different quantities in ΣF = +F_B − w equations for each case).
• t:
  Nearly correct, but argument has conceptual errors, or is incomplete. At least recognizes that the object has a greater buoyant force once it is fully submerged.
• v:
  imited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Some constructive attempt at relating the buoyant force to the density of the fluid and volume displaced (Archimedes' principle) and/or Newton's first law.
• x:
  Implementation/application of ideas, but credit given for effort rather than merit. Appeals to some other properties of fluids and densities other than Archimedes' principle and Newton's laws.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.
  

Grading distribution:
Sections 70854, 70855
Exam code: midterm02sQm5
p: 13 students
r: 12 students
t: 8 students
v: 15 students
x: 4 students
y: 0 students
z: 0 students

A sample "p" response (from student 3372):

Physics quiz question: partially submerged block resting on bottom

Physics 205A Quiz 5, fall semester 2019
Cuesta College, San Luis Obispo, CA

A solid block has 75% of its volume below water, while resting on the bottom of a water tank. The force with the largest magnitude is the:
(A) weight force of Earth on the block.
(B) buoyant force of water on the block.
(C) normal force of tank bottom on the block.
(D) (There is a tie.)
(E) (Not enough information is given.)

Correct answer (highlight to unhide): (A)

The block has three vertical forces acting on it:

Weight force of Earth on block (downwards, magnitude w = m·g).
Buoyant force of water on block (upwards, magnitude F_B = ρ_fluid·g·(Volume submerged)).
Normal force of tank bottom on block (upwards, magnitude N).

Because the block is stationary in the vertical direction, from Newton's first law all of the up and down forces must sum to zero. This means that the two upwards forces (buoyant force and normal force) are together equal to the one downwards force (weight), such that the weight force has the largest magnitude of these three forces.

Sections 70854, 70855
Exam code: quiz05Gu1L
(A) : 30 students
(B) : 2 students
(C) : 5 students
(D) : 14 students
(E) : 0 students

Success level: 58%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.67

Physics midterm question: cargo-loaded truck vs. truck-loaded cargo

Physics 205A Midterm 1, fall semester 2019
Cuesta College, San Luis Obispo, CA

A Physics 205A student draws a (correct) free body diagram for a 600 kg cargo load resting on a stationary 11,000 kg truck with these two forces[*]:

Weight force of Earth on cargo load (5,800 N, downwards),

Normal force of truck on cargo load (5,800 N, upwards).

This student additionally claims that "this [free body diagram for the cargo load] would change if the truck was on top of the cargo load." Discuss why both the magnitude and direction of the normal force of truck on the cargo load would change if the truck were instead resting on top of the cargo load, and how you know this. Explain your reasoning using free-body diagram(s), the properties of forces, and Newton's laws.


[*] waiferx.blogspot.com/2017/10/physics-midterm-question-proposed-test.html.

Solution and grading rubric:

• p:
  Correct. Complete free-body diagram(s), and discusses/demonstrates that when the truck is on top of the cargo load:
  1. the truck has two vertical forces acting on it:

     Weight force w of Earth on truck (m_truck·g = 107,800 N, downwards),
     Normal force N of cargo load on truck (107,800 N, upwards),

     and since there is no vertical motion, these two vertical forces must be equal in magnitude due to Newton's first law; and
  
  
  2. from Newton's third law, these two forces must have equal magnitudes and opposite directions:

     Normal force Nof cargo load on truck (107,800 N, upwards),
     Normal force Nof truck on cargo load (107,800 N, downwards),

     such that the normal force of truck on cargo load for the case where the truck is on top of the cargo load is both different in magnitude (107,800 N vs. 5,800 N) and direction (downwards vs. upwards) compared to the normal force of truck on cargo load in the case where the cargo load was on top of the truck.
• r:
  As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes. Typically application of Newton's third law is problematic, or only implied, but still discusses how the normal force of truck on the cargo load is both different in magnitude and direction than in the previous case.
• t:
  Nearly correct, but argument has conceptual errors, or is incomplete. Some substantive attempt at analyzing forces using Newton's laws and free-body diagrams.
• v:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Some garbled attempt at using Newton's laws and free-body diagrams.
• x:
  Implementation of ideas, but credit given for effort rather than merit. No systematic attempt at using Newton's laws and free-body diagrams.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.

Grading distribution:
Sections 70854, 70855
Exam code: midterm01duCk
p: 17 students
r: 16 students
t: 18 students
v: 4 students
x: 0 students
y: 0 students
z: 0 students

A sample "p" response (from student 1995):

Physics midterm question: pulled box pulling on table underneath

Physics 205A Midterm 1, fall semester 2019
Cuesta College, San Luis Obispo, CA

A Physics 205A student applies a force (magnitude of 32 N, directed to the left) to pull on a rope attached to a 12.0 kg box, which moves at constant speed to the left across a fixed, stationary table. Discuss why both the magnitude and direction of the kinetic friction force of the box on the table would also be 32 N, directed to the left. Explain your reasoning using free-body diagram(s), the properties of forces, and Newton's laws.

Solution and grading rubric:

• p:
  Correct. Complete free-body diagram(s), and discusses/demonstrates:
  1. the box has two horizontal forces acting on it:

     Tension force T of student on box (32 N, to the left),
     Kinetic friction force  f_k of table on box (32 N, to the right),

     and since the box has a constant velocity ("constant speed to the left"), these two horizontal forces must be equal in magnitude due to Newton's first law; and
  
  
  2. from Newton's third law, these two forces must have equal magnitudes and opposite directions:

     Kinetic friction force  f_k of table on box (32 N, to the right),
     Kinetic friction force  f_k of box on table (32 N, to the left).

• r:
  As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes.
• t:
  Nearly correct, but argument has conceptual errors, or is incomplete. Some substantive attempt at analyzing forces using Newton's laws and free-body diagrams. Typically discusses Newton's first law for the forces acting on the box, but subsequent discussion of Newton's third law is omitted or merely implied.
• v:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Some garbled attempt at using Newton's laws and free-body diagrams.
• x:
  Implementation of ideas, but credit given for effort rather than merit. No systematic attempt at using Newton's laws and free-body diagrams.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.

Grading distribution:
Sections 70854, 70855
Exam code: midterm01duCk
p: 15 students
r: 5 students
t: 14 students
v: 15 students
x: 2 students
y: 0 students
z: 0 students

A sample "p" response (from student 6900):
Another sample "p" response (from student 2533; note that the static friction force of the ground on table pointing to the right is denoted as a tension force):
A sample "t" response (from student 1995), discussing Newton's first law for the box, demonstrating that the kinetic friction force of the table on the box points to the right with a magnitude of 32 N:

Online reading assignment: impulse and momentum

Physics 205A, fall semester 2019
Cuesta College, San Luis Obispo, CA

Students have a bi-weekly online reading assignment (hosted by SurveyMonkey.com), where they answer questions based on reading their textbook, material covered in previous lectures, opinion questions, and/or asking (anonymous) questions or making (anonymous) comments. Full credit is given for completing the online reading assignment before next week's lecture, regardless if whether their answers are correct/incorrect. Selected results/questions/comments are addressed by the instructor at the start of the following lecture.

The following questions were asked on reading textbook chapters and previewing a presentation on impulse and momentum.


Selected/edited responses are given below.

Describe what you understand from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically demonstrate your level of understanding.

"I understand that to clearly determine momentum you must have mass to figure out the amount of force the object is projecting on to another object. Also, impulse is the change in momentum over time which is used to determine how much net force is applied to an object, or the momentum-impulse theory. Finally, impulse can change an object's direction from left-to-right (or vice versa)."

"Momentum takes into account mass and speed of an object. Impulse takes into account net force and duration of time. The impulse-momentum theorem reflects the order of effects."

"Impulse is related to average force times the change in time and linear momentum is equal to max times velocity."

"The definition of impulse seemed pretty straightforward and I feel like I understand that, but I honestly think I'm going to have trouble with this chapter. It didn't really make sense to me when I was reading it. I think I'm going to need more practice questions."

Describe what you found confusing from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically identify the concept(s) that you do not understand.

"How is the impulse-momentum theory useful? When do we need it?"

"I'm having trouble grasping the concept of the impulse-momentum theorem."

"This only has to do with when two objects are touching?"

"What I initially found confusing was the second example of the presentation preview and how the direction towards the left was negative. But understanding that the left direction was considered negative for all the examples, then it wasn't so confusing."

"I'm just having trouble visualizing how all this works for different situations."

"However, the confusing aspect of the textbook and presentation is going to be the examples using it. I feel like it might be difficult and get confusing with all the other equations."

"The setup of impulse-momentum theorem in relation to the examples given in the book, need some lecture and problems to work to make the connection to visual ideas."

"After going through the presentation preview I released impulse was slightly confusing. Once I read through it a second time I was able to understand it."

"This seems pretty straightforward."

For the child hitting the tee ball with a bat, if the bat is swung such that it exerts the same net force on the tee ball for a longer time (by giving the bat more "follow-through"), the impulse on the tee ball will be __________, and the change in momentum of the tee ball will be:

less; less.	***** [5]
less; greater.	******* [7]
greater; less.	***** [5]
greater; greater.	*************************** [27]
(Unsure/lost/guessing/help!)	* [1]

For this golf ball initially at rest, and then has a speed of 97 m/s (to the right) after being hit by a golf club, indicate the horizontal directions (+/– signs) for the various impulse-momentum theorem quantities. (Only correct responses shown.)

Golf ball's initial momentum p₀: no direction (0). [87%]
Golf ball's final momentum p_f: to the right (+). [87%]
Golf ball's initial-to-final change in momentum ∆p: to the right (+). [89%]
Golf club's impulse "J" on the golf ball: to the right (+). [76%]

For this F/A-18E-F Super Hornet initially at rest, and then has a speed of 74 m/s after being it is catapulted (to the left), indicate the horizontal directions (+/– signs) for these impulse-momentum theorem vectors. (Only correct responses shown.)

Super Hornet's initial momentum p₀: no direction (0). [87%]
Super Hornet's final momentum p_f: to the left (–). [76%]
Super Hornet's initial-to-final change in momentum ∆p: to the left (–). [71%]
Catapult's impulse "J" on the Super Hornet: to the left (–). [58%]

For this Ford Ranger, hitting a crash barrier with a speed of 11.0 m/s (to the right), and then rebounding (to the left) off the crash barrier with a speed of 2.2 m/s, indicate the directions (+/– signs) for the various impulse-momentum theorem quantities. (Only correct responses shown.)

Ford Ranger's initial momentum p₀: to the right (+). [87%]
Ford Ranger's final momentum p_f: to the left (–). [78%]
Ford Ranger's initial-to-final change in momentum ∆p: to the left (–). [53%]
Crash barrier's impulse "J" on the Ford Ranger: to the left (–). [67%]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.

"The impulse and the change in momentum is fun."

"I think I am not understanding the relationship between momentum and impulse."

"What actually is the impulse and how is it different than momentum?"

"I still don't understand the concept of finding the momentum or how the impulse is ever different than the change in momentum because I thought that was the same thing. I think I'm just lost." (Impulse causes the change in momentum, so calculating the impulse (caused by a force acting over a period of time) allows you to find out the change in momentum, since they're mathematically equal to each other.)

"Is it possible to have a change in momentum without an impulse?" (No. Since impulses cause changes in momentum, any change in an object's momentum (which has magnitude and direction) means that there must be an impulse acting on it. Also if there is no change in an object's momentum, the there is no impulse acting on it. (If those examples sound like Newton's second law and Newton's first law, then yes, they can be applied to the impulse-momentum theorem.)

"The concept behind 'impulse' is a little confusing. The name kind of implies that it's instantaneous and only happens at one point in time." (The common meaning of "impulse" means a sudden urge to act, but the older meaning comes from "impel," or to drive forward, urge, or command.)

"Is there a way to measure impulse in a lab setting, like with a special tool?" (Since impulse is the amount of force exerted over a duration in time, then all you would need to measure impulse is the force sensor to measure how much force is exerted, and a stopwatch to time how long you would exert that amount of force. But also since you have a motion sensor (or the video analysis tool) that can track the velocities of objects, you can use that to tell you the initial velocity and the final velocity of an object, and when you multiply those velocities with the object's mass, you can calculate the initial momentum and final momentum of the object; and the change in momentum (final momentum minus initial momentum) is also equal to the impulse.)

"Something I didn't understand is how is 'J' the impulse but it also mentions that 'F⋅Δt' is also impulse. There are many equations." (It's just a definition. You exert an impulse (denoted by the vector "J") on an object by exerting a force over a period of time.)

"Will these equations be given or do we have to memorize them?" (These impulse and momentum equations are given on the quizzes and exams.)

"I am very uncertain about my answers to these examples. I thought I understood the reading but I will definitely need some clarification of these examples in class please!"

"I hope I got these questions right."

"Please go over these in class! thx"

"This case of confusion only exists if I'm mistaken in my understanding of the relationship between impulse and momentum. Since impulse is the product of time and net force, the resulting change in momentum should also increase as is the tee ball case. If not, I'm lost and need help." (You should be okay, as your reasoning sounds good.)

"I'm a bit confused on the change in momentum example where a truck has an initial velocity of 11m/s to the right and bounces off of a crash barrier with a velocity of –2.2m/s to the left. I know that change in momentum is found by subtracting (∆p_f – ∆p₀). So, for this example, I'm assuming it would be m⋅((– 2.2m/s) – (+11m/s)) to find the direction of the truck's initial-to-final change in momentum ∆p, which would be mass times –13.2m/s to the left." (This looks good. Sounds like you aren't that confused at all.)

"When it comes to the energy transfer-balance equation, I'm struggling in knowing when to drop what is not needed and a little on how to set it up after." (If there is an energy form that doesn't apply, then you drop that term (for example, if there are no springs involved, then you can drop the ∆PE_elas term. Or if there is no net initial-to-final change in that energy form, then you would also drop that term (for example, if an object is a rest on top of a vertical spring, which releases, and the object's final position is when it is (momentarily) at the highest point of its trajectory, then its initial and final translational kinetic energies are both zero, and then you would drop that term as ∆KE_trans = 0.)

"I'm enjoying this slightly cold weather."

Physics quiz question: comparing directions and magnitudes of forces

Physics 205A Quiz 3, fall semester 2019
Cuesta College, San Luis Obispo, CA

"A 2019 Chevrolet Silverado LT Trail Boss, New York International Auto Show"
Kevauto
commons.wikimedia.org

A cargo load (of unknown mass) rests on the bed of a 10,660 kg stationary truck[*]. Newton's __________ law tells you that these two forces are equal in magnitude and opposite in direction:

Normal force of cargo load on the truck.
Normal force of ground on the truck.

(A) first.
(B) second.
(C) third.
(D) (These forces are not equal in magnitude and/or opposite in direction.)

[*] chevrolet.com/commercial/silverado-chassis-cab.

Correct answer (highlight to unhide): (D)

The truck has three forces acting on it:

Weight force of Earth on the truck (m_truck·g = 1.04×10⁵ N, downwards).
Normal force of cargo load on the truck (downwards).
Normal force of ground on the truck (upwards).

Because the truck is stationary, the magnitudes of the two downward forces added together must equal the magnitude of the upwards force, due to Newton's first law. So the normal force of cargo load on the truck and the normal force of ground on the truck, while being opposite in direction, cannot be equal in magnitude (the difference in magnitudes being equal to the weight of the truck).

Sections 70854, 70855
Exam code: quiz03Ch3V
(A) : 22 students
(B) : 1 student
(C) : 9 students
(D) : 22 students

Success level: 41%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.65

Physics quiz question: accelerating crate pulled by falling block

Physics 205A Quiz 3, fall semester 2019
Cuesta College, San Luis Obispo, CA

A 2.0 kg crate is attached to a block (of unknown mass) by use of an ideal rope and pulley. As a result, the crate slides across the table to the right with an acceleration of 1.2 m/s² (while the block descends downwards). The table is not frictionless, and exerts a kinetic friction force of 5.9 N on the crate as it slides. Newton's __________ law tells you that these two forces are equal in magnitude and opposite in direction:

Tension force of block on the crate.
Kinetic friction force of table on the crate.

(A) first.
(B) second.
(C) third.
(D) (These forces are not equal in magnitude and/or opposite in direction.)

Correct answer (highlight to unhide): (D)

The crate has two vertical forces acting on it:

Normal force of table on the crate (upwards).
Weight force of Earth on the crate (m_box·g = 4.9 N, downwards).

Because the crate is stationary in the vertical direction, these two forces are equal in magnitude and opposite in direction, due to Newton's first law.

The crate has two horizontal forces acting on it:

Tension force of block on the crate (to the right).
Kinetic friction force of table on the crate (to the left).

Since the crate is accelerating at 1.2 m/s² to the right, then according to Newton's second law the forces acting on it cannot sum to a net force of zero, and the tension force must have a larger magnitude than the kinetic friction force. Thus the tension force and the kinetic friction force, while opposite in direction, are not equal in magnitude.

Sections 70854, 70855
Exam code: quiz03Ch3V
(A) : 2 students
(B) : 10 students
(C) : 3 students
(D) : 39 students

Success level: 72%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.52

Physics quiz archive: vectors, projectile motion, forces

Physics 205A Quiz 3, fall semester 2019
Cuesta College, San Luis Obispo, CA
Sections 70854, 70855
Exam code: quiz03Ch3V



Sections 70854, 70855 results

0- 6 :	* [low = 6]
7-12 :	***
13-18 :	**************
19-24 :	******************* [mean = 21.7 +/- 5.8]
25-30 :	***************** [high = 30]

Online reading assignment: uniform circular motion

Physics 205A, fall semester 2019
Cuesta College, San Luis Obispo, CA

Students have a bi-weekly online reading assignment (hosted by SurveyMonkey.com), where they answer questions based on reading their textbook, material covered in previous lectures, opinion questions, and/or asking (anonymous) questions or making (anonymous) comments. Full credit is given for completing the online reading assignment before next week's lecture, regardless if whether their answers are correct/incorrect. Selected results/questions/comments are addressed by the instructor at the start of the following lecture.

The following questions were asked on reading textbook chapters and previewing a presentation on uniform circular motion.


Selected/edited responses are given below.

Describe what you understand from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically demonstrate your level of understanding.

"In circular motion, there is a net force that produces the acceleration of an object. The centripetal force always points towards the middle of a circle."

"That circular motion is covered by Newton's second law. Even restricted to uniform circular motion Newton's second law still applies."

"I understand that by Newton's second law, there must be some type of force to create centripetal acceleration. It makes sense to me that this force is pointing towards the center of the circle, as if the force pointed outward, the object would drift off its circular path."

"Since the word 'centripetal' means 'moving toward a center,' the centripetal acceleration vector always points toward the center of the circle and continually changes direction as the object moves. For an object in uniform circular motion, the velocity vector has different directions at different places on the circle. Uniform circular motion along the arc of an infinitely large circle entails no acceleration, because it is just like motion at a constant speed along a straight line. An object undergoing uniform circular motion can never be at equilibrium because there is acceleration since the direction is constantly changing. Sometimes the centripetal force consists of a single force such as tension, friction, the normal force or a component thereof or the gravitational force."

"The concept of centripetal force, the net force of an object undergoing circular motion with a specific direction which is in towards the center of the circle and specific magnitude m·v²/r. Centripetal means 'center-seeking.' This is the requirement for uniform circular motion to maintain a constant speed along a circular trajectory with acceleration directed in towards the center."

"Centripetal force! It's like centrifugal force but not. In all seriousness, a sample scenario is a car turning a corner. This change in direction of velocity requires a net force to keep pulling the car into the center of its curve."

Describe what you found confusing from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically identify the concept(s) that you do not understand.

"I don't understand the conceptual understanding of why the net force points towards the center of the circle."

"I'm a little confused on if this is the same or different from 'centrifugal force.'"

"I understand the majority of these situations, but the one about the girl swinging is tripping me up."

"Slightly confused on calculating ALL the net forces during circular motion."

"Friction and how it affects centripetal force (i.e. car driving in a curve along a track)."

"How circular motion works for a car drifting around a turn. I do not understand what direction the net force is going."

"Is centripetal acceleration the acceleration the object is going at each new direction in a circular motion? I'm a little perplexed by that concept."

"I guess I still haven't grasped the idea of vertical circular motion yet."

"I'm pretty sure that I understood everything in the chapter."

For the "drifting" car (skidding around a circular track at constant speed), Newton's __________ law applies to its motion, and the forces acting on it add up to a net force that:

first; is zero.	****** [6]
second; points to the left.	*************** [15]
second; points to the right.	************************ [24]
(Unsure/lost/guessing/help!)	* [1]

At the moment when the woman is at the bottom of her swinging trajectory (when the rope is vertical), Newton's __________ law applies to her motion, and the forces acting on her add up to a net force that:

first; is zero.	****** [6]
second; points upwards.	**************************** [28]
second; points downwards.	*********** [11]
(Unsure/lost/guessing/help!)	* [1]

At the moment when the motor scooter is on the left side of the screen (traveling out at you), Newton's __________ law applies to its motion, and the forces acting on it add up to a net force that:

first; is zero.	** [2]
second; points to the left.	************** [14]
second; points to the right.	************************** [26]
(Unsure/lost/guessing/help!)	**** [4]

At the moment when the car is at the very top of the loop-the-loop, Newton's __________ law applies to its motion, and the forces acting on it add up to a net force that:

first; is zero.	***** [5]
second; points upwards.	******* [7]
second; points downwards.	******************************** [32]
(Unsure/lost/guessing/help!)	** [2]

At the moment when a person is at the right edge of the screen (traveling out at you), Newton's __________ law applies to his/her motion, and the forces acting on him/her add up to a net force that:

first; is zero.	**** [4]
second; points to the left.	**************************** [28]
second; points to the right.	*********** [11]
(Unsure/lost/guessing/help!)	*** [3]

At the moment when the car is at the very top of its mid-air trajectory, Newton's __________ law applies to its motion, and the forces acting on it add up to a net force that:

first; is zero.	******** [8]
second; points upwards.	****** [6]
second; points downwards.	**************************** [28]
(Unsure/lost/guessing/help!)	**** [4]

At the moment when the skateboarder is at the very top of his mid-air trajectory, Newton's __________ law applies to his motion, and the forces acting on him add up to a net force that:

first; is zero.	****** [6]
second; points upwards.	****** [6]
second; points downwards.	******************************* [31]
(Unsure/lost/guessing/help!)	*** [3]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.

"I feel like to really grasp what these questions are asking we need a bit more clarification in class because it's easy to get mixed up." (That's okay, just try your best--I need to see how well you do on the first try, so I know how basic or advanced to tailor my lecture for this material. (Spoiler alert: very basic.)

I've got no idea how I did on those last questions. May we go over these questions in class? I think I got all of them wrong."

"How do I know if I got these questions right or not?" (We'll go over these in class, and as part of the next reading assignment, you have a link to click on so you can see these correct answers.)

"Can we go over how to tell which way the net force is pointing when objects are moving in a circle? How do you figure out which way the net force points?" (For objects undergoing uniform circular motion (moving in a circle with constant speed), the net force always points inwards, towards the center of the circle. Always. Points. Inwards.)

"So lost help!!"

"After going over the questions I am really confused on the idea of objects in motion. Do we determine the law at a specific moment in time or the entire sequence?" (At that one moment in time. Then for an entire sequence, it's just a whole bunch of consecutive moments in time, taken one by one.)

"I am still confused about the car and motorcycle skidding around a circular track--if I was to compare the motorcycle to the car, the motorcycle would almost be in a standstill, but compared to an outside frame of reference, they both move?" (Yes, since both the car and motorcycle are accelerating (constant speed, but changing direction), they are not proper inertial reference frames, and they will not be able to properly apply Newton's laws to their motions. As an outside (stationary) observer, you would be in an inertial reference frame, and would be able to properly apply Newton's laws to them as they undergo uniform circular motion.)

"I didn't really understand how when in circular motion we are feeling the forces that may feel like outward but really they are going in." (When you undergo uniform circular motion, you are not in a proper inertial reference frame, and cannot properly apply Newton's laws because you experience "pseudoforces" like something is pulling/pushing you outwards. An outside (stationary) observer would be in an inertial reference frame, and would be able to properly apply Newton's laws and describe the net force as inwards, despite what you (undergoing uniform circular motion) "perceives.")

"Is it still considered uniform circular motion if it is not a perfect circle? What about a racetrack that is an oval in shape?" (If we can approximate part of the oval as a circle (for any small enough section of an oval), then we can apply Newton's second law for uniform circular motion for that section.)

"Do we need to memorize all the weight, normal, tension, static friction, kinetic friction, Newton's laws and ΣF = m·a equations, or are they given on quizzes and exams like the kinematic equations of motion?" (You need to memorize w = m·g, f_{s, max} = μ_s·N, f_k = μ_k·N, ΣF = m·a, and "POF-OST-ITO." Instead of just giving you those equations on a quiz or exam, I think memorizing those equations will force you understand how and why to use them while applying Newton's laws on free-body diagrams. (Note that the normal force N and the tension force T don't have any equations; their magnitudes have to be deduced, as they vary depending on the particular situation.)

Online reading assignment: applications of Newton's laws (friction)

Physics 205A, fall semester 2019
Cuesta College, San Luis Obispo, CA

Students have a bi-weekly online reading assignment (hosted by SurveyMonkey.com), where they answer questions based on reading their textbook, material covered in previous lectures, opinion questions, and/or asking (anonymous) questions or making (anonymous) comments. Full credit is given for completing the online reading assignment before next week's lecture, regardless if whether their answers are correct/incorrect. Selected results/questions/comments are addressed by the instructor at the start of the following lecture.

The following questions were asked on reading textbook chapters on applications of Newton's laws (emphasizing static and kinetic friction).


Selected/edited responses are given below.

Describe what you understand from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically demonstrate your level of understanding.

"There can only be two types of motion: constant and changing. There also only two types of net forces: zero and non-zero."

"Newton's third law of motion isn't necessarily about motion or net force, but the properties of force itself."

"We can use the mnemonic device 'POF-OST-ITO' in order to test if Newton's third law applies or not. POF stands for 'pair of opposite forces,' OST is 'of same type,' ITO 'involving two objects.'"

"Newton's third law relates only two forces of the same type acting on two different objects. However Newton's first law deals with two forces acting upon one object. Additionally, if the POF-OST-ITO test fails, then it is not a N3 scenario, but it is not yet safe to assume it is a N1 scenario either, until you do further investigation."

"Newton's second law shows how the acceleration depends on both the net force and the mass. The magnitude of acceleration is proportional to the net force acting on the object, and inversely proportional to the mass. The net force includes only the forces that the environment exerts on the object of interest. The kinetic friction force opposes the relative sliding motion."

"Everything is effected on by gravity. If something is touching a surface, it will have a normal force acting on it."

"The normal force can only exist when two surfaces are making contact with one another. If two objects are not making contact, then there is no normal force."

"Friction is another component when dealing with the motions of an object. It is a force that is parallel to the surface an object is moving on. Static friction is what makes an object remain stationary even when a force is applied, and only moves when the applied force is slightly greater than the maximum force of static friction."

"Static friction force comes into play when pulling on an object at rest, and if the force applied is small enough static friction will cancel out the applied force resulting in no movement."

"That an object initially at rest interacts with a surface through static friction. As the object moves, after overcoming the maximum static friction, the resistance turns into kinetic friction. Friction is a parallel and normal force is perpendicular to the surfaces in contact."

"Static friction is the friction that a surface has on a object that is resting on the surface and is directly proportional to the normal force on said object. Next, there is kinetic friction which is friction that occurs between two sliding surfaces. Also kinetic friction is typically less than the static friction."

"Static and kinetic frictional forces seem pretty straightforward. Static refers to when there is friction acting on an object but the object is not moving. Kinetic is when there is friction acting on an object but the object is moving."

"Was a little lost, so there isn't much I understand."

Describe what you found confusing from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically identify the concept(s) that you do not understand.

"I feel like I understand Newton's laws from the textbook. However, when it comes to applying the concepts to a free body diagram, I start getting confused with the direction of the forces and how they cancel out especially when things are stacked on top of each other."

"I still have trouble distinguishing between Newton's first law and second law."

"I was slightly confused as to why the pulleys and ropes would have equal tensions for the different worksheet examples in class. But after further research I understood that neither the mass nor acceleration of gravity changed, therefore the tension forces stayed the same."

"I get confused on how to determine which law is being used between Newton's first law or third law. I also don't understand the difference between static and kinetic friction forces and the magnitudes associated with them."

"The difference between static friction and kinetic friction aren't clear to me."

"The coefficient of friction and what it stands for might need some clarification for me."

"I would like some more work in class with problems on kinetic friction and static friction. I understand that static friction is friction for stationary objects, like a box on the ground, and that kinetic friction is friction for moving objects, like a box sliding across the ground. However, I'm still unsure of how this would work in real-world situations."

"I don't know how to put friction in an equation or how to use it to solve for something."

"Why are there no SI units for static/kinetic friction coefficient?"

"For the most part it is pretty easy to visualize friction, since we have all experienced it before."

"Didn't really find anything confusing."

"I understood most of it conceptually, I think it could get more confusing when these concepts are applied in a problem."

"Most of it."

What is the meaning of the "normal" in the "normal force?"

"The perpendicular force applied between two objects contacting each other."

"It just means the force is perpendicular to the surface."

"The 'normal' force refers to the perpendicular direction with respect to the surface."

"Normal means 'perpendicular.'"

"There will almost always be normal force on any given objects in contact, regardless of its state or location? That's what makes it 'normal,' right?

The SI (Système International) units of the static friction coefficient µ_s and the kinetic friction coefficient µ_k are:

" Unitless."

The coefficients because the units cancel."

"I do not believe these coefficients have units."

"Newtons?"

"kg·m²·s²?"

"I'm not entirely sure."

Identify the magnitude of the static friction force f_s for each of the following situations of a box that is initially stationary on a horizontal floor. (Only correct responses shown.)

No external horizontal forces applied to it, so it remains stuck to the floor:
f_s = 0. [77%]

An external horizontal force applied to it, but still remains stuck to the floor:
f_s = some value between 0 and µ_s·N. [83%]

An external horizontal force applied to it, at the threshold of nearly becoming unstuck:
f_s = µ_s·N. [72%]

Identify the magnitude of the kinetic friction force f_k for each of the following situations of a box that is already sliding across a horizontal floor. (Only correct responses shown.)

No external horizontal forces applied on it, so it slows down:
f_k = µ_k·N. [21%]

An external horizontal force applied in the forward direction, but not enough to keep the box going so it still gradually slows down:
f_k = µ_k·N. [23%]

An external horizontal force applied in the forward direction, just enough to keep the box going at a constant speed:
f_k = µ_k·N. [43%]

An external horizontal force applied in the forward direction, enough to gradually increase the speed of the box:
f_k = µ_k·N. [58%]

An external horizontal force applied in the backwards direction, such that the box slows down:
f_k = µ_k·N. [32%]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.

"Can we go over the friction coefficients? Please explain static friction force and kinetic friction force in detail in class! I could use some review on the equations for the magnitudes of static and kinetic frictional force. Is the friction coefficient different between objects that do not appear smooth. For example, is the constant for ice on ice different than say, bumpy ice on ice?" (Yes, the coefficient will be larger for rougher surfaces than for smoother sliding surfaces.)

"Are there ways to increase of decrease friction?" (Change the smoothness/roughness of the surfaces, or add a lubricant (which is "smooth" on a molecular level; long hydrocarbon chains can align with each other to roll like logs) between the two surfaces.)

"What is it that causes friction? Is it simply objects hooking onto each other at microscopic levels?"

"Does static friction force become kinetic friction force once the object starts moving?" (Yes, once the object is already unstuck.)

"What I found confusing was that static friction force and the applied force on a object are directly proportional, except for the fact that static friction has a maximum amount, as opposed to applied force which can keep increasing indefinitely. If the two are directly proportional, shouldn't the static friction force increase as long as the applied force increases, or shouldn't the applied force also have a maximum if the static friction has one as well?" (You can arbitrarily exert any amount of applied force on an object, but the static friction force will have a maximum amount because at some point your applied force will "unstick" the object so it will then begin to move. I suppose if you stuck together the two surfaces magic superglue, then you could exert an infinite amount of applied force, and the static friction force would then also be infinite, provided the magic superglue still holds the object stationary.)

"I hope I'm understanding this correctly! It seems to make sense so that's either a really good sign or a really bad sign."

"Definitely need some review, this weekend fried my brain."

"I don't like this :("

Online reading assignment: forces and motion

Physics 205A, fall semester 2019
Cuesta College, San Luis Obispo, CA

Students have a bi-weekly online reading assignment (hosted by SurveyMonkey.com), where they answer questions based on reading their textbook, material covered in previous lectures, opinion questions, and/or asking (anonymous) questions or making (anonymous) comments. Full credit is given for completing the online reading assignment before next week's lecture, regardless if whether their answers are correct/incorrect. Selected results/questions/comments are addressed by the instructor at the start of the following lecture.

The following questions were asked on reading textbook chapters and previewing a presentation on forces and motion.


Selected/edited responses are given below.

Describe what you understand from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically demonstrate your level of understanding.

"I understood the motion flowcharts very well. By this, it represents when we should and when we should not use Newton's first and second law. It does this by asking whether the speed is constant and then whether the direction is constant. I understand the flowcharts and the seemingly easy two-step verification process for determining the specific Newton's law. I can observe how the flowchart could be used to decipher the correct Newton's law at play in a situation."

"I understand that in order for Newton's first law to apply, the speed and direction of an object must BOTH be constant. If either aren't, than Newton's second law applies. I also understand that Newton's first law is applicable when the net force of an object is zero, and the Newton's second law applies if the object's force is anything but zero."

"The basic postulates of Newton's first and second laws of motion. I know that the first law states that an object will continue to move at a constant velocity until an outside force causes that to change. The second law then goes on to say that acceleration of a moving object depends on the object's mass and the net force acting on the object."

"With constant direction and speed, Newton's first law is in motion; all other cases are second law. Similarly, if the net force is equal to zero, it is the first law. ΣF = m⋅a is the equation used and can end up being 0 = 0 (for the first law case)."

"Newton's first law is applied when an object is maintaining a constant velocity; Newton's second law is applied when an object’s velocity has changed and we have to take into account the net force acted upon the object and inertia of the object."

"An object will stay at rest or in motion at a constant speed unless it is compelled to a change in net force. The net force is also the vector sum of all forces acting upon an object.

Describe what you found confusing from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically identify the concept(s) that you do not understand.

"I was a bit lost when viewing the net force flowchart. It seems overly simplified. But after researching it more, I am slightly more comfortable using it for the questions."

"Something I found confusing was how vectors work in Newton's second law. If you are moving in a circle in constant motion, then would the forces be considered non-zero or zero because of the changing direction?"

"Where I get confused is when I overthink and take too many things into consideration with the object. Or when I forget to take things into consideration."

"The concept of net force. Honestly the pictures in the presentation made me more confused."

"I don't see how to calculate the net force of an object when two different forces occur in different directions, as well as the other factors such as air resistance and friction. I was confused by how to determine whether Newton's first law is being used or Newton's second law. I also was a bit confused by some of the equations for the vector components of the second law."

"I am having a difficult time understanding what we are getting to in class."

Identify whether a zero or non-zero net force corresponds to Newton's first law or Newton's second law. (Only correct responses shown.)

Zero net force (ΣF = 0): Newton's first law ("N1") [92%]
Non-zero net force (ΣF ≠ 0): Newton's second law ("N2") [88%]

For the rocket sled, Newton's __________ law applies to the motion of this object, and the forces acting on the object add up to a __________ net force.

first; zero.	************* [13]
second; non-zero.	******************************** [32]
(Unsure/guessing/lost/help!)	*** [3]

While the F-35B is descending at a steady rate (before it touches the ground), Newton's __________ law applies to the motion of this object, and the forces acting on the object add up to a __________ net force.

first; zero.	***************************** [29]
second; non-zero.	************** [14]
(Unsure/guessing/lost/help!)	***** [5]

For this car with a steady speedometer reading (and assuming the shaking is due to the person holding the camera, and not from the car itself), Newton's __________ law applies to the motion of this object, and the forces acting on the object add up to a __________ net force.

first; zero.	***************************** [29]
second; non-zero.	************** [14]
(Unsure/guessing/lost/help!)	***** [5]

For a person in the swinging chair ride moving along a circular trajectory at a constant speed, Newton's __________ law applies to the motion of this object, and the forces acting on the object add up to a __________ net force.

first; zero.	****************** [18]
second; non-zero.	************************* [25]
(Unsure/guessing/lost/help!)	***** [5]

For this car with a zero speedometer reading, Newton's __________ law applies to the motion of this object, and the forces acting on the object add up to a __________ net force.

first; zero.	****************************************** [42]
second; non-zero.	**** [4]
(Unsure/guessing/lost/help!)	** [2]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.

"Is there ever a situation when Newton's laws don't apply?" (No. Either motion is constant (Newton's first law), or is not constant (Newton's second law).)

"For the most part, this section was pretty straightforward. The only thing I would like touched upon would be the flowchart, in case I am not overthinking it. Is it as simple as it appears?" (Yes.)

Can you go over these in class with a couple of examples?"

"Will we go over the answers to the questions above in class?" (For some of these, yes.)

"Is there an answer key for these questions?" (Make sure you review these responses (with the correct answers in boldface) as part of the next reading assignment.)

"Definitely would appreciate discussion on the differences between Newton's first and second law in real physics problems."

"In the rocket sled example, is the speed constant even though the sled is accelerating when it first fires up?" (No, the speed is not constant once the rocket fires up, as it starts off stationary, and then begins to speed up, so its motion is then covered by Newton's first law (even though the direction is constant, though.)

"If an object is moving in a circular motion, is that 'constant?'" (Even if the speed is constant, the direction of the object's velocity vector is always changing, so its motion is then covered by Newton's second law.)

"What should we know about working with free body diagrams in this class?" (Nothing official, yet, but we'll get started on Wednesday.)

"what does is mean when it says forces don't 'cancel?'" (Two forces can 'cancel' in the sense that their contributions are opposite and as a result nothing happens, but it's not like they disappear or vanish--they're still there, its just that there is no net effect on the object.)

"When would vertical acceleration be negative and when would it be positive?" (If vertical acceleration is downwards/negative (say, only the force of gravity is acting on an object, and there is no air resistance), then an object moving downwards (in the direction of the net force) will speed up, and an object moving upwards will slow down. If vertical acceleration is upwards/positive (say, a rope is pulling upwards with a force greater than the weight force), then an object moving upwards (in the direction of the net force) will speed up, and an object moving downwards will slow down.)

"If Earth is spinning, how is it an inertial reference point?" (For our purposes, it's a "good enough" inertial reference frame given the situations we'll be dealing with in this course, especially in lab, where our scales, motion sensors, etc. are nowhere near sensitive enough to detect that Earth is not actually stationary. It's similar to neglecting air resistance for free fall, as that is also another "good enough" approximation to situations we'll analyze in lab.)

"Getting into forces. Things are now gonna get a little more overwhelming."

"Yee haw."