Physics quiz question: finding amount of unknown charge

Physics 205B Quiz 3, spring semester 2020
Cuesta College, San Luis Obispo, CA

A –4.5 nC point charge and an unknown positive point charge are held a distance of 3.0 cm apart. The magnitude of the electric force of the –4.5 nC charge on the unknown positive charge is 7.6×10^–5 N. The amount of the unknown positive charge is:
(A) 1.7×10^–9 C.
(B) 5.6×10^–8 C
(C) 1.7×10⁴ C.
(D) 4.5×10⁴ C.

Correct answer (highlight to unhide): (A)

The magnitude of the force on the unknown positive point charge is given by:

|F_{1 on 2}| = k·|q₁|·|q₂|/(r²),

where q₁ is the –4.5 nC point charge, and q₂ is the unknown positive charge. Then the absolute value of the unknown charge can then be solved for:

|q₂| = |F_{1 on 2}|·(r²)/(k·|q₁|),

|q₁| = |7.6×10^–5 N|·(0.030 m)²/((8.99×10⁹ N·m²/C²)·|–4.4×10^–9 C|),

|q₁| = 1.6907675195...×10^–9 C,

or to two significant figures, the amount of the unknown (positive) charge is (+)1.7×10^–9 C.

(Response (B) is |F_{1 on 2}|·r/(k·|q₁|); response (C) is |F_{1 on 2}|/|q₁| (which is the electric field of the unknown positive charge at the location of the –4.5 nC charge); and response (D) is k·|q₁|/r² (which is the electric field of the –4.5 nC charge at the location of the unknown positive charge).)

Sections 30882, 30883
Exam code: quiz03Cv1d
(A) : 28 students
(B) : 1 student
(C) : 4 students
(D) : 1 student

Success level: 82%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.18

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.)

Online reading assignment: charges, electric forces and fields

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 charges and materials and electric forces and fields.


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 differences between a conductor and an isolator. The polar molecule definition was also easy to understand from chemistry."

"Electrons move freely in a conductor but are still bound to atoms in a insulator but electrons will still orient to opposite charges on insulators. Polar molecules will also orient their opposing charge to a charged object, becoming aligned."

"Considering electrical charge, insulators result in outer electrons that are mostly fixed to their atomic location, while conductors allow electrons greater ability to move about the material. Both neutral conductors and insulators will be attracted to positively and negatively charged objects. Electrical forces and electrical fields will exert a force on a test charge."

"I think I understand Coulomb's law, it is a way to measure the force exerted from a source charge to a test charge. I understand it as the force an object is exerting on another object, but I am not entirely sure."

"Make sure to not have static electricity around a gas pump. I'm pretty sure I understand the differences between source charges and test charges as well as the electric force and electric field."

"From this section I learned that there are electric fields and electric forces that both act together to produce the force acted on something. With an electric field, the force applied to something is reliant on the type of electric field and the positive or negative test charge q."

"I quickly skimmed this and did not really understand anything. I didn't really get a good look at anything."

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.

"Not exactly sure about the insulator/conductor attractions."

"I had confusion with multiple aspects of this material starting with the definitions of isolators and conductors and what objects we might categorize as either insulators or conductors. Although, I'm proficient in the simple idea that opposite charges attract and same charges are repulsed from each other. I also was confused with the idea of electric fields and the interactions with electric forces. I feel like I'm on the verge of understanding in that the interactions of two charged objects--or possibly a single neutral object--creates an electric force either attracting or repulsing which in turn establishes an electric field in which that electric force occurs. But again, generally confused."

"The polarization aspect with insulators and conductors. It was confusing to understand the exact differences."

"I was very confused about the conductor and insulator concepts. I don't understand how the electrons come into play at all."

"I understand polarization but don't quite have a strong grasp on the characteristics of insulators and conductors. Also, fields are going over my head as a whole."

"I am kinda confused on what Coulombs law turns out to be unit-wise."

"I'm having trouble understanding electric forces and fields."

"The force model and field model did not make much sense to me."

"I found the concept behind the electric forces and field to be confusing. More specifically, the difference between the two."

"In the field model, I am confused by what is means for +q: F 'obeys' E, and for –q: F 'disobeys' E."

"Haven't got to this yet."

"I mean I honestly can't think of a specific question. I guess it would be helpful to go over as much as you can."

"I didn't find anything confusing in this topic."

A gas pump fire is inadvertently caused by the sudden discharge of an electrical spark. Describe when and how the woman became electrically charged.

"I think it is when she exits the car. I have heard of this occurring, but never understood what actually happened."

"Two insulators with different electron affinities are rubbed against each other. When she rubs her hand her shirt, it causes a reaction due to this difference in electron affinity."

"It was when she came into contact with the cloth on her seats in her car."

"The woman became electrically charged when she went back in her car and then touched the gas pump after getting out. She created friction by sliding in and out of her car, and the fire started after she touched the metal gas pump."

"When the woman stepped into her vehicle she created some exchange of electrons by rubbing tohether two different insulators with two different affinities for electrons. When she then reached for the pump, those electrons were sent toward the metal pump causing a spark."

"Static electricity allowed her to carry electrons making her negatively charged. When she got close to the pump, she transferred these electrons causing a spark."

"I am not super-positive, but I am going to guess that electrons were transferred by her rubbing her sweater and then touching the pump."

Electrically neutral polystyrene "packing peanuts" would be attracted to a cat that has __________ charge.

a positive.	**** [4]
a negative.	*** [3]
either a positive or a negative.	*********************** [23]
zero (neutral).	* [1]
(Unsure/lost/guessing/help!)	** [2]

An electrically neutral aluminum soda can would be attracted to a balloon that has __________ charge.

a positive.	*** [3]
a negative.	**** [4]
either a positive or a negative.	********************** [22]
zero (neutral).	* [1]
(Unsure/lost/guessing/help!)	*** [3]

Electrically neutral polar molecules in a water stream would be attracted to a comb that has __________ charge.

a positive.	*** [3]
a negative.	**** [4]
either a positive or a negative.	********************** [22]
zero (neutral).	* [1]
(Unsure/lost/guessing/help!)	*** [3]

Explain the difference between units for electric force, F, and the electric field, E.

"Force is in newtons, and electric field is in newtons per coulomb."

"N vs. N/C."

"The electric field units are taking into account two different charges that are exerting a force on each other (q1 and q1, both in coulombs) divided by the square of the distance between them (m²). k is a constant and has units of N·m²/C². When you calculate the force of the electric field coulombs and meters cancel out leaving newtons. For the electric field, we once again have the constant k with units N·m²/C² multiplied by the source charge Q (C) and divided by r² (m²). When you plug into the formula, all units except N/C cancel."

"I don't understand the difference in units."

Explain the conceptual difference between the electric force, F, and the electric field, E.

"An electrical force is created directly by a source charge Q and can affect a test charge q, where as an electric field is created by a source, and this electric field can cause an effect on a test."

"Electric field refers to all the space around a source while electric force is the magnitude and direction of the field being exerted on the test charge."

"The electric force is more direct and is a one-step process. While the electric field is indirect and has two steps."

"The electric field surrounds the source charge. The electric force is what is exerted onto the test charge."

"I don't understand the difference."

"Will get to this later."

"I'm honestly not sure."

Explain the conceptual difference between a source charge (±Q), and a test charge (±q).

"A source charge Q creates an electric field, and a test charge q receives the force from the field."

"A source charge Q is a charge exerting a force onto a test charge q. The test charge is the charge which is being acted upon which will either be attracted to or repulsed by the source charge dependent upon whether those two charges have opposite (attract) or the same (repulsive) ± signs."

"A source charge Q is the charge that exerts the force on the test charge q, while the test charge is being exerted on by the source charge."

"Source charge exerts a force on the test chare. Source is giver, test is receiver."

"Q defines the direction of E and q is the relationship between F and E."

"This is what I need help with. I know a test charge has a very small magnitude because it shouldn't change the source charges. But don't understand why a test charge is necessary?"

Indicate the direction of electric field lines for these ±Q source charges:
(Only correct responses shown.)

Positive source charge (+Q): E field lines point outwards [75%]
Negative source charge (–Q): E field lines point inwards [70%]

A positive point charge (+q) is placed on an electric field line (due to some other source charge ±Q). This positive point charge will always experience an electric force F:

along the E field direction.	******************** [20]
opposite the E field direction.	****** [6]
(Unsure/guessing/lost/help!)	******* [7]

A negative point charge (–q) is placed on an electric field line (due to some other source charge ±Q). This positive point charge will always experience an electric force F:

along the E field direction.	****** [6]
opposite the E field direction.	******************** [20]
(Unsure/guessing/lost/help!)	******* [7]

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

"The reading said a neutrally charged insulator's electrons would turn away from a negatively charged external material. Why are they two materials still attracted?" (The insulator's positive nuclei would then be fractionally closer to the negatively charged external material, and this attractive force would be greater than then repulsive force between the insulator's negative electrons, which are fractionally farther away from the negatively charged external material.)

"With the examples given about induction and polarization, they all seemed to have some net attraction. Is there ever a net repulsion?" (No.)

"Very cool! A huge fan of the Star Wars analogy for this material."

"Can we please review the two-step field model? Thank you!"

"I don't quite understand the difference between the different qs?"

"I feel really lost, can you please explain field lines and test charges?"

"I have very little exposure to electricity so I am having a hard time understanding this chapter. There also seems to be a LOT of information!"

"This electricity stuff is hard :("

"Are we going to go over this a lot more?" (Yes.)

"I'll have to do some more reading at home. I thought I'd be able to do this during breaks at work but I stand corrected and confused."

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:

Physics midterm problem: skateboard-launched rubber duck toy

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

"WRECKING BALL Vs. SEESAW from 45m! How High Will the Watermelon Go?"
How Ridiculous
youtu.be/1quHlRJLtgM

Brett Stanford, Derek Herron and Scott Gaunson for the "How Ridiculous" YouTube channel dropped a heavy ball on one end of a skateboard to launch a rubber duck toy from the other end. Video analysis shows that the toy was launched at an angle of 61° from the horizontal, and took 3.2 s from the moment it was launched from the ground to land back down on the ground. Determine the horizontal distance along the ground from where it was launched to where it landed. Neglect air resistance, and treat the toy as a point object that started on the ground. Show your work and explain your reasoning using properties of projectile motion.

[*] youtu.be/1quHlRJLtgM?t=335.

Solution and grading rubric:

• p:
  Correct. From the time of flight t = 3.2 s, solves for the initial vertical velocity component v_0y = +16 m/s. Next, using the launch angle of elevation θ = 61° finds the initial horizontal velocity component v_0x = +8.7 m/s, and subsequently uses that value and the time of flight t = 3.2 s to solve for the final horizontal position x = +28 m.
• r:
  Nearly correct, but includes minor math errors. At least successfully solves for the vertical v_0y and/or horizontal v_0x components of the initial velocity vector.
• t:
  Nearly correct, but approach has conceptual errors, and/or major/compounded math errors. At least some systematic attempt at using kinematic equations for projectile motion. May have made one or more erroneous assumptions about certain values, such as setting the final velocity components v_x = 0 and or v_y = 0, but still methodically solves for a (wrong) value of v_0y, and then (somehow) solves for a (wrong) value of v_0x using trigonometry to find a (wrong) value for the final horizontal position x.
• v:
  Implementation of right ideas, but in an inconsistent, incomplete, or unorganized manner. Some attempt at systematic use of kinematic equations for projectile motion.
• x:
  Implementation of ideas, but credit given for effort rather than merit. No clear attempt at kinematic equations for projectile motion.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.

Grading distribution:
Sections 70854, 70855
Exam code: midterm01duCk
p: 15 students
r: 4 students
t: 9 students
v: 21 students
x: 2 students
y: 0 students
z: 0 students

A sample "p" response (from student 5832):
Another sample "p" response (from student 2342):

Physics quiz question: comparing horizontal velocity components

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

Two velocity vectors shown at right have different speeds and directions. All angles are measured counterclockwise from the +x axis. Velocity vector __________ has the larger horizontal component magnitude.
(A) v_A.
(B) v_B.
(C) (There is a tie.)
(D) (Not enough information is given.)

Correct answer (highlight to unhide): (B)

Since these θ angles are measured counterclockwise from the +x axis, the horizontal components of these velocity vectors are given by:

v_A,x = v_A·cosθ_A,
v_B,x = v_B·cosθ_B.

Then the horizontal components are these velocity vectors can be calculated and compared:

v_A,x = v_A·cosθ_A = (10 m/s)·cos(80°) = 1.736481776669303... m/s,

or to two significant figures, the horizontal component of v_A has a magnitude of 1.7 m/s, while:

v_B,x = v_B·cosθ_B = (4.0 m/s)·cos(60°) = 2.0 m/s.

Thus the horizontal component of v_B is greater than the horizontal component of the horizontal component of v_A.

Sections 70854, 70855
Exam code: quiz03Ch3V
(A) : 4 students
(B) : 49 students
(C) : 1 student
(D) : 0 students

Success level: 91%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.19

Physics quiz question: soccer ball vertical velocity component

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

A Physics 205A student kicks a soccer ball off a cliff with an initial velocity vector that has x- and y-components:

v_0x = +6.1 m/s,
v_0y = –3.2 m/s.

Neglect air resistance. Just before it hits the ground, the magnitude of the soccer ball's vertical velocity is:
(A) 0 m/s.
(B) some value between 0 m/s and 3.2 m/s.
(C) 3.2 m/s.
(D) some value greater than 3.2 m/s.

Correct answer (highlight to unhide): (D)

While the horizontal component of the soccer ball's velocity never changes, the vertical component of the soccer ball's velocity will always keep changing, due to the acceleration due to gravity. The soccer ball already starts with an initial downwards speed, and the vertical downwards speed will increase as it keeps moving along its trajectory.

Sections 70854, 70855
Exam code: quiz03Ch3V
(A) : 6 students
(B) : 5 students
(C) : 19 students
(D) : 24 students

Success level: 44%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.86

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: 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: Newton's third law

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 Newton's third law.


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 there are five different types of forces (static, kinetic, weight, tension, and normal) as covered in the presentation. I also understand that Newton's first law requires an object to have a constant speed and direction."

"Newton's third law concept-wise is fairly straightforward, for whenever one object exerts force on a second object, the second object exerts an oppositely directed force of equal magnitude on the first object."

"What I understood from the assigned reading was POF-OST-ITO. POF stands for 'Pair of Opposite Forces.' OST stands for 'Of Same Type,' and ITO stands for 'Involving Two Objects.' If there are two forces in question satisfy all three parts of the POF-OST-ITO checklist, then Newton's third law applies."

"Newton's third law and how it has nothing to do with motion or net force but is a fundamental property of force itself. I also understand interaction pairs and how all forces are the interactions between two objects, and the POF-OST-ITO checklist."

"The POF-OST-ITO checklist and how it helps us determine if two forces are covered under Newton's third law."

"I understand, possibly, the most basic concept of this chapter--forces come in pairs. Basic but important, as it leads into the first question of the POF-OST-ITO checklist, 'Pair of Opposite Forces?' An object that receives some force by another object will exert a force back onto the other."

"I understood the differences in force types, (hopefully, assuming I got the multiple choices correct) and I found the normal force to be most interesting, as it is the force that stops stationary objects from continuously falling, counteracting the object's weight force. Even though it's a simple concept, it's interesting to have it explained in the world of physics, rather than through observation in the real world."

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.

"In all classes I've ever had, when I see the Σ summation symbol my mind just goes somewhere else. I understand it is some simple summation, but I'm sure seeing it with values will help."

"I think I understand the POF-OST-ITO checklist for Newton's third law, but I am not entirely sure. I would appreciate some review on that."

"Newton's third law and the POF-OST-ITO mnemonic kind of scares me. I understand that it's just a mnemonic used to confirm that Newton's third law applies, but it's still new and scary."

"The most confusing part was POF-OST-ITO checklist, and exactly how Newton's first law and Newton's third law differ. If any of the POF-OST-ITO checklists fail, is it automatically a Newton's first law scenario?"

"The mnemonic is a little difficult to remember. There is a kind of confusing relationship between Newton's first law and third law."

"I am having difficulty understanding the difference between Newton's third law and how it relates to the first two laws."

"Something I found confusing in our reading was the section about normal force, weight, and net force go altogether. I do not how to solve a problem like that."

"I found the statement 'the word tension is commonly used to mean the tendency of the rope to be pulled apart.' I found this confusing because I wasn't sure if tension then required two different forces pulling on an object from opposite ends."

"I do not understand tension very well. I do not know if it is based on mass or weight. Tension could also be an individual force, just not a fundamental force?"

"When it comes to solving tension problems, I start getting confused when there are angles involved. How do we use x- and y- components for tension forces?"

"I have not finished the reading but will do so as I will be caught up by the end of the week!"

Consider two categories of motion: (1) Velocity that is constant and unchanging. (2) Velocity that is changing. Discuss whether or not there would be a third category of motion not already covered under these two categories.

"NO other category of motion exists."

"Logically, and given the two choices, I would say no. (I have no idea if this is right, but here goes.) If the two categories are 'changing' and 'not changing,' I don't really there's an option for 'somewhere in the middle changing.'"

"I do not think there is another category of motion that would not be covered by these two categories. The only other option would be if an object is not in motion, however this would mean they have a (constant and unchanging) zero velocity."

"There would not be a third category because these two categories involve velocity and whether it's constant/unchanging or changing. These two categories fall under Newton's first and second laws, however his third law is more focused on equal and opposite/reactions based off force pairs, or forces in general."

According to Newton's first law, the normal force of the person's head on the stack of books is equal in magnitude and opposite in direction to the:

normal force of the stack of books on the person's head.	****************** [18]
weight force of Earth on the stack of books.	******************** [20]
(Both of the above choices.)	** [2]
(Neither of the above choices.)	***** [5]
(Unsure/guessing/lost/help!)	[0]

According to Newton's third law, the normal force of the person's head on the stack of books is equal in magnitude and opposite in direction to the:

normal force of the stack of books on the person's head.	********************** [22]
weight force of Earth on the stack of books.	*********** [11]
(Both of the above choices.)	*********** [11]
(Neither of the above choices.)	* [1]
(Unsure/guessing/lost/help!)	[0]

Identify the type of interaction ("force") with its symbol. (Only correct responses shown.)

Weight ("gravitational force") : w [91%]
Surface contact force ("normal force"): N (or F_N) [91%]
Tension ("rope/cable/string force"): T [93%]
Kinetic friction ("sliding force," or "sliption"): f_k [84%]
Static friction ("sticking force," or "stiction"): f_s [89%]

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

"Can we think of the first law as using a single object as the reference point when determining forces and the third law as using two objects as reference points when determining how the forces of each act upon each other?" (Yes, exactly.)

"This was an interesting reading! I could use a little bit of clarification on a few things, but I'm sure I'll get it after the upcoming lecture."

"Can we have a quick clear summary of each force?" (We'll introduce them in-depth one at a time, in the examples we'll go over in class.)

"Please go slow for this particular section..... explain throughly."

"Will we have a flowchart for Newton's third law?" (You have the POF-OST-ITO checklist.)

"How much will we be doing universal law of gravitation problems?" (Actually, none. We're going to stay on this planet when it comes to doing physics.)

"Can you please clarify the difference between weight and mass." (We definitely will.)

"How would Newton's laws appear in a math problem?" (They're important concepts you should apply even before setting up a free-body diagram or solving an equation.)

"I appreciate your sternness when it comes to teaching these concepts, I thought you were gonna be goofy all semester."

"Will this be on the midterm?" (On both Quiz 3 and the first midterm, which is cumulative.)