Physics midterm question: comparing relative amounts of energy changes

Physics 205A Midterm 2, 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. Starting from rest, the crate slides to the right while the block descends downwards, both with increasing speed. Ignore friction and drag. Determine which of these two energy forms undergoes a larger amount of change (increase or decrease) for this process, or if there is a tie:

translational kinetic energy of the crate;
gravitational potential energy of the block.

Explain your reasoning using the properties of energy forms, and conservation of energy.

[*] youtu.be/CgNlPOMOps0.

Solution and grading rubric:

• p:
  Correct. Applies energy forms and conservation concepts with:
  1. both the crate and the box are speeding up, such that their (same) final speed is faster than their initial speed (zero), making both their translational kinetic energy terms increase; and
  2. the box is going downwards, such that the final height is lower than the initial height, making its gravitational potential energy decrease; and
  3. sets up a transfer-balance energy conservation equation with the sum of the changes in translational kinetic energy of the crate, translational kinetic energy of the box, and gravitational potential energy of the box set to zero (as no energy is lost to non-conservative work); then
  4. since the decrease in gravitational potential energy of the box is "feeding" the increase in the translational kinetic energies of both the crate and the box; the gravitational potential energy of the box must undergo a larger change than the translational kinetic energy of the crate.
• 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. Correctly determines that no energy is lost to non-conservative work, but discusses how the decrease in gravitational potential energy of the block is transferred solely to the increase in translational kinetic energy of the crate (concluding that there is a tie in the amount of change of these two energy forms), while neglecting the increase in translational kinetic energy of the block.
• v:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Some garbled attempt at using properties of forces, work, energy forms and (non-)conservation of energy.
• x:
  Implementation of ideas, but credit given for effort rather than merit. Approach other than using properties of forces, work, energy forms and (non-)conservation of energy.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.

Grading distribution:
Sections 70854, 70855
Exam code: midterm02sQm5
p: 5 students
r: 1 student
t: 35 students
v: 9 students
x: 2 students
y: 0 students
z: 0 students

A sample "p" response (from student 1478):

Physics midterm problem: basketball rolling up ramp

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

A basketball (mass 0.43 kg, radius 0.11 m) rolls without slipping with a constant initial velocity of 2.4 m/s across a horizontal floor. The basketball begins to roll up a ramp. Determine the highest vertical height that the basketball will reach before rolling back down the ramp. Ignore friction and drag. Show your work and explain your reasoning using the properties of rotational inertia, energy forms, and conservation of energy.

The basketball is a hollow sphere (I_{hollow sphere} = (2/3)·M·R².)


Solution and grading rubric:

• p:
  Correct. Sets up a transfer-balance energy conservation equation with the sum of the changes in translational kinetic energy of the basketball, rotational kinetic energy of the basketball, and gravitational potential energy of the basketball set to zero (as no energy is lost to non-conservative work), fills in all known/given values, and solves for the unknown. (May have ± sign errors for final terms subtracting initial terms, but in a somewhat consistent manner that still results in correct final height of basketball).
• r:
  Nearly correct, but includes minor math errors. Or multiple arithmetic errors.
• t:
  Nearly correct, but approach has conceptual errors, and/or major/compounded math errors. Typically omits one of the energy terms, but attempts to apply energy conservation to the remaining two terms.
• v:
  Implementation of right ideas, but in an inconsistent, incomplete, or unorganized manner. Calculations of some energy terms, but does not sufficiently tie them together in a transfer-balance energy conservation equation. Typically has only one energy term, or relates an energy term with a non-energy quantity (such as weight, momentum, moment of inertia, etc.).
• x:
  Implementation of ideas, but credit given for effort rather than merit. Approach involving methods other than energy conservation.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.

Grading distribution:
Sections 70854, 70855
Exam code: midterm02sQm5
p: 26 students
r: 16 students
t: 7 students
v: 3 students
x: 0 students
y: 0 students
z: 0 students

A sample "p" response (from student 8520):
Another sample "p" response (from student 1203):

Physics quiz question: energy changes of barrel rolled up a ramp

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

A Physics 205A student uses a ramp and a trailer to deliver an empty barrel. Ignore friction and drag; the barrel rolls without slipping. As the barrel is rolled up the ramp with constant speed, its __________ increases.
(A) gravitational potential energy.
(B) translational kinetic energy.
(C) rotational kinetic energy.
(D) (Two of the above choices.)
(E) (All of the above choices.)
(F) (None of the above choices.)

Correct answer (highlight to unhide): (A)

The energy transfer-balance equation is given by:

W_nc = ∆KE_tr + ∆KE_rot ∆PE_grav + ∆PE_elas,

where ∆PE_elas = 0, as there is no spring involved in this process.

Just looking at the three remaining terms on the right-hand side of the energy transfer-balance equation, for the change in translational kinetic energy:

∆KE_tr = (1/2)·m·(v_f² – v₀²),

and since the speed is constant, v₀ and v_f have the same magnitude, then KE_tr is constant (∆KE_tr = 0).

Similarly for the change in rotational kinetic energy:

∆KE_rot = (1/2)·I·(ω_f² – ω₀²),

and since the angular speed is constant, ω₀ and ω_f have the same magnitude, then KE_rot is constant (∆KE_rot = 0).

Then for the change in gravitational potential energy:

∆PE_grav = m·g·(y_f – y₀),

since y_f is greater than y₀, then PE_grav increases (∆PE_grav is positive).

(In order for the equality to hold for the energy transfer-balance equation, the student must then be doing positive work on the barrel in order to increase its gravitational potential energy.)

Sections 70854, 70855
Exam code: quiz05Gu1L
(A) : 41 students
(B) : 1 student
(C) : 0 students
(D) : 7 students
(E) : 3 students
(F) : 0 students

Success level: 79%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.53

Physics quiz archive: rotations, torque, pressure, buoyancy, fluid flow

Physics 205A Quiz 5, fall semester 2019
Cuesta College, San Luis Obispo, CA
Sections 70854, 70855, version 1
Exam code: quiz05Gu1L



Sections 70854, 70855 results

0- 6 :	** [low = 6]
7-12 :	*********
13-18 :	***********
19-24 :	***************** [mean = 19.8 +/- 6.3]
25-30 :	************* [high = 27]

Online reading assignment: simple harmonic 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 simple harmonic 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.

"Simple harmonic motion is repeated motion back and forth, assuming no outside forces interact with the object, and that under ideal circumstances the motion repeats continuously. When graphed, Simple harmonic motion tends to be similar to a sine/cosine function."

"That the period T is the time it takes for an object in simple harmonic motion to complete one cycle. I also understand that the frequency is related to the period in that the frequency is the number of cycles of the motion per second and can used to find the hertz, or one cycle per second."

"That total energy conservation equals a constant because as KE translational increases then PE elastic will decrease. I also know simple pendulum period is determined by the length of the string which is why in the billiard ball example they all start to swing at different rates even though they are released at the same time."

"That springs have no kinetic energy and maximum potential energy at compression and fully stretched, while they have maximum kinetic energy and no potential energy at equilibrium."

"I don't think I understand anything in this section. I've re-read the section multiple times."

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.

"The part that confuses me is putting it all together. But lecture will help with that."

"I didn't really understand the pendulum period/length stuff."

"I did not really understand the units of certain variables in some of the new functions like L."

"Do pendulum/mass-spring periods always use 2π in the equation?"

"The mass spring period is a little confusing, I feel as though I understand the basis of it. Essentially more mass means more time or T in the mass spring period. Also a weaker spring constant would mean T would be greater. I think."

"There weren't any major things I found confusing. However, it is helpful when we go over the math of the problems and how to sub things in."

"I just need this taught to me. I don't learn well from reading."

"I first was confused on when to use the mass-spring period equation and the energy conservation equation. But then I realized that the first is used when given time and the other when focusing on energy."

"The math in how to perform these examples and the wording in the book is very confusing."

"I understand the basics but I guess the only thing I find 'confusing' is how many formulas are introduced in this chapter. I feel like normally we focus on one or two. I hope you will clarify which ones we will be using primarily."

The total energy of a mass-spring system would have zero translational kinetic energy and all elastic potential energy at the:

origin (x = 0).	******* [7]
turnaround points (x = ±A).	************************************* [37]
(Both of the above choices.)	* [1]
(Neither of the above choices.)	[0]
(Unsure/lost/guessing/help!)	[0]

The total energy of a mass-spring system would have all translational kinetic energy and zero elastic potential energy at the:

origin (x = 0).	************************************* [37]
turnaround points (x = ±A).	****** [6]
(Both of the above choices.)	[0]
(Neither of the above choices.)	** [2]
(Unsure/lost/guessing/help!)	[0]

For these identical mass billiard balls hanging from strings of different lengths, the billiard balls hanging from shorter length strings have periods that are _________ the periods of the billiard balls hanging from longer length strings.

shorter than.	***************************************** [41]
equal to.	** [2]
longer than.	[0]
(Unsure/lost/guessing/help!)	** [2]

For the different mass riders on swings of the same lengths, the more massive rider has a period __________ the period of the less massive rider.

shorter than.	***** [5]
equal to.	************** [14]
longer than.	************************** [26]
(Unsure/lost/guessing/help!)	[0]

For the overloaded truck with a vertical oscillation period of approximately half a second, after dumping its load (thus decreasing the mass connected to the springs in its suspension) would __________ the period of oscillation.

decrease.	************************* [25]
have no affect on.	* [1]
increase.	*************** [15]
(Unsure/lost/guessing/help!)	**** [4]

For this mass connected to two springs, its springs weakening over time (decreasing its spring constant) would __________ the period of oscillation.

decrease.	********************** [22]
have no affect on.	**** [4]
increase.	*************** [15]
(Unsure/lost/guessing/help!)	**** [4]

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

"Go over these."

"I didn't understand the swing question, they have different masses but appear to be swinging the same." (Yes, that is exactly what you should be seeing.)

"How much more will we be working with springs and stuff?" (You'll have a lab on this later, where you tinker around with periods of pendulum and mass-spring systems.)

"Seems simple enough."

"I'm never going to understand physics."

"Life is really getting to me now."

"I can't believe it's already November :|"

Physics quiz question: camel spider translational kinetic energy (purported)

Physics 205A Quiz 4, fall semester 2019
Cuesta College, San Luis Obispo, CA

"Mr. Solifugae (probably Eremobates)"
Dallas Krentzel
flic.kr/p/9Ki3kX

Consider the myths listed below[*] as factual for camel spiders (Eremobates pallipes, also known as "wind scorpions"), which are native to California. Ignore friction and drag.

Camel spiders can reach up to 6 inches (15 cm) in length and weigh about 2 ounces (57 grams). Some common myths...are that they can run up to 30 mph (13 m/s) and jump up to 3 feet [1 m] high.

A camel spider that starts from rest and speeds up to 13 m/s will have an increase in translational kinetic energy of:
(A) 0.4 J.
(B) 0.6 J.
(C) 0.7 J.
(D) 5 J.

[*] Jessie Szalay, "Camel Spiders: Facts & Myths" (December 17, 2014), livescience.com/40025-camel-spiders-facts.html.

Correct answer (highlight to unhide): (D)

The change in translational kinetic energy is given by:

∆KE_tr = (1/2)·m·(v_f² – v₀²),

∆KE_tr = +4.8165 J,

or to two significant figures, the increase in translational kinetic energy is 5 J.

(Response (A) is (1/2)·m·(v_f – v₀); response (B) is m·g·(y_f – y₀); response (C) is m·(v_f – v₀).)

Sections 70854, 70855
Exam code: quiz04JuR4
(A) : 11 students
(B) : 2 students
(C) : 1 student
(D) : 38 students

Success level: 73%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.56

Physics quiz question: energy changes of a dragged box speeding up a ramp

Physics 205A Quiz 5, fall semester 2018
Cuesta College, San Luis Obispo, CA

A Physics 205A student pulls a 15.0 kg box with increasing speed up a ramp with a rope that is parallel to the slope of the ramp. Friction is not negligible. Ignore drag. For this process, the __________ of the box increases.
(A) gravitational potential energy.
(B) translational kinetic energy.
(C) (Both of the above choices.)
(D) (Neither of the above choices.)

Correct answer (highlight to unhide): (C)

The change in translational kinetic energy is given by:

∆KE_tr = (1/2)·m·(v_f² – v₀²).

Since the speed of the box is increasing, v_f is faster than v₀, and so KE_tr is increasing (∆KE_tr is positive).

The change in gravitational potential energy is given by:

∆PE_grav = m·g·(y_f – y₀).

Since y_f is higher than y₀, then PE_grav also increases (∆PE_grav is positive).

Sections 70854, 70855
Exam code: quiz04JuR4
(A) : 17 students
(B) : 7 students
(C) : 28 students
(D) : 0 students

Success level: 54%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.75

Physics quiz question: completely inelastic lab cart collision

Physics 205A Quiz 4, fall semester 2019
Cuesta College, San Luis Obispo, CA

Two lab carts initially move towards the right along a horizontal track, and make a completely inelastic collision. Ignore friction and drag during this brief collision.


After the collision, the 0.200 kg cart has a speed that is __________ the speed of the 0.500 kg cart.
(A) slower than.
(B) equal to.
(C) faster than.
(D) (Not enough information is given.)

Correct answer (highlight to unhide): (B)

In a completely inelastic collision, the two colliding objects stick to each other such that they have the same speed (and direction) afterwards.

Sections 70854, 70855
Exam code: quiz04JuR4
(A) : 7 students
(B) : 40 students
(C) : 5 students
(D) : 0 students

Success level: 77%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.48

Physics quiz archive: energy conservation, momentum conservation

Physics 205A Quiz 4, fall semester 2019
Cuesta College, San Luis Obispo, CA
Sections 70854, 70855
Exam code: quiz04JuR4



Sections 70854, 70855 results

0- 6 :	** [low = 6]
7-12 :	****
13-18 :	*******
19-24 :	********************** [mean = 21.9 +/- 6.1]
25-30 :	***************** [high = 30]

Online reading assignment: rotational dynamics

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 rotational dynamics.


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.

"This section is about rolling objects in terms of rolling dynamics, rotational inertia, and angular speed. With these ideas the reading linked them to previous ideas of potential and kinetic energy."

"I somewhat understand how to recognize changes (increases/decreases) in rotational kinetic energy, and how rotational speed ω is related to translational speed v for objects that roll without slipping."

"The concept of balancing the angular speed equation v =r·ω. Larger objects have larger radii and therefore must have a smaller ω to match the velocity of a smaller object."

"As a circular object rolls down a hill without slipping, its gravitational potential energy decreases at the same rate that its (translational kinetic energy + rotational kinetic energy) INCREASES. I also understand that when a circular object that doesn't slip launches off of a ramp, it will maintain the SAME rotational kinetic energy in the air that it had at the moment it left the ramp."

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.

"Moment of inertia to be confusing."

"Rotational inertia and inertia in general. I'm also confused on how we relate inertia and energy in the equations given."

"How to find the variables in the equations, they all look like random letters and I don't know what they stand for."

"I think I just need some review to further understand the difference between equations and how to memorize what equation to actually use in a given situation. Otherwise, I feel confident on this subject."

"The concept of rolling motion is still confusing. I'm unsure of what tangential speed really is."

"I'm trying to wrap my head around the rolling motion."

"I would like a little more explanation on all the terms in the energy transfer-balance equation."

"Upon completing the presentation preview, I was very confused about the rotational kinetic energy at first but soon figured it out once given another glance. The rotational kinetic energy depends on rotational inertia and the angular speed of the object."

What is the SI (Système International) unit for rotational kinetic energy?

"J for joules."

"Like other forms of energy, the SI unit for rotational kinetic energy is the joule (J)."

"kg·m²/s²."

Describe an object that only has rotational kinetic energy, and no translational kinetic energy.

"A ceiling fan is an object that has only rotational kinetic energy and no translational kinetic energy."

"A windmill/wind turbine."

"The gear in a watch or machine that only spins and stays stationary."

"A hamster wheel has rotational kinetic energy but no translational kinetic energy."

"It would be an object that continues to spin, but remains in one place, like a Ferris wheel."

"An ice skater spinning in one place would only have rotational kinetic energy but not translational. This is because they are only rotating about an axis, but are not moving through space from one point to another."

"An object can rotate really really fast but its rotational kinetic energy can still be zero as long as it does not have any contact with a solid surface. Example: a car at an auto shop is resting on a lift like in the movie Feris Bueller's Day Off, and the rear wheels are rotating really fast, but since there no contact with the ground the car is not moving anywhere and transitional kinetic energy is zero."

"Any wheel that spins around a fixed axis."

"I'm not 100% on understanding this but I think a compact disc would have rotational kinetic energy but not translational because it isn't moving."

"I'm not sure on this one."

Describe an object that has both translational kinetic energy and rotational kinetic energy.

"A ball rolling down a hill."

"The wheels on a skateboard which are both rotating and moving forward."

"A unicycle."

"A car driving on the walls of a circle."

"The teacup ride at Disneyland is an example of an object with both rotational and translational kinetic energies because it is rotating about an axis as well as moving from one point in space to another."

"I need further instruction on this."

"I am not sure."

From starting at the top of the ramp to the bottom of the ramp, indicate the changes in each of the energy forms of the tire.
(Only correct responses shown.)

Gravitational potential energy: decreases [76%]
Translational kinetic energy: increases [78%]
Rotational kinetic energy: increases [80%]

From starting at the top of the ramp to the bottom of the ramp, the energy form that experienced the greatest amount of change (increase or decrease) was the tire's:

gravitational potential energy.	****************** [18]
translational kinetic energy.	***** [5]
rotational kinetic energy.	********* [9]
(There is a tie.)	******* [7]
(Unsure/lost/guessing/help!)	******* [7]

For the subsequent part of this stunt, from just as it leaves the second ramp to reaching the top of its trajectory, indicate the changes in each of the energy forms of the tire.
(Only correct responses shown.)

Gravitational potential energy: increases [76%]
Translational kinetic energy: decreases [57%]
Rotational kinetic energy: no change [43%]

For the subsequent part of this stunt, from just as it leaves the second ramp to reaching the top of its trajectory, the energy form that experienced the greatest amount of change (increase or decrease) was the tire's:

gravitational potential energy.	****************** [18]
translational kinetic energy.	********* [9]
rotational kinetic energy.	*** [3]
(There is a tie.)	******** [8]
(Unsure/lost/guessing/help!)	******** [8]

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

"Can we go over specific examples for each type of energy showing when each energy increases and decrease?"

"I am EXTREMELY confused how to tell the difference between the gravitational, rotational, and translational energies. May we have a quick simple break down summary in class?"

"I'm liking the colder weather."

"Q: How does a German physicist drink beer? A: With ein Stein."

Online reading assignment: collisions

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 collisions.


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.

""Total momentum of an isolated system is the same before and after a collision. An isolated system is which the vector sum of the average external impulses acting on the system is zero."

"There are three ways to identify collisions: elastic, inelastic, and completely inelastic. They are classified according to whether the total kinetic energy changes due to the collision."

"There are three types of collisions: elastic, partially elastic, and completely inelastic. Kinetic energy is only conserved in elastic collisions because there is no permanent deformation of the objects colliding."

"When objects collide and rebound off each other without causing damage the energy is conserved. However, when two objects collide and are crushed or bent, kinetic energy is lost as heat or from deformation."

"An elastic collision is when there is no damage. Partial inelastic means there is crushing or damage but the objects are no longer touching. Finally complete elastic is when there is damage and the objects are still touching."

"In an elastic collision, the two objects rebound off each other, and ideally there is no permanent damage deformation afterwards. Even though there is no cosmetic damage in this collision there was minor structural damage, but this is close to a ideal elastic collision."

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 am confused on where the kinetic energy is exactly transferred to, depending on if it's an elastic or inelastic collision."

"How kinetic energy is related to the different types of collisions. I also don't understand momentum conservation."

"I found the principle of conservation of linear momentum to be confusing for me. The textbook it states 'the total linear momentum of an isolated system remains constant. An isolated system is one for which the vector sum of the average external forces acting on the system is zero.' Something about this definition just did not fully click for me and I'm hoping lecture will help me to gain a better understanding."

"I don't understand anything about conservation of momentum. I don't understand how the internal and external forces affect the collision, or when the net force is zero or not. I also don't understand how to tell if the kinetic energy is the same or not before/ after a collision."

"The mathematical analysis of collisions are difficult to interpret correctly."

"I found using the equations confusing especially when deciding when a velocity is negative or positive."

"I'm confused on how to use the momentum and kinetic energy conservation equations."

"Is momentum the same as kinetic energy?"

Explain the difference between a (partially) inelastic collision and a completely inelastic collision.

"In a partially inelastic collision, the two objects collide and rebound off each other but there is permanent damage done afterwards. In a completely inelastic collision, the two objects will collide and stick together."

"With both inelastic collision types, the kinetic energy of the system after the collision is not the same as the kinetic energy of the system before a collision and the objects still separate, but with completely inelastic the objects will stick together."

"Partially inelastic results in damage to the objects but they do not become stuck. Completely inelastic is when the objects become stuck and are physically altered. More damage means more kinetic energy loss."

Explain why drag, friction, and other external forces do not matter during sufficiently "brief" collisions, in order for momentum to be conserved.

"The contact forces during collisions are so large compared to the external forces such as friction and air resistance that we neglect them."

"The drag, friction, and other external forces do not matter during sufficiently 'brief' collisions because those impulses would be almost negligible in at such a short time period."

"They are negligible due to the relatively brief time that collisions take place, so if we look at the left (external impulse) side of the momentum conservation equation we can set it equal to zero, so the objects are exchanging momentum."

"I don't fully understand to be honest."

For the Nissan Altima and Nissan Rogue crash test, classify the type of collision. (Neglect drag/friction/external forces during this "brief" collision.)

Completely inelastic.	[0]
(Partially) inelastic.	********************************** [34]
Elastic.	** [2]
(Unsure/lost/guessing/help!)	** [2]

For the Ford Explorer and Ford Taurus crash test, classify the type of collision. (Neglect drag/friction/external forces during this "brief" collision.)

Completely inelastic.	** [2]
(Partially) inelastic.	****** [6]
Elastic.	**************************** [28]
(Unsure/lost/guessing/help!)	** [2]

For the train and minivan crash, classify the type of collision. (Neglect drag/friction/external forces during this "brief" collision.)

Completely inelastic.	**************************** [28]
(Partially) inelastic.	*** [3]
Elastic.	***** [5]
(Unsure/lost/guessing/help!)	** [2]

For the bullet burrowing through and back out of the baseball, classify the type of collision. (Neglect drag/friction/external forces during this "brief" collision.)

Completely inelastic.	***** [5]
(Partially) inelastic.	*************************** [27]
Elastic.	*** [3]
(Unsure/lost/guessing/help!)	*** [3]

For the bullet shot out of this gun, classify the type of collision. (Neglect drag/friction/external forces during this "brief" collision.)

Completely inelastic.	*** [3]
(Partially) inelastic.	************ [12]
Elastic.	***************** [17]
(Unsure/lost/guessing/help!)	****** [6]

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

"Which type of collision consists of one object being damaged but the other object was not altered in any way?" (Even if only one object was damaged, then that would make it either a partially inelastic collision (if they separated afterwards) or a completely inelastic collision (if they stuck together afterwards). An elastic collision is only if both objects are undamaged and separated afterwards.)

"Is there such a thing as a partially elastic collision?" (That would be covered under the partially inelastic collision category.)

"Can a collision actually be elastic even with drag air resistance?" (On the smallest scale, gas atoms and molecules bounce off each other elastically with no energy loss.)

"It was harder than I thought to determine which one of those were elastic or inelastic or completely inelastic."

"Will we be looking at car crashes and how it relates to the three types of collisions in lab?" (Yes, specifically the completely inelastic collision type.)

"I'm a little confused when deciding whether or not a collision is completely inelastic or partially inelastic. What happens if one object goes completely through the other?" (As in the bullet goes into the baseball and comes out the other side, there is permanent damage done, so some kinetic energy was lost, such that this makes it a (partially) inelastic collision.)

"I think this is the first thing I have somewhat understood all semester so far."

Online reading assignment: energy conservation

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 energy conservation.


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.

"The easiest concept for me to understand from the reading and slide is the fact that translational kinetic energy is directly correlated to speed, in that if speed increases, so does translational kinetic energy."

"Gravitational potential energy is the energy that an object of mass has by its position in relation to the surface of Earth measured by the height of the object and an arbitrary zero level."

"I think gravitational potential energy is the easiest energy type to identify. If an object is 2 m off the ground then it has more gravitational potential energy than if the same object were 1 m off the ground."

"The gravitational force can do negative and positive work and that only the initial and final position should be considered when solving problems. This is also why it is considered a conservative force. Also, the greater the height, the greater gravitational potential energy is."

"A conservative force is not dependent on the path between start and finish points. Therefore, these types of forces also equate to zero work if the start and finish are the same."

"Conservative force is when the work done on an object is independent from the path taken between the initial and final heights, or when it does no net work on an object on a closed path. Nonconservative force is when the work done on an object moving between two points does depend on the path of motion, as we see with force of friction, air resistance, tension, etc."

"The basics of gravitational and elastic potential energy. And how they are combined with the change in translational kinetic energy in transfer balance equation."

"When a spring is pulled back it causes the energy to be stored and once released it shoots out."

"I have learned that elastic potential energy, translational kinetic energy and gravitational potential energy all play a part in non-conservative and conservative work. As the height of where an object is increases, so does its gravitational potential energy. As a slingshot is stretched back with a rock in the sling, the elastic potential energy increases. As it is fired off, the translational kinetic energy increases, and elastic potential energy decreases."

"This specific topic was kind of a blur, not gonna lie. I understand conservative forces and how how they can store energy and later retrieve without loss, and the opposite for nonconservative energy. That makes sense, but what doesn't is the application of information into the equations."

"Not much."

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 am a little unsure if the translational kinetic energy is simply increased as an object's movement or speed increases."

"Translational kinetic energy is difficult to comprehend on a practical level."

"Applying the equations to gravitational potential energy and elastic potential energy. Also need better clarification of conservative and non-conservative work."

"The transfer-balance equation and how to use it."

"I would like an example of using the transfer-balance equation in class because I am still confused on how to properly implement it into my problem-solving."

"I found the transfer-balance equation slightly confusing at first but then soon realized that its a total energy conservation equation. Other than that I did not find anything else confusing."

"I found it confusing to apply the certain types of energies to certain problems. For example, when do you use potential energy, kinetic energy, and/or mechanical energy? I understand the differences of the three. However, will a question on a test ask us to find one of these or a few of these or will we have to decide that based on the problem? "

For the woman moving upwards after being catapulted, her translational kinetic energy __________ while her gravitational potential energy __________.

decreases; increases.	*************************************** [39]
increases; decreases.	********* [9]
(Unsure/lost/guessing/help!)	* [1]

For the ball bearing being launched by the slingshot, its translational kinetic energy __________ while the elastic potential energy of the slingshot bands __________.

decreases; increases.	********** [10]
increases; decreases.	************************************** [38]
(Unsure/lost/guessing/help!)	* [1]

For the woman falling off the building starting from the edge of the roof to just before reaching her lowest point of descent, indicate the changes in each her of energy forms. (Only correct responses shown.)

Translational kinetic energy: increases. [80%]
Gravitational potential energy: decreases. [74%]
Elastic potential energy (of the bungee cords): increases. [76%]

For the woman falling off the building starting from the edge of the roof to just before reaching her lowest point of descent, the energy form that experienced the greatest amount of change (increase or decrease) was:

her translational kinetic energy.	************* [13]
her gravitational potential energy.	*************** [15]
the elastic potential energy of the bungee cords.	**************** [16]
(Unsure/lost/guessing/help!)	***** [5]

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

"How do we know when something is non-conservative and conservative?"

"For the examples above, they were cases of either an increase of kinetic energy and decrease of gravitational potential energy, or vice versa. Is it possible the have increase-increase or decrease-decrease?" (Only if there was (positive) work be done to add energy to the system, or (negative) work being done to remove energy from the system. Otherwise if there are only two types of energy and no work being transferred to/from the system, then an increase in one type of energy must be fed by a corresponding decrease in the other type of energy, and vice versa.)

"For the woman on the bungee swing, how are we supposed to know which energy experiences the greatest amount of change without numeric values?"

"Will there be a study guide for the midterm that's a little more structured on the key concepts we need to know? Or do we just have to shuffle through our quizzes and homework, etc.?" (Go through the list of example midterm questions on next week's announcements page. The idea is that the methods and concepts used to answer those examples is what you would need to apply to answer your upcoming midterm.)

"Having a hard time understanding the textbook, YouTube is my friend but also sometimes my enemy."

"Can we do an end of the year potluck?" (The final exam is basically a "knowledge potluck." Everyone should bring something.)