Physics quiz question: "superhero landing" impulse

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

"Deadpool | 'Superhero Landing' | Official HD Clip 2016"
20th Century Fox UK
youtu.be/EwUilIo036g

In the movie Deadpool (2016), Angel Dust (portrayed by Gina Carano, mass 66 kg[*]) falls vertically downwards for a "superhero landing," coming to a complete stop from a downwards speed of 34 m/s in 0.24 s as she contacts the ground[**]. Ignore friction and drag. The magnitude of the stopping impulse of the ground on her was:
(A) 0 N·s.
(B) 2.2×10³ N·s.
(C) 9.4×10³ N·s.
(D) 3.8×10⁴ N·s.

[*] imdb.com/name/nm2442289/.
[**] Assuming an initial vertical speed of zero as she steps off the edge of a derelict helicarrier, with a flight deck assumed to be 60 m high above the ground.

Correct answer (highlight to unhide): (B)

From the impulse-momentum theorem, the impulse J on an object causes its initial-to-final change in momentum ∆p:

J = ∆p,

where ∆p = m·(v_f – v₀).

The initial velocity vector is v₀ = –34 m/s (traveling downwards), and the final velocity vector is v_f = 0 m/s ("a complete stop"). Then:

J = (66 kg)·((0 m/s) – (–34 m/s)) = (66 kg)·(+34 m/s) = +2,244 N·s,

or to two significant figures, the magnitude of the impulse is 2.2×10³ N·s (and the "+" sign indicates that it is in the upwards direction).

(Response (C) is the magnitude of the average stopping force over the 0.24 s stopping time interval; response (D) is the amount of translational kinetic energy dissipated by the landing.)

Sections 70854, 70855
Exam code: quiz04JuR4
(A) : 1 student
(B) : 27 students
(C) : 20 students
(D) : 4 students

Success level: 52%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.73

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: 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: 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 midterm problem: categorizing a cart collision

Physics 205A Midterm 2, fall semester 2018
Cuesta College, San Luis Obispo, CA

A 0.90 kg cart traveling in the +x direction at 0.45 m/s collides with a 0.15 kg cart that is initially at rest. The carts are not stuck together after the collision. After the collision, the 0.90 kg cart continues traveling in the +x direction at 0.35 m/s. Ignore friction, drag and other external forces during this brief collision. Find (a) the final velocity of the 0.15 kg cart, and (b) classify this collision as elastic, inelastic, or completely inelastic. Show your work and explain your reasoning using properties of collisions, energy (non-)conservation, and momentum conservation.


Solution and grading rubric:

• p:
  Correct. Finds final speed v_f2 = +0.60 m/s of the 0.15 kg cart, using conservation of momentum (as there is negligible drag/friction for this brief collision). It is not known whether the carts are permanently deformed and/or energy was lost to thermal/sound systems, so collision could be either inelastic or elastic (but cannot be completely inelastic because the carts are not stuck together after the collision). Explicitly tests for whether or not translational kinetic energy is conserved, and finds that since it is not conserved, this collision must be inelastic.
  
• r:
  Nearly correct, but includes minor math errors.
  
• t:
  Nearly correct, but approach has conceptual errors, and/or major/compounded math errors. Has correct v_f2 from momentum conservation, and at least some attempt at testing for translational kinetic energy conservation.
  
• v:
  Implementation of right ideas, but in an inconsistent, incomplete, or unorganized manner. Application of momentum conservation, but v_f2 is incorrect, with little or no test of kinetic energy conservation.
  
• x:
  Implementation of ideas, but credit given for effort rather than merit.
  
• y:
  Irrelevant discussion/effectively blank.
  
• z:
  Blank.
  

Grading distribution:
Sections 70854, 70855
Exam code: midterm02r3iN
p: 28 students
r: 7 students
t: 13 students
v: 4 students
x: 5 students
y: 0 students
z: 0 students

A sample "p" response (from student 1408):

Physics quiz question: impulse on Kenny Bräck

Physics 205A Quiz 4, fall semester 2018
Cuesta College, San Luis Obispo, CA

IndyCar race driver Kenny Bräck (mass 70 kg) blogged the details of his notorious accident at the Texas Motor Speedway in 2003:[*],[**]

My car caught air at [100 m/s], got airborne and smashed straight into a massive steel pole in the catch fence... It recorded a record [150,000 N force] impact [on me] and left me seriously injured.

Ignore friction and drag, and assume the car was completely stopped by crashing into the steel pole. (Kenny Bräck was later able to make a full recovery and return to racing.)

The magnitude of the stopping impulse was:
(A) 1.5×10³ N·s.
(B) 7.0×10³ N·s.
(C) 1.5×10⁵ N·s.
(D) 1.1×10⁷ N·s.

[*] kennybrack.com/pages/personal-info/2003.html.
[**] motorsportmagazine.com/archive/article/november-2014/102/lunch-kenny-br-ck.

Correct answer (highlight to unhide): (B)

The impulse J can be calculated as the initial-to-final change in momentum:

J = ∆p = m·∆v,

where ∆v = v_f – v₀.

The initial velocity vector is v₀ = +100 m/s (traveling to the right, in the +x direction), and the final velocity vector is v_f = 0 ("completely stopped"). Then with a mass m = 70 kg:

J = (70 kg)·((0 m/s) – (+100 m/s)) = –7,000 N·s,

or to two significant figures, the magnitude of the impulse is 7.0×10³ N·s (and the "–" sign indicates that it is directed to the left, opposite the direction of the initial velocity).

(Response (A) is the impact force divided by the initial velocity; response (C) is the impact force; response (D) is impact force multiplied by the mass.)

Sections 70854, 70855
Exam code: quiz04W3rK
(A) : 3 students
(B) : 34 students
(C) : 13 students
(D) : 1 student

Success level: 67%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.74

Physics quiz archive: energy conservation, momentum conservation

Physics 205A Quiz 4, fall semester 2018
Cuesta College, San Luis Obispo, CA
Sections 70854, 70855, version 1
Exam code: quiz04W3rK



Sections 70854, 70855 results

0- 6 :	* [low = 3]
7-12 :	********
13-18 :	**************
19-24 :	****************** [mean = 19.6 +/- 6.4]
25-30 :	********** [high = 30]

Online reading assignment: collisions

Physics 205A, fall semester 2018
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.

"There are three different types of collisions: completely inelastic, partially inelastic, and elastic. And each of these collisions can either conserve or not conserve kinetic energy."

"An inelastic collision is where the kinetic energy is not conserved. A completely inelastic collision is where it sticks together afterward, while partially inelastic collisions deform but don't stick together afterwards."

"The differences between inelastic and elastic collisions and how to identify them. Inelastic collisions (partial or complete) do not conserve kinetic energy, while elastic collisions do."

"An elastic collision is where two objects rebound and there is no permanent damage. Also, only elastic collisions conserve kinetic energy since there is no damage."

"Momentum is conserved for any collision as long as it is brief, the time is so short it becomes zero. Kinetic energy is conserved during elastic conclusions."

"When collisions happen during a very short period of time the left side of the momentum conservation equation can be set to 0. Basically when you are looking at a collision that happens in a short period of time you are just examining the exchange in momentum between the two objects."

"The different types of collisions and the rules for colisions. The conservation laws and the flowchart for them made perfect sense to me."

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 flowchart in the presentation was hard to understand. Also, the equation for momentum conservation look intimidating."

"I was confused by the principle of conservation of linear momentum."

"The equation for the conservations kind of confusing. I would just like to see examples worked out in class."

"I had a little trouble understanding the difference between elastic and inelastic collision. Elastic collision is when two objects collide head on and no heat or other form of energy is produced during the collision. Inelastic collision have some thermal and sound energy produced so kinetic energy is not conserved?"

"Categorizing collisions and determining kinetic energy conservation (I did read kinetic energy is only conserved for elastic collisions because none is permanently lost in deformation). Applying conservation law(s) for collisions."

"What I found confusing was why partially inelastic has kinetic energy lost but I think I understand it now."

"When we read the question it is very important to understand what happens after colliding. Sometimes I get confused about this."

"I understood everything in the presentation/textbook...for now anyway."

"I actually understood what this reading assignment was about."

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

"A partially inelastic collision is where two objects bounce off of each other. A completely inelastic collision is where objects do stick to each other. Permanent damage is present in both cases."

"A partially inelastic collision is when two objects rebound off each other but there is some permanent damage afterwards and a completely inelastic collision is when two objects collide but they stick together."

"A partially inelastic collision has a small rebound with damage and a completely elastic collision has no rebound and the two crashing objects become 'stuck' to each other."

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

"The time is so small that the friction is negligent."

"We can neglect their impulses because the brief collision occurs in an instant, so δt would be zero."

"External forces/impulses would be negligible in the relatively brief time that collisions occur."

"Because collisions happen in such a brief time, external forces do not factor."

"These 'brief' collisions happen in a fraction of a second so even though these forces can be quite large their impulses are negligible because of the amount of time they take place in. So momentum is conserved because the objects will only exchange momentum."

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.	* [1]
(Partially) inelastic.	********************************* [33]
Elastic.	** [2]
(Unsure/lost/guessing/help!)	* [1]

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.	***** [5]
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.	******************************* [31]
(Partially) inelastic.	*** [3]
Elastic.	** [2]
(Unsure/lost/guessing/help!)	* [1]

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.	*** [3]
(Partially) inelastic.	*********************** [23]
Elastic.	******* [7]
(Unsure/lost/guessing/help!)	**** [4]

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

Completely inelastic.	***** [5]
(Partially) inelastic.	************ [12]
Elastic.	*************** [15]
(Unsure/lost/guessing/help!)	***** [5]

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

"How can you tell if something gets damaged during a collision, such as when firing a gun? Should we just have real-world knowledge or should we know this from examples?" (You'll practice this in class; basically any process that removes or adds kinetic energy counts as "damage.")

What would be the collision if the object went through the other object, for example a bullet going through another object." (That counts as "damage," and since the bullet and object are not stuck together afterwards, then this would be a partially inelastic collision. (If the bullet was embedded in the object, such that they are "stuck together," then this would be a completely inelastic collision.)

"Kinetic energy is all lost when you have completely inelastic collision?" (Well, not all of it is lost, but enough of it is lost such that the objects stick together (rather than rebounding) after the collision, even if they both still have some motion together afterwards.)

Online reading assignment: impulse and momentum

Physics 205A, fall semester 2018
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.

"The concept of momentum and that its magnitude depends on both the mass and speed of an object, and how impulse is the product of the net force acting on an object and the duration of time that the net force acted on this object."

"That momentum is the mass of an object times the speed of that object and it is a vector because of the direction of that object's speed. Impulse is dependent on the average net force applied on an object and how long that force is applied."

"According to the impulse-momentum theorem, impulse causes a change in momentum."

"Both momentum and impulse are vector quantities. Momentum is the product of mass and velocity and change in momentum could be caused by impulse which is equal to the product of force and time."

"How to find the change in momentum. The change in momentum is the final momentum minus the initial momentum."

"It seemed like I was applying momentum and impulse the correct way during the examples but now going back over it I don't think I have a clear understanding of what either of them are."

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 found the impulse-momentum theorem a bit confusing."

"I didn't understand the concept of impulse, which is the product of the net force acting on an object and the duration of time that the net force acted on this object."

"I'm struggling to understand what exactly impulse and momentum are. More like what they do, and how to describe them in a problem."

"I don't understand why time plays a role in impulse and the change in momentum, but I think I understand now."

"Figuring out the signs (+ or –) when finding the change in momentum. I think it depends on what direction the object is going, but on some of the examples I was not sure what the signs would be."

"I honestly didn't find much in this chapter confusing. The subjects of momentum and impulse were not very difficult to me."

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.	[0]
less; greater.	******* [7]
greater; less.	***** [5]
greater; greater.	***************************** [29]
(Unsure/lost/guessing/help!)	*** [3]

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). [86%]
Golf ball's final momentum p_f: to the right (+). [86%]
Golf ball's initial-to-final change in momentum ∆p: to the right (+). [81%]
Golf club's impulse "J" on the golf ball: to the right (+). [70%]

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). [86%]
Super Hornet's final momentum p_f: to the left (–). [39%]
Super Hornet's initial-to-final change in momentum ∆p: to the left (–). [36%]
Catapult's impulse "J" on the Super Hornet: to the left (–). [45%]

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 (+). [86%]
Ford Ranger's final momentum p_f: to the left (–). [39%]
Ford Ranger's initial-to-final change in momentum ∆p: to the left (–). [36%]
Crash barrier's impulse "J" on the Ford Ranger: to the left (–). [45%]

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

"Can you go over the impulse-momentum theorem?"

"Is it possible to go over more conceptual aspects of this rather than examples? I'm having trouble figuring out from the reading what each of these concepts mean and I feel that I should know better what they are before I do example practice problems."

"Can you do more problems in class?"

"No questions on today's assignment, I think I got this so far!"

"I think I understood this stuff...after answering these questions, let's hope!"

"What?"

"Which is a scarier thought? That the human race is the most advanced form of life in the universe, or that we are mere amoebae compared to other life forms?"

"PANICKING!! WORRIED ABOUT MIDTERM!! Practice midterm NOT helpful. Sure we have the long weekend to study, but it will be difficult to ask you for help face-to-face (which is more effective than email)." (I will specifically go over what you need to know for the midterm, and target the Wednesday's review session such that ideally you can spend the long weekend how to apply the tools that you're given for the midterm, rather than trying to determine what tools are needed for the midterm.)