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]

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]

Physics quiz archive: energy conservation, momentum conservation

Physics 205A Quiz 4, fall semester 2017
Cuesta College, San Luis Obispo, CA
Sections 70854, 70855, version 1
Exam code: quiz04w33N



Sections 70854, 70855 results

0- 6 :
7-12 :	***** [low = 9]
13-18 :	****************
19-24 :	**************** [mean = 20.8 +/- 5.7]
25-30 :	*********** [high = 30]

Physics presentation: impulse and momentum

Whuuuuuuut. (Video link: "bowling strike with a ping pong ball.")

In this presentation we will introduce another new connection between forces and motion, in terms of how the net force can exert an impulse on an object in order to change its momentum. This is yet another new approach to connecting forces and motion, compared to the previous discussion in this course of using Newton's laws to relate how forces on an object result in a net force that may or many not change its motion, and analyzing how forces can do work on or against an object in order to speed up or slow down its motion.

First, defining the momentum of an object, and then expressing how the net force can exert an impulse on this object.

The introduction slide showing a ping-pong ball knocking over all ten bowling pins should seem very strange to you, as the mass of the ping-pong ball is too small to effectively bowl a strike, even if it were traveling with a supersonic speed. In order to fully account for the "knocking-over" strength of a moving object, then, we must include mass as well as its speed (and direction) to define its momentum p.

Momentum p is a vector quantity (so don't forget to draw an arrow over it) whose magnitude depends both on the mass and speed of the object, with the combined units of both mass and speed (kg·m/s).

We also need to introduce the concept of impulse J, 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 (whether for a brief instant, or for a prolonged period). (Video link: "Teaching Tee Ball Hitting.")

Impulse has the combined units of both force and time (N·s). Here we use the somewhat obscure (but totally legit) "J" symbol for impulse, remembering to draw an arrow over it (as it is a vector quantity). (It turns out that "I" is already reserved for rotational inertia in the next chapter.)

Second, let's now explicitly make the connection between the impulse acting on an object, and the resulting change in the momentum of the object.

This "impulse-momentum theorem" emphasizes how the impulse (exerted by the net force acting over a specific duration of time) causes a corresponding initial-to-final change in the momentum of the object. And vice versa, where the initial-to-final change in the momentum of an object is caused by the impulse on the object.

Let's apply these concepts to several objects that undergo changes in momentum, with an emphasis on the directions (+/– signs) of these quantities, and how they all must be consistent with each other, starting with a golf ball initially at rest, and then has a speed of 97 m/s after being hit by a golf club. (Video link: "The Moment of Impact. An Inside Look at Titleist Golf Ball R&D.")

This golf ball is initially at rest, so its initial momentum p₀ (mass times its initial velocity) is 0.

We'll define the horizontal direction to be positive to the right (and negative to the left). After it is hit by the golf club, its final momentum (mass times its final velocity) p_f points to the right (and will be a positive quantity).

The initial-to-final change in momentum ∆p of the golf ball is given by:

∆p = p_f – p₀,

and since get a positive quantity minus zero, then ∆p must be positive (thus pointing to the right).

Since the impulse "J" on the golf ball causes this initial-to-final change in momentum:

"J" = ∆p,

the impulse must also have the same direction as ∆p, and so it must also point to the right. (Also since the impulse "J" is the net force ΣF on the golf ball times the contact time ∆t, the net force of the golf club on the golf ball is also directed to the right.)

Now let's have you look at the directions involved in the impulse-momentum theorem for this catapult-launched F/A-18E-F Super Hornet, initially at rest, and then has a speed of 74 m/s after being it is catapulted. (Video link: "F/A-18E-F Super Hornet Catapult Launches.")

Super Hornet's initial momentum p₀ direction? (left (–), none (0), or right (+)?)
Super Hornet's final momentum p_f direction?
Direction of Super Hornet's initial-to-final change in momentum ∆p?
Direction of catapult's impulse "J" on the Super Hornet?

Finally, consider the directions involved in the impulse-momentum theorem for this Ford Ranger, hitting a crash barrier with a speed of 11.0 m/s, and then rebounding off the crash barrier with a speed of 2.2 m/s. (Video link: "Crash Test Ford Ranger 2012....")

Ford Ranger's initial momentum p₀ direction? (left (–), none (0), or right (+)?)
Ford Ranger's final momentum p_f direction?
Direction of Ford Ranger's initial-to-final change in momentum ∆p?
      (Hint: watch your signs!)
Direction of crash barrier's impulse "J" on the Ford Ranger?

Physics quiz archive: energy conservation, momentum conservation

Physics 205A Quiz 4, fall semester 2016
Cuesta College, San Luis Obispo, CA
Sections 70854, 70855, 73320, version 1
Exam code: quiz04th1R



Sections 70854, 70855, 73320 results

0- 6 :
7-12 :	**** [low = 9]
13-18 :	****************
19-24 :	*********************** [mean = 20.5 +/- 4.8]
25-30 :	******** [high = 27]

Physics quiz archive: energy conservation, momentum conservation

Physics 205A Quiz 4, fall semester 2015
Cuesta College, San Luis Obispo, CA
Sections 70854, 70855, 73320, version 1
Exam code: quiz04w04K



Sections 70854, 70855, 73320 results

0- 6 :	** [low = 6]
7-12 :	*******************
13-18 :	************************
19-24 :	****************************** [mean = 18.1 +/- 5.6]
25-30 :	*** [high = 30]

Physics quiz archive: energy conservation, momentum conservation

Physics 205A Quiz 4, fall semester 2014
Cuesta College, San Luis Obispo, CA
Sections 70854, 70855, 73320, version 1
Exam code: quiz04w3Rc



Sections 70854, 70855, 73320 results

0- 6 :	**** [low = 3]
7-12 :	********
13-18 :	******************
19-24 :	************************ [mean = 19.5 +/- 6.6]
25-30 :	*********** [high = 30]

Physics quiz archive: energy conservation, momentum conservation

Physics 205A Quiz 4, fall semester 2013
Cuesta College, San Luis Obispo, CA
Sections 70854, 70855, 73320, version 1
Exam code: quiz04iSs5



Sections 70854, 70855, 73320 results

0- 6 :	** [low = 6]
7-12 :	***********
13-18 :	************************* [mean = 18.6 +/- 6.0]
19-24 :	******************
25-30 :	********** [high = 30]

Physics quiz archive: energy conservation, momentum conservation

Physics 205A Quiz 4, fall semester 2012
Cuesta College, San Luis Obispo, CA
Sections 70854, 70855, version 1
Exam code: quiz04Bmw3



Sections 70854, 70855 results

0- 6 :
7-12 :	******** [low = 9]
13-18 :	**********
19-24 :	****************** [mean = 21.3 +/- 6.0]
25-30 :	**************** [high = 30]

Physics quiz archive: energy conservation, momentum conservation

Physics 205A Quiz 4, fall semester 2011
Cuesta College, San Luis Obispo, CA
Sections 70854, 70855, version 1

Sections 70854, 70855 results
Exam code: quiz04iMpL

 0- 6 : **  [low = 6]
 7-12 : *****
13-18 : ******************
19-24 : ******************  [mean = 19.4 +/- 5.5]
25-30 : ********  [high = 30]

Aftermarket after-accident airbag deployment

The Simpsons
Episode 13 (413), Season 19
"The Debarted"
Screencaps from hulu.com

Well after Hans Moleman's car is hit from behind by Marge Simpson, its airbag deploys to comedic results. (As Hans' car appears to be an AMC Gremlin, would that airbag an aftermarket add-on?)

Related posts:
Physics clicker question: airbag effect
Airbags make babies cry

Physics clicker questions: kinetic energy-momentum sequence

Physics 5A, Fall Semester 2007
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 1/e, Problem 7.7

Students were asked the following clicker questions (Classroom Performance System, einstruction.com) near the beginning of their learning cycle:

Mass of (bowling?) ball = 5.0 kg

[0.6 participation points.] The ball is initially at rest, and then you push it such that it has a final speed of 3.0 m/s in the +x direction. What was the change in kinetic energy of the ball?
(A) –23 J.
(B) –15 J.
(C) 0 J.
(D) +15 J.
(E) +23 J.
(F) (I'm lost, and don't know how to answer this.)

Sections 0906, 0907
(A) : 1 student
(B) : 1 student
(C) : 1 student
(D) : 9 students
(E) : 22 students

Correct answer: (E)

[0.6 participation points.] What was the change in momentum of the ball?
(A) –23 kg*m/s.
(B) –15 kg*m/s.
(C) 0 kg*m/s.
(D) +15 kg*m/s.
(E) +23 kg*m/s.
(F) (I'm lost, and don't know how to answer this.)

Sections 0906, 0907
(A) : 0 students
(B) : 0 students
(C) : 0 students
(D) : 32 students
(E) : 2 students
(F) : 0 students

Correct answer: (D)

So far, so good--students seem to understand how to calculate changes in kinetic energy, and changes in momentum, especially when the initial velocity is zero.

---

Mass of (bowling) ball = 5.0 kg

[0.6 participation points.] The ball is moving with a speed of 3.0 m/s in the +x direction, and then hits a wall and bounces back with the same speed in the –x direction. What was the change in kinetic energy of the ball?
(A) –23 J.
(B) –15 J.
(C) 0 J.
(D) +15 J.
(E) +23 J.
(F) (I'm lost, and don't know how to answer this.)

Sections 0906, 0907
(A) : 1 student
(B) : 2 students
(C) : 25 students
(D) : 0 students
(E) : 5 students
(F) : 0 students

Correct answer: (C)

The final and initial speed is the same, so there is no change in kinetic energy.

[0.6 participation points.] What was the change in momentum of the ball?
(A) –23 kg*m/s.
(B) –15 kg*m/s.
(C) 0 kg*m/s.
(D) +15 kg*m/s.
(E) +23 kg*m/s.
(F) (I'm lost, and don't know how to answer this.)

Sections 0906, 0907
(A) : 27 student
(B) : 2 students
(C) : 5 students
(D) : 0 students
(E) : 0 students
(F) : 0 students

Correct answer: (A)

The final velocity is negative, and the initial velocity is positive, such that the change in momentum (the impulse) points in the -x direction. (Note that a negative change in kinetic energy corresponds to a decrease in speed; a negative change in momentum corresponds to the -x direction.)

Physics clicker question: airbag effect

Physics 5A, Fall Semester 2007
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 1/e, Example 7.3

Students were asked the following clicker question (Classroom Performance System, einstruction.com) in the middle of their learning cycle:

[0.6 participation points.] An airbag (in conjunction with a seatbelt) will lessen the injuries caused by a car accident. How does the airbag do this, compared to wearing a seatbelt only?
(A) By decreasing the change in momentum.
(B) By decreasing the amount of impulse.
(C) By increasing the time duration of impact.
(D) (More than one of the above choices apply.)
(E) (I'm lost, and don't know how to answer this.)

Sections 0906, 0907
(A) : 5 students
(B) : 5 students
(C) : 11 students
(D) : 14 students
(E) : 0 students

Correct answer: (C)
Assuming that the same type of accident is involved for the same mass occupant, the momentum change (m*(v_f - v_i) will be the same, as will be the impulse on the driver. The time duration (delta_t) for the impulse (F*delta_t) to be applied will be longer with the airbag, thus the force applied on the occupant will be lessened.

Airbags make babies cry

L: Ralph Nader and Toddler, (© UPI/Bettman)
achievement.org
July 5, 1977

R: Ralph Nader and Toddler, unknown photographer
scienceservingsociety.com
July 5, 1977

Sure, airbags save lives, but do airbags make babies cry?

Two photos from the same event. Ralph Nader coaxing little Betsy into the airbag demonstrator, placing his finger over the airbag detonator.

"Watch out for that tree, Betsy! Watch out! Don't you see it? Betsy, what are you doing?!? BETSY..."

<INFLATE>Poof.</INFLATE>

A good 30 seconds passes before Betsy begins bawling her eyes out.

Well, that's the way it might have happened.

American crash-test dummies

Bizarro, by Dan Piraro
www.bizarro.com
March 30, 2007

Considering the same velocity change for a typical accident (v_f - v_i), a larger mass will require a greater impulse to come to a complete stop.