1. Chapter 9-10 Clicker Questions
Center of Mass
and
Linear Momentum

2. The center of mass is
a) the region of an object where the density has the largest value.
b) the region of an object where most of the mass is located.
c) the point within an object that moves as if all of the object’s mass where located there.
d) the point at the geometrical center of an object.
e) the only point on an object at which the gravitational force acts.

3. Complete the following statement: The center of mass of a system of particles has a constant velocity if
a) the velocity of the center of mass is initially zero.
b) the particles are distributed symmetrically around the center of mass.
c) the forces exerted by the particles on each other sum to zero.
d) the center of mass is at the geometric center of the system.
e) the external forces acting on particles of the system sum to zero.

4. Which one of the following quantities is equal to the change in momentum of an object during a collision?
a) impulse
b) net force
c) work
d) change in kinetic energy
e) maximum force

5. A rifle of mass M is initially at rest
A rifle of mass M is initially at rest. A bullet of mass m is fired from the rifle with a velocity v relative to the ground. Which one of the following expressions gives the velocity of the rifle relative to the ground after the bullet is fired.
a) mv
b) mv
c) Mv/m
d) mv/M
e) mv/M

6. A collision between two objects is elastic
A collision between two objects is elastic. Which one of the following statements concerning this situation is true?
a) The total kinetic energy of the objects is the same before and after the collision.
b) The total momentum of the objects is zero kgm/s after the collision.
c) The objects stick together and move as one object after the collision.
d) The kinetic energy of the objects is zero joules after the collision.
e) The vector sum of the velocities of the two objects is equal to zero m/s after the collision.

7. Which one of the following provides a basis to distinguish different types of collisions?
a) conservation of linear momentum
b) conservation of mechanical energy
c) conservation of kinetic energy
d) conservation of impulse
e) conservation of mass

8. A boy of mass m runs with a speed v and jumps onto a sled on an icy pond. The sled was at rest before the boy jumped onto it. After the jump, the sled and boy move at a speed v/2. What is the mass of the sled?
a) m/2
b) m
c) 2m
d) 3m
e) 4m

9. Car A of mass 2m is traveling due west at 40 m/s when it collides with car B of mass m traveling due west at 10 m/s. If the collision occurred on an icy road, so that the surface is essentially frictionless, what is the final velocity of the cars, which became locked together during the collision?
a) The cars are stationary.
b) 10 m/s, due east
c) 10 m/s, due west
d) 20 m/s, due west
e) 30 m/s, due west.

10. Two pucks on an air hockey table collide elastically
Two pucks on an air hockey table collide elastically. Complete the following statement: when such a collision occurs in two dimensions, the before and after velocities are best determined by
a) using the fact that momentum is conserved and that the initial speeds of the objects must equal the final speeds of the objects.
b) remembering that momentum is a vector quantity that is conserved in each direction.
c) applying Newton’s second law of motion and setting the net force equal to zero newtons.
d) making use of the work-energy theorem.
e) using the fact that the total energy is conserved.

11. Two carts are placed on a horizontal air track
Two carts are placed on a horizontal air track. The mass of the first cart is m and the mass of the second cart is 1.5m. The first cart has a speed v just before it collides with the second cart at rest. What is the speed of the center of mass of the system containing the two carts after the collision?
a) v
b) 4v/5
c) 2v/5
d) v/2
e) This cannot be determined without knowing if the collision is elastic or inelastic.

12. A rod of very small diameter with a mass m and length L is placed along the x axis with one end at the origin. An identical rod is placed along the y axis with one end at the origin, so that the two rods form an L-shape. What are the coordinates of the center of mass for these two rods?
a) (L/2, L/2)
b) (L/4, L/2)
c) (L/2, L/4)
d) (L/4, L/4)
e) (L/8, L/8)

13. Three events are observed at a baseball game:
I. A baseball is thrown by a pitcher. It starts from rest and is traveling at +38 m/s as it flies toward the catcher.
II. A baseball is traveling at +38 m/s when it enters the catcher’s glove and stops.
A baseball is traveling at +38 m/s when it hits a wall and bounces away from the wall at 38 m/s.
The change in the momentum of the baseball has the largest magnitude in which case(s)?
a) I only
b) II only
c) III only
d) I and II only
e) II and III only

14. A stone of mass m is dropped and falls freely under the influence of gravity. Ignoring any effects of air resistance, which one of the following expressions gives the momentum of the stone as a function of time t? The local acceleration due to gravity is g.
a) ½mgt2
b) gt
c) ½gt2
d) mgt
e) G dm/dt

15. A football of mass m, initially at rest, is kicked so that leaves the foot at a speed v. If t represents the duration of the collision between the ball and the foot, which one of the following expressions determines the magnitude of the average force exerted on the ball?
a) mvt
b) ½mv2
c) ½mv2/t
d) mv/t
e) ½mvt2

16. During the filming of a movie, a stunt person jumps from the roof of a tall building, but no injury occurs because the person lands on a large, air-filled bag. Which one of the following best describes why no injury occurs?
a) The bag increases the amount of time the force acts on the person and reduces the change in momentum.
b) The bag increases the amount of time during which the momentum is changing and reduces the average force on the person.
c) The bag decreases the amount of time during which the momentum is changing and reduces the average force on the person.
d) The bag provides the necessary force to stop the person.
e) The bag reduces the impulse to the person.

17. A force that varies with time is applied to an object
A force that varies with time is applied to an object. The force increases linearly from zero newtons to twenty-four newtons during the first two seconds. Then, the force remains constant for four seconds. During the last two seconds, the force linearly decreases to zero newtons. What is the total impulse exerted on the object?
a) This cannot be determined because neither the initial momentum nor the final momentum is known.
b) This cannot be determined because the mass of the object is not given.
c) The total impulse is 96 N  s.
d) The total impulse is 128 N  s.
e) The total impulse is 144 N  s.

18. A 9-kg object is at rest. Suddenly, it explodes and breaks into two pieces. The mass of one piece is 6 kg and the other is a 3-kg piece. Which one of the following statements concerning these two pieces is correct?
a) The speed of the 6-kg piece will be one eighth that of the 3-kg piece.
b) The speed of the 3-kg piece will be one fourth that of the 6-kg piece.
c) The speed of the 6-kg piece will be one forth that of the 3-kg piece.
d) The speed of the 3-kg piece will be one half that of the 6-kg piece.
e) The speed of the 6-kg piece will be one half that of the 3-kg piece.

19. What position did Mr. Pennington play on his high school football team?
a) Quarterback
b) Safety
c) Offensive Guard
d) Linebacker
e) Wide Receiver

20. Where did Mr. Pennington finish academically in his high school class?
a) Bottom 25%
b) Top 25%
c) Top 10
d) #2
e) #1

21. At which university did Mr. Pennington get his B.S. in Physics?
a) Brown
b) Stanford
c) Oregon
d) Richmond
e) UVM

22. How many Nobel prize winners did Mr
How many Nobel prize winners did Mr. Pennington have for instructors at Stanford?
a) 0
b) 1
c) 2
d) 3
e) 4

23. Mr. Pennington has a Master of Science degree in which subject(s)?
a) Astronomy
b) Aeronautical Engineering
c) Materials Science Engineering
d) a & b
e) a & c

24. Where did Mr. Pennington work before teaching at Essex High School?
a) Dunkin Donuts for 24 years
b) Fletcher Allen Hospital for 24 years
c) GE for 24 years
d) IBM for 24 years
e) He has only worked at Essex High School