Question 1

At the same instant that a 0.50-kg ball is dropped from 25 m above Earth, a second ball, with a mass of 0.25 kg, is thrown straight upward from Earth's surface with an initial speed of 15 m/s. They move along nearby lines and pass without colliding. At the end of 2.0 s the acceleration of the center of mass of the two-ball system is:

Accepted Answer

A)   0.25g
B)   0.50g
C)   0.75g
D)   g
E)   g/0.75 D

Question 2

A projectile in flight explodes into several fragments. The total momentum of the fragments immediately after this explosion:

Accepted Answer

A)   is the same as the momentum of the projectile immediately before the explosion
B)   has been changed into kinetic energy of the fragments
C)   is less than the momentum of the projectile immediately before the explosion
D)   is more than the momentum of the projectile immediately before the explosion
E)   has been changed into radiant energy A

Question 3

A very massive object traveling at 10 m/s strikes a light object, initially at rest, and the light object moves off in the direction of travel of the heavy object. If the collision is elastic, the speed of the lighter object is

Accepted Answer

A)   5.0 m/s
B)   10 m/s
C)   15 m/s
D)   20 m/s
E)   Can't tell from the information given. D

Question 4

Bullets from two revolvers are fired with the same velocity. The bullet from gun #1 is twice as heavy as the bullet from gun #2. Gun #1 weighs three times as much as gun #2. The ratio of the momentum imparted to gun #1 to that imparted to gun #2 is:

Accepted Answer

A)   2:3
B)   3:2
C)   2:1
D)   3:1
E)   6:1

Question 5

Object A strikes the stationary object B head-on in an elastic collision. The mass of A is fixed, you may choose the mass of B appropriately. Then:

Accepted Answer

A)   for B to have the greatest recoil speed, choose mA = mB
B)   for B to have the greatest recoil momentum, choose mB A
C)   for B to have the greatest recoil kinetic energy, choose mB >> mA
D)   for B to have the least recoil speed, choose mB = mA
E)   for B to have the greatest recoil kinetic energy, choose mB = mA

Question 6

Two bodies of unequal mass, placed at rest on a frictionless surface, are acted on by equal horizontal forces for equal times. Just after these forces are removed, the body of greater mass will have:

Accepted Answer

A)   the greater speed
B)   the greater acceleration
C)   the smaller momentum
D)   the greater momentum
E)   the same momentum as the other body

Question 7

A man is marooned at rest on level frictionless ice. In desperation, he hurls his shoe to the right at 15 m/s. If the man weighs 720 N and the shoe weighs 4.0 N, the man moves to the left at approximately:

Accepted Answer

A)   0
B)   2.1 * 10-2 m/s
C)   8.3* 10-2 m/s
D)   15 m/s
E)   2.7 * 10-3 m/s

Question 8

Two identical carts travel at 1 m/s on a common surface. They collide head-on and are reported to rebound, each with a speed of 2 m/s. Then:

Accepted Answer

A)   momentum was not conserved, therefore the report must be false
B)   if some other form of energy were changed to kinetic during the collision, the report could be true
C)   if the collision were elastic, the report could be true
D)   if the surface were inclined, the report could be true
E)   if the duration of the collision were short enough, the report could be true

Question 9

A 10-kg block of ice is at rest on a frictionless horizontal surface. A 1.0-N force is applied in an easterly direction for 1.0 s. During this time interval, the block:

Accepted Answer

A)   acquires a speed of 1 m/s
B)   moves 10 cm
C)   acquires a momentum of 1.0 kg .m/s
D)   acquires a kinetic energy of 0.1 J
E)   none of the above

Question 10

A 2.0-kg block is attached to one end of a spring with a spring constant of 100 N/m and a 4.0-kg block is attached to the other end. The blocks are placed on a horizontal frictionless surface and set into motion. At one instant the 2.0-kg block is observed to be traveling to the right with a speed of 0.50 m/s and the 4.0-kg block is observed to be traveling to the left with a speed of 0.30 m/s. Since the only forces on the blocks are the force of gravity, the normal force of the surface, and the force of the spring, we conclude that:

Accepted Answer

A)   the spring is compressed at the time of the observation
B)   the spring is not compressed at the time of observation
C)   the motion was started with the masses at rest
D)   the motion was started with at least one of masses moving
E)   the motion was started by compressing the spring

Question 11

A uniform narrow bar, resting on ice, is given a transverse horizontal impulse  at one end as shown. The center of mass of the bar COM will then:

Accepted Answer

A)   remain at rest
B)   move in a circle
C)   move in a straight line
D)   move in a parabola
E)   move along some other curve

Question 12

Blocks A and B are moving toward each other along the x axis. A has a mass of 2.0 kg and a velocity of 50 m/s, while B has a mass of 4.0 kg and a velocity of -25 m/s. They suffer an elastic collision and move off along the x axis. The kinetic energy transferred from A to B during the collision is:

Accepted Answer

A)   0
B)   2500 J
C)   5000 J
D)   7500 J
E)   10000 J

Question 13

Sphere X, of mass 2 kg, is moving to the right at 10 m/s. Sphere Y, of mass 4 kg, is moving to the left at 10 m/s. The two spheres collide head-on. The magnitude of the impulse of X on Y is:

Accepted Answer

A)   twice the magnitude of the impulse of Y on X
B)   half the magnitude of the impulse of Y on X
C)   one-fourth the magnitude of the impulse of Y on X
D)   four times the magnitude of the impulse of Y on X
E)   the same as the magnitude of the impulse of Y on X

Question 14

A ball hits a wall and rebounds with the same speed, as diagrammed below. The changes in the components of the momentum of the ball are:

Accepted Answer

A)    Δ\DeltaΔ px > 0,  Δ\DeltaΔ py > 0
B)    Δ\DeltaΔ px Δ\DeltaΔ py > 0
C)    Δ\DeltaΔ px = 0,  Δ\DeltaΔ py > 0
D)    Δ\DeltaΔ px = 0,  Δ\DeltaΔ py 
E)    Δ\DeltaΔ px > 0,  Δ\DeltaΔ py < 0

Question 15

Two bodies, A and B, have equal kinetic energies. The mass of A is nine times that of B. The ratio of the momentum of A to that of B is:

Accepted Answer

A)   1:9
B)   1:3
C)   1:1
D)   3:1
E)   9:1

Question 16

A 75-kg man is riding in a 30-kg cart at 2.0 m/s. He jumps off in such a way as to land on the ground with no horizontal velocity. The resulting change in speed of the cart is:

Accepted Answer

A)   zero
B)   2.0 m/s
C)   3.0 m/s
D)   5.0 m/s
E)   7.0 m/s

Question 17

Momentum may be expressed in:

Accepted Answer

A)   kg/m
B)   gram.s
C)   N.s
D)   kg/(m.s) 
E)   N/s

Question 18

A cart loaded with sand slides forward along a horizontal frictionless track. As the cart moves, sand trickles out at a constant rate through a hole in the back of the cart. The acceleration of the cart is:

Accepted Answer

A)   constant and in the forward direction
B)   constant and in the backward direction
C)   variable and in the forward direction
D)   variable and in the backward direction
E)   zero

Question 19

At one instant of time a rocket is traveling in outer space at 2500 m/s and is exhausting fuel at a rate of 100 kg/s. If the speed of the fuel as it leaves the rocket is 1500 m/s, relative to the rocket, the thrust is:

Accepted Answer

A)   0
B)   1.0 * 105 N
C)   1.5 * 105 N
D)   2.9 * 105 N
E)   2.5* 105 N

Question 20

The momentum of an object at a given instant is independent of its:

Accepted Answer

A)   inertia
B)   mass
C)   speed
D)   velocity
E)   acceleration

Exam 9: Center of Mass and Linear Momentum