AP Physics 1 - Conceptual Test on Momentum

Momentum

Mass in motion

The amount of momentum an object has depends on

1. how much stuff is moving (mass)

2. how fast stuff is moving (speed/velocity)

Standard metric unit of momentum is

kg * m / s

Is momentum scalar or vector?

Vector

A car accelerates from rest, changing its momentum. The Earth's momentum changes by

a) a larger amount
b) the same amount
c) a smaller amount
d) depends on interaction of two

b) the same amount

p = F * t
Force exerted by car and Earth are equal and opposite, hence the Force cancels out.

The Time these forces were exerted are the same.

same time * same force = same momentum

Suppose the entire population of the world gathers in ONE spot and everyone jumps at the sound of a prearranged signal. While everyone is in the air, does the Earth gain momentum in the opposite direction?

a) No, the inertial mass of Earth is so large that the planet's change in motion is imperceptible
b) Yes, however the change in momentum of Earth is much less than that of all the jumping people because of Earth's large inertial mass
c) Yes, Earth recoils like that of a rifle firing a bullet with a change in momentum equal to and opposite of people
d) It depends

c) Yes, Earth recoils like that of a rifle firing a bullet with a change in momentum equal to and opposite of people

Earth is a closed system (the planet with people inside it), and according to Law of Conservation of Momentum, momentum will not change.

1. Momentum of Earth + people on ground

2. Momentum of Earth + people in air

Momentum Before Jump: 0 Momentum After Jump: Also 0

Suppose the entire population of the world gathers in one spot and, at the sound of a prearranged signal, everyone jumps up. About a second later, 5 billion people land back on the ground. After the people have landed, Earth's momentum is

a) Same as before people jumped
b) Different from when people jumped

a) Same as before people jumped

It's impossible to change momentum of closed system

Suppose rain falls vertically into an open cart rolling along a straight horizontal track with negligible friction. As a result of the accumulating water, the momentum of the cart

a) momentum of the cart increases, and the kinetic energy of the cart increases.

b) momentum of the cart increases, and the kinetic energy of the cart does not change

c) momentum of the cart increases, and the kinetic energy of the cart does decreases

d) momentum of the cart does not change ,and the kinetic energy of the cart increases
.
e) momentum of the cart does not change , and the kinetic energy of the cart does not change

f) momentum of the cart does not change , and the kinetic energy of the cart decreases

g) momentum of the cart decreases, and the kinetic energy of the cart increases.

h) momentum of the cart does decreases, and the kinetic energy of the cart does not change

i) momentum of the cart decreases, and the kineti

f) momentum of the cart does not change , and the kinetic energy of the cart decreases


Because rain falls into cart, the mass increases. Because the mass increases, the velocity decreases.
KE = 0.5mv²
By squaring the decreased velocity, the speed decreases even more. Thus KE is much smaller than when the cart was lighter.

Momentum doesn't change because mass increases and velocity decreases.

The vertically falling rain does NOT affect the horizontal momentum. The rain doesn't have a horizontal component, so the cart will just cancel out its (rain's) vertical component. The increased mass can be calculated with the force (when multiplied by acceleration), and this force cancels out, so the mass component of horizontal momentum doesn't change, and neither does velocity for some reason....

In which case(s) does the ball undergo the largest change in momentum?

(i) a ball moving at speed v is brought to rest
(ii) the same ball is projected from rest so that it moves at speed v
(iii) the same ball moving at speed v is brought to rest and then projected backward to its original speed.

a) (i)
b) (i) and (ii)
c) (i), (ii), and (iii)
d) (ii)
e) (ii) and (iii)
f) (iii)

f) (iii)

Change in Momentum: Final - Initial
(i): 0 - mv = -mv
(ii): mv - 0 = mv

(iii): Add both (final - initial) momentums
a) (0 - mv) = -mv
b) m(-v) - 0 = -mv (-v because projected backwards)
-mv + (-mv) = -2mv

Consider two carts, of masses m and 2m, at rest on an air track. If you push first one cart for 3 s and then the other for the same length of time, exerting equal force on each, the momentum of the light cart is ________ the momentum of the heavy cart.

a) four times
b) twice
c) equal to
d) one-half
e) one-quarter

c) equal to

Momentum = mass * velocity
-We don't know velocity
Momentum also = Force * time
Forces are same for both, time is same fro both
Momentums are equal

Consider two carts, of masses m and 2m, at rest on an air track. If you push first one cart for 3 s and then the other for the same length of time, exerting equal force on each, the kinetic energy of the light cart is ______________ the kinetic energy of the heavy cart.

1. larger than

2. equal to

3. smaller than

1. larger than (by 2 times)

KE = 0.5mv²

-We don't know velocities, but we know momentum of both carts are equal (from previous flashcard)

If momentums of both are equal, then SPEED of light cart must be twice that of heavy cart.

Light Cart: 0.5mv² ---- 0.5(m)((2v)²) = 2mv²
Heavy Cart: 0.5mv² ----- 0.5(2m)(v²) = 1mv²

Suppose a ping-pong ball and a bowling ball are rolling toward you. Both have the same momentum, and you exert the same force to stop each. How do the distances needed to stop them compare?

1. It takes a shorter distance to stop the ping-pong ball.

2. Both take the same distance.

3. It takes a longer distance to stop the ping-pong ball.

3. It takes a longer distance to stop the ping-pong ball.

Both have same momentums
Ping pong ball has smaller mass, thus larger velocity
With larger velocity, KE is also greater for ping pong ball
Because KE is greater, it will take more work to stop it
Work = Force * Displacement
Exerting same force on both, displacement is greater for ping pong ball

Newton's Cradle The number of balls bouncing on each side is equal because

1. of conservation of momentum.

2. the collisions are all elastic.

3. neither of the above

2. There are many different final states that conserve momentum, but this is the only one that also conserves kinetic energy. Since conservation of kinetic energy is the same thing as saying "the collisions are elastic," choice 2 is correct.

A cart moving at speed v collides with an identical stationary cart on an air track, and the two stick together after the collision. What is their velocity after colliding?

1. v

2. 0.5 v

3. zero

4. -0.5 v

5. -v

6. need more information

2. 0.5 v

Law of Conservation of Momentum
mv = 2m(u)
0.5v = u

A person attempts to knock down a large wooden bowling pin by throwing a ball at it. The person has two balls of equal size and mass, one made of rubber and the other of putty. The rubber ball bounces back, while the ball of putty sticks to the pin. Which ball is most likely to topple the bowling pin?

1. the rubber ball

2. the ball of putty

3. makes no difference

4. need more information

1. the rubber ball

• Rubber ball has more velocity = more momentum

Carr Collision Question

Damage is done to that which has greater velocity (greater KE).

A golf ball is fired at a bowling ball initially at rest and sticks to it. Compared to the bowling ball, the golf ball after the collision has

1. more momentum but less kinetic energy.

2. more momentum and more kinetic energy.

3. less momentum and less kinetic energy.

4. less momentum but more kinetic energy.

5. none of the above

3. Both balls move at the same speed after the collision. So the ball with the larger inertial mass has both the larger momentum and the larger kinetic energy.

Suppose you are on a cart, initially at rest on a track with very little friction. You throw balls at a partition that is rigidly mounted on the cart. If the balls bounce straight back as shown in the figure, is the cart put in motion?

1. Yes, it moves to the right.

2. Yes, it moves to the left.

3. No, it remains in place.

2. Because all the balls bounce back to the right, then in order to conserve momentum, the cart must move forward.
(You're throwing at left)