A Ball Bounces Off the Floor As Shown. the Direction of the Impulse of the Ball, , Is

CT11-1

A ball bounces off the floor as shown. The direction of the impulse of the ball, , is ...

Pink: straight up ­

Yellow: straight down ¯

Green: to the right ®

Blue: to the left ¬

Answer: straight up.

Also, since , the direction of the force from the floor is the same as the direction of the impulse

CT11-2

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..

Pink: four times Green: twice Yellow: equal to

Blue: one-half Purple: one-quarter

the momentum of the heavy cart.

Answer: the momenta are equal. so if we apply the same force for the same time, we will get the same increase in momentum. Applying the same force to the heavier cart will make it go more slowly, but it has more mass, so it will get the same momentum.

CT11-3

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 time intervals to stop them compare?

Pink: It takes less time to stop the ping-pong ball.

Green: Both take the same time.

Yellow: It takes more time to stop the ping-pong ball.

Correct answer is both take the same time. Both have the same momentum (the bowling ball must be going very slow or else the ping-pong ball is going really fast) and . If we apply the same force to each, it will take the same time to stop both.

CT11-4

A big ball, mass M=10m, speed v, strikes a small ball, mass m, at rest. Could the following occur: The big ball comes to a complete stop and the small ball takes off with speed 10v?

Pink: Yes, this can occur.

Green: No, it cannot occur because it would violate momentum conservation

Yellow: No, it cannot occur because it would violate conservation of energy.

Answer: It cannot occur because it would violate conservation of energy. Although momentum is conserved in this collision,

Ptot,i = (10m)v = Ptot,f = m(10v),

Energy is not conserved, since the KE after is 10 times bigger than the KE before. If any heat were generated in the collision, energy conservation would be violated even more.

KEi = (1/2)(10m)v2 ¹ KEf= (1/2)m(10v)2 =(1/2)m(100)v2

In real life, it would not be possible for the big mass (10m) to come to a complete stop as a result of the collision.