Homework Help: Physics

Posted by Soly on Sunday, October 14, 2007 at 11:07pm.

Will you check my work and help me with the last two problems please?

A 4 kg ball has a momentum of 12 kg m/s. What is the ball's speed?
-(12 kg/m/s) / 4kg=3m/s

A ball is moving at 4 m/s and has a momentum of 48 kg m/s. What is the ball's mass?
-(48kg/m/s) / (4m/s) =12 kg

A 1-kg chunk of putty moving at 1 m/s collides with and sticks to a 5-kg bowling ball initially at rest. The bowling ball and putty then move with a momentum of? (This one I dont know how to work)

A 1000-kg car moving at 10 m/s brakes to a stop in 5 s. The average braking force is? (This one I dont know how to work)

Physics - Quidditch, Monday, October 15, 2007 at 12:46am
Your first two answers look OK.

3rd question:
Assuming that all momentum is conserved, the total momentum after the collision is the same as before the collision. The bowling ball is at rest, so no momentum there. The putty has a momentum of 1kg * 1m/s=1kg*m/s. After the collision, the putty and the bowling ball are stuck together for a total mass of 6kg and have the same total momentum that the putty alone had before the collision. While not asked, the velocity after the collision is 1/6 m/s.

4th question:
Force = dp/dt where p is the momentum.
dp= 1000kg * 10m/s
dt= 5s

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