Help please! Can't get these two physics problems?

Question

1.  0.260-kg billiard ball that is moving at 3.80 m/s strikes the bumper of a pool table and bounces straight back at 3.04 m/s (80% of its original speed). The collision lasts 0.0160 s. (Assume that the ball moves in the positive direction initially.)
(a) Calculate the average force exerted on the ball by the bumper. (Indicate the direction with the sign of your answer.)
 N
(b) How much kinetic energy in joules is lost during the collision? (Enter the magnitude.)
 J
(c) What percent of the original energy is left?

2. 0.0270 kg bullet moving horizontally at 500 m/s embeds itself into an initially stationary 0.500 kg block.
(a) What is their velocity just after the collision?
 m/s
(b) The bullet-embedded block slides 8.0 m on a horizontal surface with a 0.30 kinetic coefficient of friction. Now what is its velocity?
 m/s
(c) The bullet-embedded block now strikes and sticks to a stationary 2.00 kg block. How far does this combination travel before stopping?
 m

Amy · Answer

1a)  average force = change in momentum / change in time
Remember that momentum is a vector.
b) KE is not a vector, so this is just the difference between initial and final KE.
c) final / initial write as a percent

2a) Assume a perfectly inelastic collision, where no momentum was lost to the surrounding air.
Final momentum = Initial momentum
b) F = μN = μmg
You can either use the kinetics equations to relate acceleration, distance, and initial velocity to final velocity,
or you can equate the work done by friction to change in kinetic energy and use that to find the final velocity.
c) Repeat part (a) with the new masses and velocity. Then repeat part (b), except instead of being given the distance you are given the final velocity/energy is 0.

Help please! Can't get these two physics problems?

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