Please help! I can't get these two physics problems.?

Question

1.A 77.0-kg ice hockey goalie, originally at rest, catches a 0.150-kg hockey puck slapped at him at a velocity of 19.0 m/s. Suppose the goalie and the ice puck have an elastic collision and the puck is reflected back in the direction from which it came. What would their final velocities be in this case? (Assume the original direction of the ice puck toward the goalie is in the positive direction. Indicate the direction with the sign of your answer.)

a.puck ____ m/s
b.goalie ____ m/s
2.A 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.)
(b) How much kinetic energy in joules is lost during the collision? (Enter the magnitude.)
(c) What percent of the original energy is lef

NCS · Accepted Answer

1. There's a trick of sorts for this one. In a perfectly elastic collision, the relative velocity of approach is equal to the relative velocity of separation. So here
19.0 m/s = v - u
where v is the post-collision speed of the goalie
and u, that of the puck.
v = u + 19.0m/s
Conserving momentum and making that substitution:
0.150kg*19.0m/s + 0 = 0.150kg*u + 77.0kg*(u + 19.0m/s)
which solves to
u = -18.9 m/s
and so v = 0.0739 m/s

2. (a) avg F = m*dv/dt = 0.260kg * (-3.04 - 3.80)m/s / 0.0160s
avg F = -111 N

(b) KEi = ½*0.260kg*(3.80m/s)² = 1.88 J
KEf = 0.8² * KEi = 1.20 J ... (since rebound v = 0.8*Vo)
lost: 0.68 J

(c) easiest is 100%*0.8² = 64%

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Please help! I can't get these two physics problems.?

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