3. A lacrosse ball was thrown and is moving at 25 m/s immediately before it stri

Question

3. A lacrosse ball was thrown and is moving at 25 m/s immediately before it strikes the pocket (net) of a teammate’s stick. The mass of the lacrosse ball is 0.15 kg. The displacement of the ball due to movement of the pocket is 0.05 m from the instant it makes contact with the pocket until it stops. Ignore any strain or gravitational potential energy as you answer the questions below.

a. How much kinetic energy does the lacrosse ball possess just before it strikes the pocket?

b. How much work does the teammate who catches the ball do on the lacrosse ball during the catch?

c. Is the work done positive or negative?

d. What is the average impact force exerted by the pocket (net) of the teammate’s stick on the lacrosse ball?

Explanation / Answer

1. KE = 0.5mv^2 = 0.5*0.15*25^2 = 46.875 J
2. to bring the ball from 25 m/s to 0 m/s in 0.05 m
F = ma
but 2*a*s = v^2
a = 25^2/2*0.05 = 6250 m/s/s
F = 937.5 N
W = fs = 46.875 J
3. Work done is negative , as the ball moves in opposite direction to the direction of applied force
4. Avg force, F = 937.5 N

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