A hockey goalie is standing on ice. Another player fires a puck ( m = 0.170 kg)

Question

A hockey goalie is standing on ice. Another player fires a puck (m = 0.170 kg) at the goalie with a velocity of +45.2 m/s. (a) If the goalie catches the puck with his glove in a time of 5.79 x 10-3 s, what is the magnitude of the average force exerted on the goalie by the puck? (b) Instead of catching the puck, the goalie slaps it with his stick and returns the puck straight back to the player with a velocity of -45.2 m/s. The puck and stick are in contact for a time of 5.79 x 10-3 s. Now, what is the magnitude of the average force exerted on the goalie by the puck? Verify that your answers to parts (a) and (b) are consistent with the conclusion of Conceptual Example 3.

Explanation / Answer

a.) calculate the momentum before the puck hits the goalie:

momentum = mass*velocity
p = 0.17*45.2
p = 7.684 kg.m/s

Because the final momentum is zero, the change in momentum is also 7.684, which is also called impulse.

Apply impulse = Force*time
7.684 = 5.79*10^-3*F
F = 1327N

Therefore the average force exerted is 1327N

b.) the original momentum is 7.684 kg.m/s.

However, this time the final momentum is not zero:

apply momentum = mass*velocity:

p = 0.17*-45.2

p = -7.684 kg.m/s

So this time the change in momentum is 7.684- (-7.684) = 15.368 kg.m/s, this is also the impulse.

apply impulse = Force*time
15.368 = 5.79*10^-3*F
F = 15.368 / 5.79*10^-3
F = 2654N

So now the average force is 2654N

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