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

Question

A hockey goalie is standing on ice. Another player fires a puck (m = 0.170 kg) at the goalie with a velocity of +67.1 m/s. (a) If the goalie catches the puck with his glove in a time of 3.35 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 -67.1 m/s. The puck and stick are in contact for a time of 3.35 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

mass of puck = m = 0.170 kg

initial velocity of puck = Vi = 67.1 m/s

final velocity of puck = Vf = 0 m/s                 (since puck comes to rest in the glove)

time = 3.35 x 10-3 sec

a)

change in momentum of puck can be given as ::

change in momentum = m (Vf - Vi) = (0.170kg) ( 0 - 67.1 ) = -11.41 kgm/s

using impulse -change in momentum equation ::

impulse = change in momentum

we know that , impulse = force x time

average force x time = change in momentum

average force x (3.35 x 10-3 sec ) = 11.41 kgm/s

force = 3405.07 N

b)

mass of puck = m = 0.170 kg

initial velocity of puck = Vi = 67.1 m/s

final velocity of puck = Vf = -67.1 m/s                

time = 3.35 x 10-3 sec

change in momentum of puck can be given as ::

change in momentum = m (Vf- Vi) = (0.170kg) ( -67.1 - 67.1 ) = -22.814 kgm/s

using impulse -change in momentum equation ::

impulse = change in momentum

we know that , impulse = force x time

average force x time = change in momentum

average force x (3.35 x 10-3 sec ) = -22.814 kgm/s

force = -6810.15 N

magnitude of force = 6810.15 N

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