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Question
A hockey ball of mass 200 g travelling at 10 m s−1 is struck by a hockey stick so as to return it along its original path with a velocity at 5 m s−1. Calculate the change of momentum occurred in the motion of the hockey ball by the force applied by the hockey stick.
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Solution
Mass of ball (m) = 200 g = 0.2 kg
Initial velocity of ball (u1) = 10 ms-1
∴ The initial momentum of the ball = mu1
= 0.2 kg × 10 ms-1
= 2 N s
Final velocity of ball (u2) = - 5 ms-1
(The minimum sign denotes that the ball is moving in the opposite direction.)
∴ Final momentum of ball = mu2
= 0.2 kg × (- 5 ms-1)
= - 1 N s
∴ Change in momentum = Final momentum - Initial momentum
= - 1Ns - 2Ns
= - 3Ns
The Minus sign denotes that the change in momentum is opposite to the direction of the initial momentum of the ball.
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