A hockey ball of mass 200 g travelling at 10 ms–1 is struck

Question.
A hockey ball of mass 200 g travelling at 10 ms–1 is struck by a hockey stick so as to return it along its original path with a velocity of 5 ms–1. Calculate the change of momentum which occurred in the motion of the hockey ball by the force applied by the hockey stick.

Solution:

Mass of ball, $\mathrm{m}=200 \mathrm{~g}=0.2 \mathrm{~kg} ;$ initial velocity of ball, $\mathrm{u}_{1}=10 \mathrm{~ms}^{-1} ;$ final velocity of ball,

$\mathrm{u}_{2}=-5 \mathrm{~ms}^{-1}$

(Negative sign denotes that ball is moving in opposite direction)

Initial momentum of ball $=\mathrm{mu}_{1}=0.2 \times 10=2 \mathrm{Ns}$ Final momentum of ball

$=\mathrm{mu}_{2}=0.2 \times(-5)=-1 \mathrm{Ns}$

$\therefore$ Change in momentum $=$

Final momentum - initial momentum $=$

$(-1)-(2)=-3 \mathrm{Ns}$

Negative sign denotes that change in momentum is in the direction opposite to the direction of initial momentum of the ball.