# NCERT Solutions Class 9 Science Solutions for Exercises 1 in Chapter 9 - Force and Laws of Motion

Question 5 Exercises 1

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 at 5 ms^{-1}. Calculate the magnitude of change of momentum occurred in the motion of the hockey ball by the force applied by the hockey stick.

Given, mass of the ball (m) = 200g

Initial velocity of the ball (u) = 10 m/s

Final velocity of the ball (v) = 5m/s

\text { Initial momentum of the ball }=m u=200 g \times 10 m s^{-1}=2000 g . m \cdot s^{-1}

\text { Final momentum of the ball }=m v=200 g \times 5 m s^{-1}=1000 g . m \cdot s^{-1}

\text { Therefore, the change in momentum }(\mathrm{mv}-\mathrm{mu})=1000 \mathrm{g} \cdot \mathrm{m} \cdot \mathrm{s}^{-1}-2000 \mathrm{g} \cdot \mathrm{m} \cdot \mathrm{s}^{-1}=-1000 \mathrm{g} \cdot \mathrm{m} \cdot \mathrm{s}^{-1}

This implies that the momentum of the ball reduces by 1000 g.m.s^{-1} after being struck by the hockey stick.

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