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| Applications of Newton's Second Law of Motion |
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| In a cricket match a fielder moves his arms back while trying to catch a cricket ball because if he tries to stop the fast moving ball suddenly then the speed decreases to zero in a very short time. Therefore the retardation of the ball will be very large. As a result the fielder has to apply a larger force to stop the ball. Thus, if he tries to stop a fast moving cricket ball the fielder may get hurt as the ball exerts a great pressure on the hands but if he tries to stop it gradually by moving his arms back then the velocity decreases gradually in a longer interval of time and hence retardation decreases. Thus the force exerted by ball on the hand decreases and the fielder does not get hurt. |
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| A Fielder Moving his Hands Backward WhileTrying to Catch a Ball |
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| An athlete before jumping runs some distance because this forward momentum may help him to jump high. |
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| 1) Calculate the momentum of a toy car of 150 g moving with a speed of 5 m/s. |
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| Solution: |
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| Mass of the toy car (m) = 150 g |
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| Velocity (v) = 5 m/s |
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| Momentum (p) = mv |
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| Momentum = 0.75 kg m/s |
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| 2) Calculate the change in momentum of a body weighing 10 kg when its velocity decreases from 20 m/s to 0.2 m/s. |
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| Solution: |
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| Mass of the body (m) = 10 kg |
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| Initial velocity (u) = 20 m/s |
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| Final velocity (v)= 0.2 m/s |
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| Change in momentum = m (v-u) |
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| = 10 (0.2 - 20) |
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| = 10 (-19.8) |
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| Change in momentum = -198 N s |
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| 3) A body of mass 50 kg has a momentum of 225 kgm/s. Calculate the velocity of the body. |
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| Solution: |
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| Mass (m) = 50 kg |
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| Momentum (p) = 225 kgm/s |
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| Velocity (v) = ? |
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| Velocity = 4.5 m/s |
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