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Momentum

A 60 kg per...

Question

A $60 k g$ person holding a $20 k g$ box riding on a skateboard moving at a speed of $7 m / s$ on a flat horizontal surface. He throws the box behind him, giving it a velocity of $5 m / s$ with respect to the ground. Calculate his velocity after throwing the object?

8 m / s

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9 m / s

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10 m / s

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11 m / s

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Solution

The correct option is C $11 m / s$
Given : $M = 60 k g$ $m = 20 k g$ $v = 7 m / s$
Initial momentum of the system $P_{i} = (m + M) v = (20 + 60) \times 7 = 560$ $k g$ $m / s$
Let the velocity of the man just after he throws the box be $V$ .
Final momentum of the system $P_{f} = 60 V - 20 \times 5 = 60 V - 100$
Using conservation of momentum : $P_{i} = P_{f}$
$∴$ $560 = 60 V - 100$ $⟹ V = 11$ $m / s$

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