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Units and Measurement
Question
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A child sits stationary at one end of a long trolley moving uniformly with a speed v on a smooth horizontal floor. If the child gets up and runs about on the trolley in any manner, what is the speed of the centre of mass of the (trolley + child) system?

Answer

In the absence of external forces, the centre of mass moves with constant velocity. That is, the velocity of the centre of mass of trolley and child remains unchanged, i.e., v. 
When the child is moving on the trolley, the force exerted by the child on the trolley is the internal force. 

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Some More Questions From Units and Measurement Chapter

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Give four examples of physical quantities.

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