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

(a) The speed of the centre of mass increases

(b) The speed of the centre of mass decreases

(c) The speed of the centre of mass will not change

(d) The speed of the centre of mass will first increase and then decrease

I had been asked this question during an interview.

My enquiry is from Angular Momentum topic in portion System of Particles and Rotational Motion of Engineering Physics I

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Right option is (c) The speed of the centre of mass will not change

For explanation I would say: The force involved in the given problem is the internal forces of the system. No external force acts on the system when the child runs So, there will be no change in the speed of the centre of mass of the (trolley+child) system.

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