A free 238U nucleus kept in a train emits an alpha particle. When the train is stationary, a nucleus decays and a passenger measures that the separation between the alpha particle and the recoiling nucleus becomes x at time t after the decay. If the decay takes place while the train is moving at a uniform velocity u, the distance between the alpha particle and the recoiling nucleus at a time after the decay as measured by the passenger

There will be no relative motion since passenger and the nucleus are moving in the train and thus the distance measured by the passenger between the alpha particle and the nucleus will be x at time t. Hence option C will be correct.

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