A ball hangs from a string inside a rail road car moving along a straight track. The string is observed to be inclined towards the rear of the car making a constant small angle with the vertical. It shows that the car is

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moving with a uniform retardation

moving with an acceleration which is increasing uniformly

moving with a uniform acceleration

moving with a uniform velocity

answer is A.

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The ball is acted upon by three forces , pseudo force ma, tension T and weight mg.

Pseudo force 'ma' is acting on the ball in a direction opposite to the acceleration of the train and because of this pseudo force, the string cannot remain vertical.If the acceleration of the train uniformly increases , then the angle of inclination of the string with vertical will also keep on increasing . tan $θ$ =a/g

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