question_answer 3)

Give the magnitude and direction of the net force acting on a stone of 0.1 kg, (i) just after it is dropped from the window of a stationary train. (ii) just after it is dropped from the window of a train running at a constant velocity of . (iii) just after it is dropped from the window of a train accelerating with . (iv) lying on the floor of a train which is accelerating with  the stone being at rest relative to the train. Neglect air resistance throughout, and take .  

Answer:

Here (i) When the stone is just dropped from the window of a stationary train,  vertically downwards (ii) When the stone is dropped from the window of a train running at a constant velocity, no force acts on the stone due to the motion of the train.  F = mg = 1 N, vertically downwards. (iii) In the train accelerating with  the stone experiences an additional force,  along horizontal. As the stone is dropped, the force F' no longer acts on the stone and so net force on the stone is F = mg = 1 N, vertically downwards (iv) Here weight of the stone is balanced by the normal reaction of the floor. Acceleration of the stone = Acceleration of the train   along horizontal.