Give the magnitude and direction of the force acting on a stone of mass 0.1 kg (a) just after it is dropped from the window of a stationary train
(b) just after it is dropped from the window of a train running at a constant velocity of `36 km//hr`
(c) just after it is dropped from the window of a train accelerating with `1 ms^(-2)`
(d) lying on the floor of a train which is accelerating with `1 ms^(-2)` the stone being at rest relative to the train .
Neglect the resistance of air throughout .

Explanation

(a) Mass of stone = 0.1 kg
Net force , F = mg = `0.1 xx 10 N` (vertically downwards)
(b) When the train is running at a constant velocity, its acceleration is zero. No force acts on the stone due to this motion. Therefore, the force on the stone is the same (1.0 N) .
(c ) The stone will experience an additional force F' (along horizontal ) i.e. F = ma = 0.1 `xx` 1 = 0.1 N
As the stone is dropped , the force F' no longer acts and the net force acting on the stone F = mg = `0.1 xx 10 = 1.0 N` ( vertically downwards )
(d) As the stone is lying on the floor of the train, its acceleration is same as that of the train .
`` force acting on the stone , F = ma = `0.1 xx 1` = 0.1 N
It acts along the direction of motion of the train .