Give the magnitude and direction of the net force acting on a stone of mass 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 36 km/h.

(iii) Just after it is dropped from the window of a train accelerating with 1 m/${{s}^{2}}$.

(iv) Lying on the floor of a train which is accelerating with 1 m/s2, the stone being at rest relative to the train. Neglect air resistance throughout.

Mass of stone (m) = 0.1 kg

(i) When stone is dropped from the window of a stationary train, it falls freely under gravity.

.’. Net force acting on stone (F)

= mg =0.1 x 10

= 1.0 N (vertical downward) 1

(ii) Just after the stone is dropped from the window of a train running at a constant velocity i.e. acceleration of the train is zero. So, no force acts on the stone due to motion it falls freely under gravity.

The force acts on it is due to its weight only.

i.e. acceleration of the train is zero, so no force acts on

the stone due to this motion.

(iii) The train accelerates horizontally by 1 m/${{s}^{2}}$, but as the stone is left it move under gravity only so the net force on it will be due to gravity only.

(iv) As the stone is accelerating with the train, so there must be a net horizontal force on it.

F = 0.1 x 1 = 0.1N