Question

The magnitude of net force acting on a car of mass 88 kg moving on a rough road with constant velocity 36 km/h.

Answer 1

a=g=10m/s2

Net force, F=ma=1N

This force acts vertically downwards.

(b) When the train is running at a constant velocity, its acceleration =0, No force acts on the stone due to this motion. 

Therefore, force on the stone F= weight of stone=mg=0.1×10=1.0N 

This force also acts vertically downwards.

(c) When the train is accelerating with 1 m/s2,

 an additional force F′=ma=0.1×1=0.1N acts on the stone in the horizontal direction.

 But once the stone is dropped from the train, 

F' becomes zero and the net force on the stone is F=mg=0.1\times 10=1.0N, acting vertically downwards.

(d) As the stone is lying on the floor of the train, its acceleration is the same as that of the train.

force acting on stone, F=ma=0.1×1=0.1N

This force is along the horizontal direction of motion of the train.

Note that in each case, the weight of the stone is being balanced by the normal reaction.