: As the rain drop is falling down with a constant speed, its acceleration, a=0 Hence, net force on the drop = ma=0. As the cork is floating on the surface of water, its weight is balanced by the up thrust. Hence net force on the cork is zero.

The force exerted by the engine is balanced by the friction due to rough road. As the car is moving with constant velocity, it’s acceleration a=0 Hence. Net force on the car, F=ma=0

Whenever a body is thrown vertically upwards gravitational pull of earth gives it a uniform acceleration a=g in downward direction.

Hence, net force on the pebble

vertically downwards