In which of the following cases the net force acting on the body is not zero?

Options

A. A drop of rain falling down with a constant speed.
B. A cork of mass 10g floating on the surface of water.
C. A car moving with a contant speed of `20kmh^(-1)` on a rough road.
D. A pebble of mass `0.05kg` is thrown vertically upwards.

Answer

D

Explanation

As the rain drop is falling down with a constant speed, its acceleration, `a=0` . Hence, net force on the drop is `ma = 0` . As the cork is floating on the surface of water, its weight is balanced by the upthrust. 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, its acceleration `a=0` . Hence, net force on the car, `F=ma=0` .
Whenever a body is thrown vertically upward gravitational pull of earth gives it a uniform acceleration, `a=g` in downward direction.
Hence, net force on the pebble=mg `=>0.05xx10=0.5N` vertically downwards.