Question

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

• A. A drop of rain falling down with a constant speed
• B. A cork of mass 10 g floating on the surface of water
• C. A car moving with a constant speed of $20kmh^{- 1}$on a Rough road:
• D. A pebble of mass 0.05 kg is thrown vertically upwards

Answer 1

Answer: (D)

A pebble of mass 0.05 kg is thrown vertically upwards

Answer 2

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 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, 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

$= mg = 0.05 \times 10 = 0.5N$vertically downwards