Question

The velocity of fall of a man jumping with a parachute first increases and then becomes constant.
Reason: The constant velocity of fall of man is called terminal velocity

Answer 1

Hint: As the body comes down its velocity will increase. Hence the viscous force will also increase. When the viscous force is equal to the weight of the body in the medium, the net force on the body will become zero. Now the body will fall down without acceleration. It moves with a constant velocity.

Step by step solution
When the man jumps out with a parachute, the velocity will increase due the gravitational force. Initially the gravitational pull will be dominating the viscous force and the buoyant force offered by the air that will oppose the downward motion of the man. But according to Stoke’s law the viscous force will increase with the increase in velocity. After some time the viscous force and buoyant force will become equal to the gravitational pull. When the viscous force and the gravitational pull becomes equal the body falls down with a constant velocity. This constant velocity attained by a body as it falls down through a fluid medium is called the terminal velocity.
Here both the assertion and reason are correct, but the reason is not the correct explanation for the assertion.
The reason for the assertion is that the velocity becomes constant when the viscous force becomes equal to the weight of the body.

The answer is: Both the reason and assertion are correct. But the reason is not the correct explanation for the assertion.

Note
The viscous force is the force which tries to reduce the relative motion between two liquid layers in contact. This property of the fluid is called viscosity. According to molecular theory the molecules in one layer oppose the motion of molecules in the adjacent layer because of the molecular force of attraction.