Question

A bicycle pump is used to fill a tyre with air. When the piston is pushed, air is filled. When the piston is pulled back, why doesn’t the tyre deflate?
(A). Pressure inside the tyre equals atmospheric pressure, hence air cannot leave
(B). The valve is open allowing the free flow of air through it
(C). The valve is closed blocking the free flow of air through it
(D). The valve is open when piston is pushed and closed when piston is pulled

Answer 1

Hint: In order to fill a tyre with air, a bicycle pump contains a piston that is pushed and pulled to force air inside the tire until a certain pressure is obtained. The tyre is fitted with a valve through which the air enters inside it. This valve is also responsible for holding the air inside.

Complete answer:
When a bicycle pump is used to fill air in a tyre, the piston is pushed to force air into the tyre and pulled back. This process is repeated until most of the space inside the tyre is filled with air.
The tyre is fitted with a special valve which allows the flow of air only in one direction. Because of the valve, air can go inside the tyre but cannot come back out. Therefore, when the piston pushes air inside the tyre, the valve is open but when the air tries to escape the tyre, the valve is closed.
Therefore, due to the valve the air can enter but cannot escape the tyre as it is open when air enters but closed when air escapes.

Hence, the correct option is (D).

Note:
The gas filled inside tyres is generally nitrogen along with some natural gas. When the internal pressure of a body is equal to the atmospheric pressure, it is in equilibrium with the atmosphere. Filling tyres with air ensures that the tyres are more rigid and ride smoother. The pressure in a tyre is measured as pressure per square inch.