Question

An empty glass jar is submerged in a tank of water with an open mouth of the jar downwards so that air inside the jar is trapped and cannot get out. As the jar is pushed down slowly, the magnitude of the net buoyant force on the system of the volume of gas trapped in the jar and the jar.

A. Increases

B. Decreases

C. Remains the same

D. Information is insufficient to draw the inference

Answer 1

Whenever an object is immersed in a liquid, it experiences an upward force which is known as Buoyant force and the process of experiencing an upward force is called Buoyancy. The unit of Buoyant force is Newton.

Complete answer:

Given that an empty glass jar is submerged in the tank of water with the open mouth of the jar downwards. Thus the air inside the jar is trapped and it will not go out.

The pressure in the fluid column increases with the depth. Thus the pressure of the object which is submerged in the fluid will be greater at the bottom than that at the top of the object. This pressure difference will result in a net upward force which can be called Buoyancy or the Buoyant force.

Now we know that the buoyant force ${F_B}$ acting on the object will be in a direction opposite to the weight of the object. We have:

$\Rightarrow {F_B}=-{\rho}{g}{V}$

Where,

$\rho$- The density of the liquid

$g$- The acceleration due to gravity

$V$- The volume of the liquid displaced

Now, as soon as we submerge the glass in the water tank due to increased hydrostatic pressure, the air trapped inside the glass will start decreasing. Thus, the buoyant force will also decrease.

Thus, we say that the Buoyant force increases whenever we apply the pressure in the air jar system which is in the water to compensate for the external pressure applied by us.

Hence, the correct option is B.

Note:

1. When an object is submerged in a liquid, the object experiences an upward force. This force which is applied by the fluid on the object makes it rise up and thus this force is called the Force of Buoyancy.

2. Center of Buoyancy is defined as the point where the force of buoyancy is applied to the object.

3. Thrust is a force and it is a vector quantity that has both magnitude and direction. Thrust can be defined as the force which is exerted by an object perpendicular to the surface.