To keep it beneath the surface of water, we have to exert downward force.
This means that, when the cork is under water, despite its weight, there is some force which pushes it upwards. When it is under water, the upward push is a force exerted by water. The downward force we exert, counter acts this upward push of the water on the cork. When we are swimming we feel that we are kind of supported by water. This is also due to the upward force exerted by water on our body. A man can lift a heavier stone more comfortably under water than he could in air. This is because the upward force of the water reduces his effort.Thus in all the three cases when the objects are in water they experience a lifting force within the water and this force is called the upthrust.
The force of the upthrust is equal to the weight of liquid displaced by the body.When a body is placed in water, the upthrust or buoyant force acting on it depends upon the following factors:
- Volume of the body submerged in the liquid - (V), or volume of the liquid displaced - (V)
- Density of the liquid - (d)
- Acceleration due to gravity - (g)
i.e., Upthrust = V x d x g
The upward force which any liquid exerts upon a body placed in it is called the buoyant force.