What is the cause of upthrust? At which point it can be considered to act?

A liquid always takes the shape of its container. Suppose a ball is dropped into a tank full of water. According to Pascal’s law, the pressure exerted by a liquid is transmitted equally in all directions.

When the ball is placed in water, it exerts a downward force on the liquid due to its weight. In response, the liquid applies force in all directions:

• Sideways forces are exerted by the walls of the container, and since they are equal and opposite, they cancel each other.
• Upward force (provided by the liquid) is not canceled because it is not balanced by the ball’s downward force.

The net effect is an upward force acting on the ball, known as upthrust (buoyant force). This force is directed through the centre of buoyancy, which is the point where the buoyant force is considered to act.