Why is the difference between top and bottom pressures is upthrust
Answers
Buoyant force does not have any fixed direction. (Refer to the submarine's working to see negative buoyancy.)
The case that is relevant to the question is as follows:
[Note: The 'downward' direction is the direction of earth's gravity.]
When an object is immersed in a fluid, its weight (force caused by the gravity) will tend to pull it downwards.
A fluid tends to exert pressure (due to the constant motion of its particles).
When an object is immersed, its base will be acted upon by the upward pressure of fluid. If the object is completely immersed, its upper half will also experience a pressure in the downward direction. The pressure pushing the object downwards will be lesser than the pressure upwards because of the difference in depth. (As the depth increases, the pressure increases.)
Hence, the net buoyancy is experienced in the upwards direction.
The object floats (in partial or complete submersion) if the buoyant force is equal to the weight of the object that was dragging it down.