- When a foreign object is submerged in a fluid, the surrounding fluid will exert pressure forces perpendicularly into each point on the surface of the foreign object.
- Because pressure increases with depth, the resultant of these pressure forces is an vertically upward force. This force is called the force of upthrust,
*U*.

- With some simple reasoning, we can deduce that the magnitude of the upthrust must be equal to the weight of the displaced fluid. Hence

- To generate a larger upthrust, a body must displace more fluid.
- Similar to weight, upthrust appears to act at a single point, the centre of gravity of the
**displaced fluid**.

- Compare the upthrust
*U*

- with the weight of the object
*W*

- When an object is fully submerged,
*V*=_{f}*V*_{o}. This also represents the maximum upthrust the object can generate.

- If
*ρ*>_{o }*ρ*, then_{f }*W*>*U*._{max}- The object will sink.

- If
*ρ*=_{o }*ρ*, then_{f}*W*=*U*._{max}- The object will neither sink nor float.
- It can hover at equilibrium at any depth in the fluid as long as it is fully submerged.

- If
*ρ*<_{o }*ρ*, then_{f}*W*<*U*._{max}- The object will float partially submerged.
- It will displace just enough fluid to generate the amount of upthrust exactly equal to its weight.
- The lower the object’s density, the higher it will float.