- 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, Vf = Vo. This also represents the maximum upthrust the object can generate.
- If ρo > ρf , then W > Umax.
- The object will sink all the way down.
- If ρo = ρf, then W = Umax.
- 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 ρo < ρf, then W < Umax.
- 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.
xmPhysics 