Upthrust (CIE AS Physics)

• Upthrust is a force which pushes upwards on an object submerged in a fluid (liquids and gases)
• Also known as buoyancy force, upthrust is due to the difference in hydrostatic pressure at the top and bottom of the immersed object
• The force of upthrust is significantly greater in liquids than in gases, this is because liquids are much denser than gases
• Recall that hydrostatic pressure depends on the height (h) or depth that an object is submerged in from p = ρgh
• Therefore, the water pressure at the bottom of an object is greater than the water pressure at the top

Upthrust on a cylinder immersed in water

The cylindrical object will experience upthrust due to the difference in hydrostatic pressure

• Upthrust is a force and is directly proportional to the pressure. The force on the bottom of the can will be greater than the force on top of the can
• This resultant pressure causes a resultant upward force on the can known as upthrust
• Upthrust is why objects appear to weigh less when immersed in a liquid. If the upthrust is greater than the weight of the object, the object will rise up
• For an object to float, it must have a density less than the density of the fluid it is immersed in