Buoyancy is the upward force, called upthrust, that a fluid exerts on an object. An object floats when the upthrust equals its weight, and sinks when its weight is greater.
This lesson is part of force and pressure. It relies on the idea from comparing pressure in fluids that pressure increases with depth.
Why is there an upthrust?
The bottom of a submerged object is deeper than its top, so the pressure pushing up on the bottom is greater than the pressure pushing down on the top. The difference is a net upward force: the upthrust.
Archimedes’ principle puts a number on it. The upthrust equals the weight of the fluid the object pushes aside, which is called the displaced fluid.
Worked example: an invented block
An invented block weighs 8 N in air. When fully submerged in water it pushes aside 5 N of water, so the upthrust is 5 N.
Step 1: compare the forces. Weight 8 N acts down and upthrust 5 N acts up.
Step 2: find the net force. 8 − 5 = 3 N downwards.
Step 3: state what happens. The net force is down, so the block sinks.
Now suppose the block is reshaped into a hollow box with the same 8 N weight but a larger volume. Fully submerged it would displace 12 N of water, giving an upthrust of 12 N.
The upthrust is now greater than the weight, so the box rises until part of it is out of the water. It settles where the displaced water weighs 8 N, equal to its weight. The box floats.
The mistake that costs marks
The common slip is to write “the block sinks because it is heavy”. A heavy steel ship floats, so heaviness alone cannot be the reason.
A correct sentence compares two forces: “The block sinks because its weight of 8 N is greater than the upthrust of 5 N”. To explain the ship, name the volume of water displaced: “the hollow shape displaces a large volume, so the upthrust equals the ship’s weight”. Both answers name weight and upthrust.
A quick decision rule
- Upthrust greater than weight: the object rises.
- Upthrust equal to weight: the object floats or hovers.
- Upthrust less than weight: the object sinks.
Each statement is a comparison of two numbers. If the question gives them, compare them. If it does not, reason from the volume displaced.
Check yourself
A small invented toy boat weighs 4 N. When pressed fully under water, it would displace 9 N of water. State whether it floats when released, and explain using forces.
Answer
Upthrust when fully submerged = 9 N, which is greater than the weight of 4 N, so the net force is upwards and the boat rises to the surface.
It floats when the displaced water weighs 4 N, which means only part of the boat stays under. At that point the upthrust equals the weight.
What to study next
Test the ideas in the force and pressure practice set, and return to the topic overview for the full order.
If you want a teacher to question your buoyancy explanations until they hold, see online one-to-one Science tuition. Use the mistake log and paper-error review to record which forces you leave out.