Floating, sinking and upthrust
Lower objects on a spring balance into fresh water, salt water and oil. Measure the upthrust and find out why some float and some sink.
Core concept
A liquid pushes up on an object with a force (upthrust) equal to the weight of liquid it displaces; an object floats if it is less dense than the liquid
Topic: Pressure
When an object is put into a liquid, the liquid pushes up on it. This upward force: A push or a pull. Forces can start, stop or turn things. All key words is called upthrust (or buoyant force). Archimedes' principle says the upthrust equals the weight: The pull of gravity on something, measured in newtons. It is less on the Moon. All key words of the liquid the object displaces. Weigh an object in air and then in water: the reading drops by the upthrust. An object floats when it is less dense than the liquid, because it can displace its own weight of liquid before it is completely under. Salt water is denser than fresh water, so it gives more upthrust.
What you will learn
- Measure upthrust as the loss of weight in a liquid.
- State Archimedes' principle.
- Use density to predict whether an object floats or sinks.
Before you start
- Calculate density = mass ÷ volume.
- Read a spring balance in newtons.
Materials
- A wooden block, a plastic toy, an apple, an egg, a stone and an iron nail cluster
- Fresh water, salt water and cooking oil
Equipment
- Spring balance (0 to 2 N)
- Large beaker
Procedure
- Hang the stone on the spring balance and record its weight in air.
- Lower it into fresh water and record the new reading. The difference is the upthrust.
- Lower the wooden block into the water and record what happens.
- Try the egg in fresh water and in salt water.
How this simulation is simplified
- g is taken as 10 N/kg.
- Densities: fresh water 1.00 g/cm³, salt water 1.10 g/cm³, cooking oil 0.92 g/cm³.