Archimedes' principle: upthrust and the liquid pushed aside
Hang blocks of iron, aluminium and wood from a spring balance and lower them into water, salt water and kerosene. Compare the loss of weight with the weight of the liquid that overflows.
Core concept
A body in a liquid feels an upthrust equal to the weight of the liquid it pushes aside (Archimedes' principle); it sinks if its density is more than the liquid's and floats if it is less, pushing aside its own weight of liquid
Topic: Pressure
A stone feels lighter in water. The liquid presses on every side of it, and the push from below is bigger than the push from above, because pressure: How much force presses on an area. A sharp knife has high pressure at its edge. All key words grows with depth. The difference is an upward force: A push or a pull. Forces can start, stop or turn things. All key words called upthrust (or buoyancy). Archimedes discovered that 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 body pushes aside. So the spring balance reads less in the liquid: apparent weight = real weight − upthrust. A body sinks if its density: How much mass is packed into a space. Things less dense than water float. All key words is more than the liquid's; if its density is less, it floats, and then it pushes aside just its own weight of liquid (the law of flotation). The liquid it pushes aside overflows from a full can into a measuring cylinder, so you can measure it.
What you will learn
- State Archimedes' principle and the law of flotation.
- Calculate upthrust from the weight in air and the apparent weight.
- Explain why a body sinks or floats using density.
- Explain how the density of the liquid changes the upthrust.
Before you start
- Know that density = mass ÷ volume and weight = mass × g.
Materials
- Blocks of iron, aluminium and wood, water, salt, kerosene
Equipment
- A spring balance, an overflow can, a measuring cylinder
Procedure
- Lower the 100 cm³ iron block into water and record.
- Record with a 50 cm³ and a 200 cm³ iron block.
- Record the 100 cm³ iron block in salt water and in kerosene.
- Record the wooden block in water.
How this simulation is simplified
- g = 9.8 N/kg; the densities are iron 7.9, aluminium 2.7 and wood 0.6 g/cm³.
- The liquids' densities are water 1.0, salt water 1.2 and kerosene 0.8 g/cm³.