About this experiment
Hang a block from a spring scale and lower it into an overflow can. As the block goes in, the scale reading drops and liquid spills into the catch cup. The loss of weight on the scale always equals the weight of the liquid in the cup, which is Archimedes' principle. Change the material, the liquid and the size of the block to find out what makes things float or sink.
Learning goals
- Measure the buoyant force as the difference between weight in air and the scale reading in liquid
- Show that the buoyant force equals the weight of the displaced liquid
- Find that the buoyant force grows with submerged volume but not with depth once the block is fully under
- Predict floating or sinking by comparing the density of the block with the density of the liquid
How to use it
- 1With the aluminium block in air, read its weight on the spring scale.
- 2Drag the block down in small steps and watch the scale reading, the buoyant force and the catch cup.
- 3Keep lowering after the block is fully submerged. Notice which force stays constant on the graph.
- 4Try wood and wax in different liquids and find the depth where the string goes slack and the block floats.
The model
Assumptions
- The block is a solid cube of uniform density and stays level as it goes in.
- The overflow can stays full to the spout, so the catch cup holds exactly the liquid the block displaces.
- The liquid is still and has the same density at every depth. Surface tension is ignored.
- The string and the scale hook have no mass, and g = 9.8 N/kg.
Try this in class
- Q1The aluminium block weighs 13.2 N in air and 8.3 N in water. What volume of water does it push aside?
- Q2Why does the scale reading stop changing once the block is completely under the surface?
- Q3Wax floats in water but sinks in ethanol. Use the densities to explain why.
- Q4Acrylic sinks in water but floats in salt water. How much of it stays above the surface?
Frequently asked questions
Does a deeper object feel a bigger buoyant force?
No. Pressure grows with depth, but so does the pressure on the top face. The difference between the top and bottom pushes stays the same, so a fully submerged object feels the same buoyant force at any depth.
Why does a heavy steel ship float?
Its hull encloses a large volume of air, so the average density of the ship is lower than that of water. It sinks until it has displaced its own weight of water.
Does the mass of the block change the buoyant force?
No. The buoyant force depends only on the submerged volume and the density of the liquid. Mass only decides whether that force is enough to hold the block up.