What it shows
Archimedes' principle states that the buoyant force or upthrust is equal to the weight of fluid displaced. An object with equal mass but a lower density occupies more volume so displaces more water; it therefore experiences a greater upthrust.
How it works
This demo compares the buoyant force acting on two 1kg masses, one of aluminum and one of brass. Each in turn is lowered into a beaker of water using a spring balance (figure 1). The aluminum, having the lower density, experiences the greater upthrust and a reduction in weight from 10N to about 6N, compared to the brass whose weight drops to 8N.
figure 1. immersing the mass
Setting it up
Requirements are a 2 liter beaker, the two masses and a 20N spring scale. The masses will probably need cotton loops tied so they can hang from the balance.
Archimedes' original problem was to determine whether the king's crown was genuinely made of gold, and figured out how to solve the problem while taking a bath. The density of the metal determined the buoyant force and therefore the apparent weight loss when submerged.