The liquid makes a dish shape known as a circular paraboloid. The horizontal cross-sections are circles. The vertical cross sections are parabolas, the same curve a ball makes when thrown through the air.
When the cylinder spins, the liquid furthest away from the centre is moving faster, which requires a larger horizontal force to keep it moving in a circle. You may have felt this force in a fast car making a turn.
Specifically, the liquid is a parabolic curve. This is because the liquid that is twice as far away from the centre, needs twice the horizontal force, which makes the curve is twice as steep.
Parabolas have been studied since Ancient Greece. Greek mathematician Diocles used a parabolic mirror (a dish shaped mirror in the shape of paraboloid) to focus sunlight to create a burning mirror.
In 1663, Scottish astronomer and mathematician, James Gregory designed a telescope that used a parabolic mirror to focus light.
Parabolas appear in the physical world when throwing a projectile through the air, such as a ball or a jet of water.
Parabolic mirrors are widely used in astronomy to focus the light and radio signals onto one point. Parabolic mirrors are made by spinning molten glass, and then allowing it to cool as it continue to rotate, created a paraboloid.
Unfortunately, parabolic mirrors are expensive to make. A cheaper alternative, used for some large telescopes, is to use a pool of liquid mercury. The mercury is then rotated as a constant speed, creating a paraboloid. This is called a liquid mirror.
Maths at Home
Here’s a way to make a parabola at home, using post-it notes (or similar).
This works because each point of the parabola you make is the same distance from the focal point as it is from the bottom edge. This is one of the definitions of a parabola.
How does changing the height of the focal point affect the parabola?