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Granite Ball


Hydraulic pressure and lubrication allows very large masses to be moved.
How it works
Push on the large granite ball to make it spin within its cradle.
Things to try or ask around the exhibit
- How easy is it to spin the granite ball in its cradle?
- What do you notice about the granite ball and its cradle?
- Would you be able to spin this granite ball if it was sitting in a dry dish?
Background
The granite ball weighs 633 kilograms and has a diameter of 750 millimetres. It sits within a cradle that has water being piped from its base. Mains pressure water is pushed up under the ball and spreads out across the base of the granite ball. The push of the water is just enough to lift the ball about one tenth of a millimetre against the pull of gravity. The water also acts as a lubricant when the ball spins.
Finding the science in your world
Even very smooth surfaces have bumps that create friction when one surface moves across the other. In engineering workshops, liquid or air is often forced underneath heavy objects, to act as a lubricant and allow objects to move easily across or past each other.