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Granite Ball

A large granite stone ball sitting on a large slab of stone, which intern sits within a stainless steel bin. Water flows from under the granite and over the stone to the bin.
A large granite stone ball sitting on a large slab of stone, which intern sits within a stainless steel bin. Water flows from under the granite and over the stone to the bin.

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.