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Gallery 7

Wobbly Wheel

A wheel can move along a tightrope cable without falling off, if the wheel's centre of mass is altered.

How it works

A wheel sits on a cable tightrope, with two masses extending from the wheel so they hang below the cable tightrope. Turn the handle to tilt the cable tightrope upwards and cause the wheel to roll 'downhill'.

Things to try or ask around the exhibit

• Tilt the cable at different angles to make the wheel roll more quickly or slowly.
• Why doesn't the wheel fall off the tightrope?
• Does the wheel slow down or speed up when it wobbles?
• What else uses masses like this to stay balanced?

Background

The total mass of the Wobbly Wheel includes the wheel itself and the two masses that hang below the cable tightrope. As the wheel moves along the cable, the wheel's mass shifts, causing it to wobble. However because the majority of the wheel's mass hangs under the cable, its centre of mass is quite low, giving the wheel stability and making the wheel behave as though it is hanging on the cable like a hook, rather than sitting on top of the cable.

The wheel relies on the unique position of its centre of mass to remain stable on the cable. Centre of mass is the mathematically weighted average location of an object’s mass, the point at which the mass of each part of an object perfectly balances against the mass of each other part.

A normal wheel's centre of mass would be situated somewhere in the wheel or frame, placing its centre of mass above the cable. As it will fall down towards the centre of the Earth, only the cable is holding up the wheel. Any slight movement will result in a shift of the majority of the wheel’s mass and it will fall sideways off the cable.

The Wobbly Wheel however has most of its mass at the ends of the arms, placing its centre of mass below the cable. The mass on one arm balances the mass on the other, resulting in the entire wheel hanging from the cable. You would almost have to turn the wheel upside down before it fell off the cable, as the masses would pull it back into position.

Finding the science in your world

Tightrope walkers often carry poles with masses on each end. These poles help to lower the walker's centre of mass and also reduces their rotational inertia. This means that if the walker starts to wobble off-balance, they can adjust the position of the pole (and the masses) and 'buy some time' to regain their balance.

If the pole does not have masses at either end, the pole still lowers the walker's centre of mass, but they have less time to regain their balance. Hence, more experienced walkers tend to use plain poles, or even simply extend their arms outwards (just as you might when you're walking along a garden beam).