Defy Gravity

How to make a gyroscope with a bicycle wheel

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Materials:

• Bicycle wheel
• Strong string (approx 1 metre)
• Student volunteer

Instructions

This demonstration has terrific “wow” factor. For the best dramatic effect, perform it twice – once with a stationary wheel, and again with the wheel spinning

1. Tie the string securely to one side of the wheel’s axle

2. Ask a volunteer hold the wheel firmly by the axle –spin the wheel, gradually increasing the speed (hold the string in your spare hand)

3. With the wheel is spinning, hold the string so that it is taught and let go of the wheel – the wheel will stay upright

4. The wheel rotates around the string – this motion is called precession and is due to the interaction between the wheel’s spin (angular momentum) and gravity The direction of the wheel’s precession depends on the direction of the wheel’s spin

Teacher notes

The spinning bike wheel appears to be defying gravity, but it is not. It is certainly very stable and resists falling into a horizontal position, but this is not defying gravity. Instead of falling flat, the effect of gravity on the spinning wheel is a slow rotation around the string – physicists call this rotating motion of a spinning wheel precession. The reason the bike wheel does not fall flat when it is spinning is because, like all moving objects, it has momentum. To distinguish the momentum due to the spinning of an object, it is called angular momentum – a concept students first encounter in senior physics. For younger students, it is suffice to say that spinning objects are very stable and resist changes in direction. This helps Frisbees stay flat (when thrown properly), yo-yos stay and spinning tops stay upright and spinning footballs on target.

More information
How Stuff Works – Gyroscopes http://www.howstuffworks.com/gyroscope.htm

Safety notes

Practice this demonstration with an adult friend before trying it in the classroom – ensure you are using strong string