• What keeps this stick in balance,…

• …while you move your fingers…

• …under it?

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Physics
age: 12 – 16

# Self-balancing wand

A quick and surprising experiment that combines friction, forces and the center of gravity. Suitable especially to recap and connect these topics before moving on to another chapter.

Learning objective

Linking friction, weight and the center of gravity.

Observing how different physical effects can play together to keep a system in balance.

Ruler (also works with broomsticks)

Hold a ruler on your index fingers near its ends. Then slowly move your fingers towards each other.

1. Why does the ruler stay balanced?

Guiding Questions (if needed)

What happens as you bring your fingers together?
› The ruler slides over the fingers but doesn't fall off.

Over which finger does the ruler slide?
› Over the one that has the lower friction.

On which finger is the friction lower?
› The finger that carries less of the ruler's weight.

Which finger carries less of the ruler's weight?
› The one from which the object protrudes less.

Findings

The ruler will slide over the finger from which it protrudes less, because the lower weight on that finger leads to a lower friction. As long as the ruler remains horizontal, the sliding will therefore alternate between the sides, and the center of gravity stay between the fingers.

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