A well balanced ruler How to use Thinking Skills to involve pupils in discussion and debate.

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A well balanced ruler How to use Thinking Skills to involve pupils in discussion and debate.

A well balanced ruler Set up the metre ruler with your hands, upright, below the 10cm mark and the 90cm mark. Explain to the pupils that you will move only one hand and keep the other in position. i.e. you will only move your right hand, not your left. You may wish to use a marker to prove that your left hand does not move.

A well balanced ruler Ask pupils to predict what they think will happen as your right hand moves towards your left hand. Ask them to be definite about where they think the ruler will topple, because they will think the ruler will fall. By the way, what do you think will happen?

Be amazed! Click the picture  to view what happens

Why does it happen? The Centre of Mass, CoM, at the start of experiment was in the middle of the ruler, but as you move your hand towards the centre of the ruler, the CoM changes and moves more towards your moving hand. This increases the amount of friction over the moving hand until eventually it overcomes the friction holding the ruler over your left hand(90cm) which then allows the ruler to move.

A magic ruler The process repeats itself, as the Centre of Mass, becomes greater over the moving hand and it causes the ruler to move, until your hands meet in the centre of the ruler.

The diagrams shown on the slide show indicate that both hands move, please don’t do this during your experiment as they are for illustrative purposes only. Enjoy the arguments that this experiment will provoke.

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