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Falling Mugs
Scare all your friends by dropping a mug with seemingly nothing to stop it hitting the floor, and find out what it has to do with ice skating.
What you need
| A mug or something of a similar weight |
| A nut weighing about 5-10g |
| A pencil |
| Some string |
What to Do
Cut a piece of string that will not quite reach the ground from about shoulder height.
Tie the mug on one end of the string and the nut on the other.
Hold the nut in one hand and the pencil in the other. Put the string over the pencil and hold the pencil at about shoulder height (with the far end higher than the close end or the string can fall off!)
Pull the nut away until the mug is near the pencil, and they string is horizontal.
Let go!
You may want to practice with something less breakable than a mug...
What may Happen
You should find that the string wraps itself around the pencil and the mug doesn't hit the ground.
What is going on?
When you let go of the nut it isn't just pulled towards the pencil it is also falling downwards under gravity. This means that it starts to rotate around the pencil like a pendulum. However it is a pendulum which is getting shorter all the time.
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The nut feels both the string pulling it towards the pencil and gravity pulling it down. | So as the nut falls it starts to rotate around the pencil. |
When something is rotating around a point and you move its mass towards the centre of rotation, it will rotate faster. This is because as the radius of the circle reduces the distance around the circle the mass has to travel decreases so it takes less time and the mass rotates quicker. Also as you pull the mass inwards you do work against what is known as centrifugal force but is actually the inertia of your mass. This work therefore has the effect of speeding up the mass.
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Ice skaters start off spinning slowly with their weight spread out. | Then they move their weight into the centre. |
This is the same effect as an ice skater doing an immensely fast spin. When they start their weight is spread out, but as they pull their weight inwards they spin faster and faster.
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As the nut moves closer to the pencil it rotates around it faster. | This means that the nut wraps itself around the pencil enough to hold the mug's weight. |
Thankyou to Richard Ellam of LM Interactive for showing me this trick.
Written by Dave Ansell
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