Can you make a string swing without touching it? We’ll show you how in this simple physics experiment.
You will need
- String
- Scissors
- Two chairs
- Handful of plasticine
Safety
This activity requires the use of scissors to cut string. Adults can help younger scientists by cutting the string for them prior to the activity.
What to do
Place two chairs back to back. Move them apart until there is a space about as long as your arm between them.
Cut a piece of string twice as long as this distance.
Tie one end of the string onto the back of one chair. Tie the other end to the other chair. Make sure that when you’re done that there is still about an arms-length between the two chairs.
Cut two smaller pieces of string, each about 30 centimetres long.
Tie the end of one of these pieces of string to the middle of the string between the chairs.
Tie the end of the second piece of string next to the first string. Create a space between them about as wide as your hand.
Break the plasticine into two pieces, each about the size of a small plum.
Mould one of the pieces around the end of one of the pieces of string, making sure it is secure. Repeat this with the other piece of plasticine around the second piece of string.
Adjust the distance between the two chairs so the string between them sags slightly.
Pull one of the lumps of plasticine back and let it go so it swings freely. Watch what happens to the second string.
What’s happening?
This 660-metric-ton steel pendulum helps keep the Tapei 101 skyscraper stable in earthquakes and in cyclones. Credit Wikimedia Commons/Armand du Plessis CC BY-SA 4.0
A pendulum is a weight pulled that’s anchored at a point which lets it swing freely under its own momentum when pulled back. The timing of each swing is determined by the distance between the bottom of the weight and the point it’s anchored to. You might have seen pendulums on clocks, which use the pendulum’s length and swing to keep time.
The weight pulls on the anchor, transferring its moving (or ‘kinetic’) energy into the support above it. In this case, each swing sends energy into the string, which moves into the second pendulum, causing it to gain energy and move back and forth with similar timing much like an adult timing the push of a child on a swing.
The location of the steel pendulum pictured above in the Taipei 101 skyscraper. Credit: Wikimedia Commons/Someformofhuman CC BY-SA 4.0
We described these two pendulums as coupled, meaning each pendulum’s swing is linked to the other’s movements. Newton’s laws describe how each pendulum pushes back on the other through the string.
Engineers can couple moving weights together and use these laws to reduce movement in objects they don’t want to move. For example, to keep buildings from swaying too much in earthquakes, pendulums can be coupled to them and adjusted to make them swing less rather than swing more.
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