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What happens when you drop a suspended slinky?
We slowed down footage of a falling slinky to show you. |
Curriculum links KS3 &4 Physics: Gravity, Waves
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Slow motion footage of a falling slinky shows very curious behaviour. The slinky appears to defy gravity. The bottom of the slinky remains stationary, suspended in mid-air until the coils have fully collapsed. How can we explain this?
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Before it is dropped, the forces acting on the suspended slinky are in equilibrium. The downwards pull of gravity is balanced by the upwards pull of the tension in the coils.
When the slinky is released, the coils collapse downwards from the top in a compression wave. The bottom of the slinky remains motionless until the coils have fully collapsed, and the compression wave reaches it. At this point the slinky falls. In their 2012 paper in the American Journal of Physics, Cross and Wheatland modelled this behaviour and compared it to real data from metal and plastic slinkies, captured on high-speed camera. They observed that the time taken for the compression to reach the bottom of the slinky is typically 0.3 seconds. |