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Child on a Swing

Imagine sitting on a swing. Without touching your legs to the ground, it is possible to get into motion by leaning forwards and backwards with the body and the legs.

So your velocity has changed without action of any external force. How to explain this?
Original post by esrever
Imagine sitting on a swing. Without touching your legs to the ground, it is possible to get into motion by leaning forwards and backwards with the body and the legs.

So your velocity has changed without action of any external force. How to explain this?

yep, I think it's that you're moving your centre of mass (slightly) wrt the suspension point of the spring... could be wrong though
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M4cc4n4
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Original post by esrever
Imagine sitting on a swing. Without touching your legs to the ground, it is possible to get into motion by leaning forwards and backwards with the body and the legs.

So your velocity has changed without action of any external force. How to explain this?

But you are displacing your centre of mass from the point of equilibrium and therefore you are putting a driving force into the system and therefore you get oscillations. You put the energy into the system and have expended energy in doing so, so work is done.
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esrever
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Original post by M4cc4n4
But you are displacing your centre of mass from the point of equilibrium and therefore you are putting a driving force into the system and therefore you get oscillations. You put the energy into the system and have expended energy in doing so, so work is done.


But how can my internal forces cause a net force on myself?
Reply 4
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M4cc4n4
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Original post by esrever
But how can my internal forces cause a net force on myself?

By contracting your muscles you are moving your Center of mass away from the point of equilibrium, then your Center of mass will want to return to the equilibrium point and this will cause a restoring force , your muscles exert the force to move you and the aim of the swing is to move you back to equilibrium, however if you move you Center of mass back and forth so that the Driving force which is caused by you purposefully shifting your Center of mass you can create some fairly big oscillations in your own.
you will be doing work against gravity by raising part of your body above its resting level. this increase in GPE can then be turned into KE by lowering the part again.

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