# The Naked Scientists Forum

### Author Topic: How does a swing work?  (Read 3521 times)

#### cheryl j

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##### How does a swing work?
« on: 02/08/2014 03:11:54 »
This is probably a very silly question. I was sitting in my hammock swing that hangs from a tree over a river behind my house, and swinging a bit, and I thought about playing on the swings at recess when I was a little kid -how every kid quickly figures out that if you extend your legs and lean back on the up swing, and flex your legs and lean forward on the way back down, you can increase your altitude. But I can't really figure out why it should work that way.

Also is there any machine that works like a kid on a swing?

#### PmbPhy

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##### Re: How does a swing work?
« Reply #1 on: 02/08/2014 05:28:47 »
This is probably a very silly question. I was sitting in my hammock swing that hangs from a tree over a river behind my house, and swinging a bit, and I thought about playing on the swings at recess when I was a little kid -how every kid quickly figures out that if you extend your legs and lean back on the up swing, and flex your legs and lean forward on the way back down, you can increase your altitude. But I can't really figure out why it should work that way.

Also is there any machine that works like a kid on a swing?
I'm not 100% certain on this but I'll take a shot at it. When you set the swing in motion one way to put energy into it is to exert a torque. That means to exert a force in the direction of the motion towards the bottom and this can be done by raising or lowering the legs. The rate at which you do this is timed so that resonance occurs and eventually you'd go over the top if you didn't slow down. The energy that is put into the swing comes from your body exerting forces on your legs in order to do keep the up and down pumping action going.

#### RD

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##### Re: How does a swing work?
« Reply #2 on: 02/08/2014 05:45:30 »
The swing is like a pendulum.
By changing your position* on the swing you are effectively changing the length of the pendulum : changing the distance from the pivot point to the centre-of-mass of the bob at the end.
If this is done at the correct time it drives the pendulum ...

[ * kicking your legs out changes the position of your centre-of-mass ]
« Last Edit: 02/08/2014 06:01:41 by RD »

#### PmbPhy

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##### Re: How does a swing work?
« Reply #3 on: 02/08/2014 05:59:32 »
Cheryl - Thank God RD is here. I never knew that. This is something I should be able to check by calculating it. In this case we're fortunate to have a system which is relatively simple and should be easy to model and therefore it might be easy to solve. I'm very busy doing other things so please don't expect an answer soon, okay my dear?
« Last Edit: 02/08/2014 06:33:49 by PmbPhy »

#### RD

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##### Re: How does a swing work?
« Reply #4 on: 02/08/2014 06:13:20 »
It's more obvious on a standing-trapeze-swing where they bend at the knee at the extremities of deflection of the swing , effectively lengthening the pendulum at that point by moving their centre-of-mass away from the pivot point  ...
« Last Edit: 02/08/2014 06:20:09 by RD »

#### cheryl j

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##### Re: How does a swing work?
« Reply #5 on: 03/08/2014 22:18:12 »
Thanks, everyone. I thought all one would be doing was somehow adding more force to the movement, but then thrusting your feet forward would also push you backwards, and it didn't seem to make sense (I don't understand torques very well), but the change the length of the pendulum makes sense to me.
« Last Edit: 03/08/2014 22:21:13 by cheryl j »

#### alancalverd

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##### Re: How does a swing work?
« Reply #6 on: 03/08/2014 22:52:56 »
Just altering the length of the pendulum won't make it start swinging from its equilibrium position.
Ignoring the effect of legs, lets just consider leaning backwards. Your center of mass - effectively the CoM of the entire swing assembly - is now behind the seat so the seat moves forwards until the CoM is under the pivot point. Now pull yourself forward quickly with your arms. You have moved the CoM away from its equilibrium position  so the swing starts to .... swing.

The assembly swings past its equilibrium position until the CoM is some way behind the centre, and at maximum amplitude it starts to move forward again at which point you lean back, thus moving the CoM even further from equilibrium and increasing the amplitude of the next oscillation.

#### cheryl j

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##### Re: How does a swing work?
« Reply #7 on: 04/08/2014 16:52:15 »
So then the legs really don't have much to do with it other than partially balancing yourself so you don't slide off the swing?

#### jccc

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##### Re: How does a swing work?
« Reply #8 on: 04/08/2014 17:18:52 »
Your force interact with gravity by the rope so you can swing.

According to momentum conservation, without your body interact with the rope, you can move your body but cannot swing it/move mass center.

A toy kid can swing? Could be pricy.

#### alancalverd

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##### Re: How does a swing work?
« Reply #9 on: 04/08/2014 20:04:38 »
So then the legs really don't have much to do with it other than partially balancing yourself so you don't slide off the swing?

I think it's related to human biophysics. Having leaned back to get the swing moving, or at the top of the backswing, you then need to start moving your CoM forward again to reverse the process at the top of the forward arc. I'm pretty sure the abdominal muscles work better at this when your legs are straight, and straightening them moves the CoM forward too. But given the degree of suspicion which surrounds unaccompanied adults in a playground, I'm now looking round the garden for a suitable tree to do some experiments!

#### The Naked Scientists Forum

##### Re: How does a swing work?
« Reply #9 on: 04/08/2014 20:04:38 »

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