How Swings Work - Sixty Symbols

Swings, pendulums, parametric resonance and all that.
Featuring Professor Roger Bowley.

This project features scientists from The University of Nottingham

Sixty Symbols videos by Brady Haran

[LayZeeBro]

"there, there, there, dear, it's getting a bit too violent for me."

[sci-piaskun]

Well, pretty sure that shortening pendulum is not main reason to increase amplitude of swing.
Proof? On rigid bar swing you can use same motion to increase amplitude, and not only by doing sit-ups like professor said in extra footage video.
Also, you can switch position when swing is at maximum positions, and that shouldn't work if shortening pendulum was main driving force.

I think swing works by shifting center of mass behind chair when moving forward, and in front of when moving backward.
And because pendulum _center of mass_ is reaching same height on both ends (ignoring air resistance), when you shift center of mass behind and in front of chair, you make chair go higher during each oscillation.

[RimstarOrg]

Neat how every kid knows exactly what to do when they get on a swing without knowing why it works, until now of course. (And by kid I mean me. :))

[RinoaL]

PROOF that even scientists and filmmakers can't resist swinging on a nice day.

one of my friends used to live on a farm and they had a swing in a big oak tree, was about 60 feet tall. it was equal parts awesome and scary when you got it going good. would take like 7 seconds just to swing from one side to the other.

[Sperzel]

"Oh dear, it's getting a bit to violent for me." Priceless!

[michaelm]

Excellent point highlighting the shortening chain contributing to a swing's acceleration. But there's a lot more to swing acceleration and momentum than just a shortening and lengthening chain.

[skalpathal]

Loved this. What are the chances of getting all the professors to do their talks while on a swing in the future? :)

[o.tsonev]

But professor, this same technique with the body and legs in a sitting position works with a swing with rigid bars too! I spent my childhood on such swings. in your explanation you say that you should shorten the pendulum at the mid point, but wen you swing you make changes only on the end points. Surely your interpretation of the dynamics of the swing should be wrong!
I think the reason this works is not the changing length but that this motion changes the height of your center of mass. 
So in the back end point you put your body back, but your seat is tilted, so your center of mass is going actually higher in respect of the ground. Then in the front end position, your center of mass is really below the seat, so you tilt forward and effectively bringing the center of mass higher than the end point, You are pumping more energy in the swing system by adding a little bit of potential energy on the high points.
Try it out on a rigid bars swing, you'll sea it's more clear what's going on. With the chain swing you basically have 2 connected pendulums and you know how unpredictable are those.

[phelpsio]

I'm so glad to see Professor Bowley is still making videos!

[kerendn]

I love Professor Bowley's videos, he's so much fun! :-)
Eagerly awaiting more sixty symbols videos - wish they were more frequent!
About the swing - I thought that moving your feet outward caused some force like when you throw a ball, but your feet are attached to you and so they pull on you, outwards, a little bit, making you swing higher... Something like that.

[shawnchin]

Look at that passion! WOW! for such an old professor playing on a swing with such passion! This is what physics can do to people

[logiclrd]

I'm quite certain that this explanation is missing a significant part of the actual physics. When you lean backward, you are not only applying some shift to the *height* of the centre of gravity, but you are also shifting the centre of gravity out of the plane of the swing itself. To imagine what I believe is going on, imagine a pendulum with a rigid bar. You set it swinging, it goes back and forth, great.

Now, with it stationary, imagine that you take another metal bar and attach it to the bottom end of the pendulum at right angles, so that it sticks out to the side, say to the right. When you release this, the pendulum will swing to the left, because that allows the *new* centre of mass to be closer to the ground than with the pendulum vertical. Similarly, if the bar sticks out to the left, the pendulum will swing to the right to accommodate.

Imagine, then, a system whereby you could instantaneously change which side the metal bar was on right at the apex of each swing. You start out at rest with the pendulum straight up and down, and the metal bar sticks out to the right. The pendulum swings to the left. In fact, it swings *past* the point of equilibrium thanks to the momentum it gains. Just as it is turning around to swing back the other way (it would eventually return to the starting point, of course), you switch the metal bar to stick out the left side of the pendulum. Now you're imparting acceleration to the right, and it will go substantially further than the starting point. Again, when it turns around, move the weight to hang off to the right, and you'll swing further than the previous apex on the left.

I believe this is the true explanation of what's going on, because I've found myself that if you *reverse* the usual process for swinging, and lean back when you're forward and forward when you're back, you actually decelerate the swinging. To make an analogy back to the simplified pendulum in the previous paragraph, then, normally what you're doing at the apex of each swing is shifting the centre of gravity so that the pendulum's rest point is on the *other* side of the Y axis. If you do it backwards, though, you're putting the centre of gravity on the *same* side of the Y axis, so that the pendulum wants to come to rest on the side you're already on and doesn't swing as far on the opposite side.

[rogg]

I spent my entire childhood trying to figure this out... I never could.... thankyou

[neutrino]

Glad to see Professor Bowley again, I was worried we wouldn't get any more videos with him since he retired.

[SSSS-eypm]

Such a great video and demonstration.... love the passion this person has and how that passion pulls the listener-watcher into learning something new. If I had teachers like him - I’d be a scientist today lol

[Epaminaidos]

I'm not sure if this is the only explanation. It is also possible to explain the swinging via the conservance of angular momentum: When being at the highest point, you rotate your upper body and legs. This angular momentum is transferred to the swing itself and lifts you up a little more. When being on the other end, you create the angular momentum in the other direction and lift yourself up a little more.
Would be interesting to check it on a swing with rigid bars...

[Niwles]

Finally!
Make more sixty symbols videos, please Brady!

[nbsr]

This method works (mostly) because of the angular momentum added when changing the body angle. The one that shortens the pendulum length requires standing on the swing (as in the extra footage).

[Hexxoone]

I smiled way too much when he was playing with the swings. 

[elephamoose]

You've just inspired a new inquiry lab for my physics students this fall...