Physics Question: How a swing works **Not Homework

[Zorba]

I've been thinking about a swing lately. I've had all the physics and dynamics, etc (I'm a senior Mechanical Engineering student) but I can't really figure out how a person swings (in my head anyways). What I mean, is how does the movement of a person cause the swing to, well, swing? I know it very hard to get a swing started if you can't push off, but once you start swing a few degrees off center it becomes very easy to gain hieght. Can anyone show me how this work (with physics, not just "that is how it works, stupid" )? Or does this question make any sense at all?

 

[WinkOsmosis]

Something to do with reinforcing the waveform with constructive interference. You can cancel your motion too. It's But what do I know?

 

[silverpig]

I think it has to do with the fact that you pull on the chains some, thereby effectively decreasing the pendulum length and the period, thus increasing the angular velocity of the swing.

 

[WinkOsmosis]

Originally posted by: silverpig
I think it has to do with the fact that you pull on the chains some, thereby effectively decreasing the pendulum length and the period, thus increasing the angular velocity of the swing.

What good is extending the chains by .001mm going to do?

 

[TechnoKid]

I think by you shifting center of mass (when you tilt back and forth to keep the swing going), it changes kinetic energy (and potential at the top of the swing?) during the period of the swing. I did a lab in physics today that delt with pendulums (timing gates), although not as in depth as a lab specifically of this nature would be.

 

[sillymofo]

I think it has a lot to do with inertia. Most of the time, people on swings use their legs to increase velocity/height. So to apply that to "a body in motion" theory, when you reach your optimal height, if you swing your legs in the right directions, that suppose to be that extra force (that is still in motion) to pull you higher.

 

[Zorba]

Originally posted by: WinkOsmosis
Something to do with reinforcing the waveform with constructive interference. You can cancel your motion too. It's But what do I know?

So you are basically saying that you are inputting into the swing at it's resonate frequency thereby causing the swing to multiply the ouput?

I think it has to do with the fact that you pull on the chains some, thereby effectively decreasing the pendulum length and the period, thus increasing the angular velocity of the swing.

I like the idea, but since the chains are usually at least ten feet long, shortening them by a few inches would have little effect.

I think by you shifting center of mass (when you tilt back and forth to keep the swing going), it changes kinetic energy (and potential at the top of the swing?) during the period of the swing. I did a lab in physics today that delt with pendulums (timing gates), although not as in depth as a lab specifically of this nature would be.

If you move your CG backwards, the swing goes forwards. If you move your CG forward then the swing move backwards (I am assuming a person's CG is in the lower back region). So effectively for small angels off the vertical your CG would stay in the same spot (from you moving it backwards and the swing moving forwards). The if you are exciting it's resonate frequency then the output of the swing will slowly gain in magnitude (on a log scale, I am assuming) until you reach you maxium height.

Does that make any sense, or am I completely off with this?

 

[PowerMacG5]

I think it has to do something with changing the moment of intertia of the swing. I=int(m*r^2)dr. THink about rotating in a chair with your arms extended, and then bringing them in; you speed up. I think that changing the moment of inertia combined with pendulum motion affects the period of the swing.

 

[Zorba]

Anybody else has some idea? I am really interested in how this works.

 

[RossGr]

When you lean back you lower your center of gravity, this means you have reduced the potential energy of the system, energy is conserved, of course, so the reduction in potential energy must show up as an increase in kinetic energy. Thus you start to move. The change in position must be timed to the period of the swing to get a reinforcing motion. Thus the amplitude grows, If you time it right, if the timing is not right you do not reinforce the motion, so the amplitude of motion does not grow.

 

[dman]

I brought my daughter to the park a few times now and I was wondering the same thing. I miss the days of not thinking about those things.

 

[LordMorpheus]

Originally posted by: dman
I brought my daughter to the park a few times now and I was wondering the same thing. I miss the days of not thinking about those things.

Don't be in too big a rush to return to them: the second time around you are called an "old fart" instead of a "child."

Its been too long since I've been on a swing . . at my gradeschool I used to be able to get the swing over above the bar that supported it: therefore, my return arc wouldn't be smooth, because I had some velocity towards the pole but gravity checked my upward velocity . . . . .

And so I was in freefall untill about halfway to the ground. What a good time. I spent so much time on those back in my day. I almost killed myself a few times, but oh well.

 

[bonkers325]

by shifting the center of gravity in curvilinear motion, you can manipulate the tangential and normal accelerations and decrease/increase your speed as you please.

take a 10 lb weight in your hand, sit on a chair that can spin and spin around while your arm is extended. while u are spinning, pull the weight back to your body, and you will begin to accelerate. nothing to do with swings, but its fun if u like to spin yourself dizzy.

-bonk's bro

 

[Zorba]

Anyone ever tried starting a swing without pushing off the ground at all? It takes forever to get going.

 

[Zorba]

Bonk's Bro - That comes from conservation of angular momentum, so when you pull the weight in you do indeed speed up but your total energy is the same. Whereas on a swing the total energy increases.

 

[bonkers325]

im just a sophomore engineering major, but im pretty sure that by shifting your weight by using your legs you can change radial acceleration. i've never even heard of the conservation of angular momentum. must be a variation of the plain old conservation of momentum.

 

[Zorba]

Yeah I see your point about shifting the legs :beer: .

Cons. of Ang. Momentum = Cons. of Momentum, they just try to make it sound cooler, so they can derive it again in Physics and Dynamics.

 

[phatj]

extending/contracting legs and back = affecting moment of inertia ?

 

[Crappopotamus]

im just a lowly high school kid, but when you lift your legs at the top, doesnt that increase the gravitational energy? every time you hit the peak of your swing, you push your feet higher than they were before, resulting in more energy.

hell. i dont know. *brain explodes* good question, jeez.

 

[edro]

It's the same idea as trying to roll a cart while standing on it. If you move your center of gravity to a new point slowly, then rapidly move it to another point, there is friction there... that = teh movement.

 

[Zorba]

Originally posted by: phatj
extending/contracting legs and back = affecting moment of inertia ?

It would because moment of inertia has to do with the distance of mass from a given axis. So then if you are all balled up you have a lower 'I' than if you are standing up.

good point edro

 

[silverpig]

Originally posted by: WinkOsmosis

Originally posted by: silverpig
I think it has to do with the fact that you pull on the chains some, thereby effectively decreasing the pendulum length and the period, thus increasing the angular velocity of the swing.

What good is extending the chains by .001mm going to do?

You pull on them with your arms, putting some of your weight up higher on the chains, effectively decreasing the pendulum length.

 

[silverpig]

Originally posted by: Marauder911
I think it has to do something with changing the moment of intertia of the swing. I=int(m*r^2)dr. THink about rotating in a chair with your arms extended, and then bringing them in; you speed up. I think that changing the moment of inertia combined with pendulum motion affects the period of the swing.

Bingo. Lower moment of inertia resulting in placing some of your weight higher on the chains (like 2-3 feet depending on your height, the distance from where your hands contact the chains to where your ass sits on the seat).

 

[silverpig]

The position of your feet has very little to do with it. Ever try swiginging without pulling on the chains at all? By just moving your feet back and forth? You ain't gonna go anywhere.

 

[DrPizza]

no offense meant... but I find it scary that a senior mechanical engineer can't figure out how a swing works...