It seems that everyone intuitively knows how to make a playground swing go: legs forward when moving forward, legs back when moving back.
Without continuing to move your legs in that way, you would swing less high each time and eventually come to rest. Therefore there is some force that will bring you to a halt.
Whatever it is that you do with your legs generates some force that is able to overcome what would otherwise gradually bring you to a stop.
What is it about the way you move your legs that makes the swing go faster? You must have enough force of some kind to overcome the force (whatever it may be) that would otherwise stop you. How does moving your legs the way everyone seems to intuitively know how to do generate this force? What kind of force is it?
When you shift your legs and body forward, you shift the center of balance forward. This adds energy to the system, energy that you produce. When you begin to relax again, the center of mass begins to shift back, and the swing falls. As it passes center, you lean back and pull your legs in fully, shifting the center of mass back the other way, and the process repeats itself. If you do this at the natural frequency of the swing, and time the input of energy to the forward and rearward maximums a process called resonance causes the oscillations to increase in amplitude, and you go higher. Resonance is the same reason an organ pipe is as loud as it is, and why tuning forks work to help tune instruments.
I recall that this paper was WRONG, and that later papers pointed out that kids do not crouch repeatedly, instead they tilt their torsos back and their legs forward, rotating round their centers of mass. Perhaps QED is right, and this ends up shifting their center of mass forwards and back.
Another site points out that nobody bothered to analyze playground swing dynamics until 1968. (And then the analyses were misguided for years because they focused only on the upper torso!) I wonder how many other interesting physics questions are hiding on kids’ playgrounds yet unexplored?
That is not the way I (or anyone I have seen) used a swing - You don’t lean back when the swing returns - you lean back when going forward. When I put my legs forward, I leaned my body back (as indicated by bbeatty and pictured here). When returning, I pulled my legs in and leaned my body forward. So I don’t buy the moving of center of balance theory - I don’t think it shifts much at all, and if it does I’d be more inclined to think it is opposite to the way Q.E.D. states as the mass of the body that has moved backwards is greater than the legs that moved forwards. In the picture I linked to, the centre of balance is clearly behind the seat.
I think the answer lies in that the person is not specifically pushing their legs out, they are using their behind to push the seat forward while holding the rope/chain to create a fulcrum (if that’s the right word) from which just the lower part of the chain can rotate. In that sense, the center of gravity of everything below the fulcrum is moved forwards compared with the chain from the hands upwards, so maybe in that sense I agree with Q.E.D.
Well, a combination of gravity and friction would be my guess.
Think about what would happen if you hold an empty swing parallel to the ground and even with the bar, then let it go. It’ll fall until the chain “catches” and then it’ll fall in an arc towards the ground under the bar. It’ll overswing, probably, then repeat until it stops.
Gravity is pulling it down, the friction (mostly the chain on the bar) is slowing it down. Without friction, it would keep swinging. Without gravity, it would stay where you let it go.
Apparently air resistance isn’t exactly friction in the same way that, say, skidding along the ground on your ass is friction. - The slowing effect of air mostly isn’t because it is ‘rubbing’ against you.
Air resistance consists largely of the work required to stir the air - air has mass and setting it in motion requires an energy input.
Spacecraft during reentry experience yet another kind of “friction,” neither caused by turbulence nor caused by viscosity.
If you travel faster than the speed of sound, then air becomes compressed in front of you, and it cannot easily get out of the way before more air piles on top. You end up with flaming hot high-pressure gas trapped against your leading edge. If your leading edge is not sharp, the problem is much worse.
Apollo capsules, the Shuttle, etc. get hot for the same reason that the coils on the back of your fridge get hot: a compressor does it.
People on playground swings just lose energy by stirring the movable air and by shearing the viscous air.
You compress the air in front of you as you swing too–that’s mostly how your momentum is transferred to the air. It just doesn’t gain enough energy to glow red hot. Or swingin’ would be a lot less popular amongst the kiddies.
Sure, but unlike friction with a solid surface, the moving body isn’t just interacting with a surface layer; it is having to accelerate volumes of matter and is thus transferring some of its energy away in the form of moving vortices.
I believe it was 1969 when I performed experiments on the phenomenon of swinging. Upon being taught the swinging technique, I realized that the higher one’s hands were positioned on the chains holding the swing, the less speed one gained (leg movements didn’t seem to impact much). So I moved my hands lower and lower on the chains, picked up more and more speed, and then went for maximum thrust by placing my hands on the seat and leaning back. Thud! This was back in the days of asphalt-covered playgrounds, which is why my skull was fractured. Moral of story: 4-year-olds don’t make good scientists.
That’s the difference between solids and fluids, pretty much. Regardless, we interact with the surface layer, the Ekman layer, pretty much, and heat is dissipated from there.
Nope. As long as you’re moving at significantly less than the speed of sound, you DON’T compress the air in front of you. Instead it gets out of your way (it moves.)
Fluid dynamics to the first approximation assumes a constant density of fluid. Textbooks all say that this approximation is very close to reality for velocities far less than the speed of sound. The rule of thumb seems to be: if you try to compress some air (by waving your hand around, for example,) instead you just create wind, not compression.
That’s the “DC” case. You can also create compressed air by sudden accelerations, but where the velocities are quite slow. For vibrations, as long as the size of the object is much less than the sound wavelength at the frequency in question, the air compression is insignificant. If the kids could swing back and forth at a few hundred times per second, then they WOULD compress the air, and they’d radiate sound waves (and be slowed down by radiation resistance.)
If children on playground swings did compress the air significantly, then they would radiate intense subsonic sound. At a half-hertz swing frequency, the kids would have to be pretty large to do this (like a few hundred feet in diameter.)
No one else mentioned it, so in the interest of accuracy, the swing is slowed 100% by friction. Gravity plays no role whatsoever in slowing the swing (other than defining the path).
hey, thanks again for the answers ! This OP was the product of a Christmas Day argument with the hubby that turned into a family debate. I have printed this out for all their benefits.
