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Puzzle 4
- Thread starter zekeman
- Start date
Greg - If the swing is hanginging vertical and stationary, where do you get a horizontal reaction from in order to move your body forward and back?
Doug Jenkins
Interactive Design Services
Doug Jenkins
Interactive Design Services
EspElement
Mechanical
could a shuttle propel itself through space by this method?
not at all.. you need a gravitational pull to generate it.. in space there isnt gravity
not at all.. you need a gravitational pull to generate it.. in space there isnt gravity
The gravitational acceleration at the orbital height of a space shuttle is not that much less than on the surface of the Earth. That's what keeps it in orbit, rather than flying off into space.
The difference is that on a swing you can get the rope inclined to vertical, by standing and leaning forward for instance, but your centre of gravity is still directly vertical under the pivot. If you now change the tension in the rope, by squatting down for instance, this will generate a torque which will set up a small swing which can be amplified.
You can't do this in a shuttle because the line of action of the gravitational force is always exactly through the centre of gravity of the craft. The same applies to a swing with a vertical rope. You need to get the rope off vertical before changes in the elevation of your centre of gravity will have any effect.
Doug Jenkins
Interactive Design Services
EspElement
Mechanical
dahhh, wasnt thinking.. but you wouldnt beable to do the same things as you do on a swing in space.. you dont have a fixed point to rotate on.. without sounding to stupid i should say much more.. LOL :-X
EngineerTex
Mechanical
This is a good one.
I think the reason is that the swing is constructed of a rope (or chain) that is of finite thickness. By leaning forwards and backwards, the kid is actually creating higher tension in the fore, followed by the aft fibers of the member, and in so doing, he (or she) is able to get tiny oscillations started in the system, at which point it's off we go. In a perfect system where the attachment to the rigid structure is frictionless and meets at an infinitely small point, the kid goes nowhere.
It's the only thing that I can think of.
Engineering is not the science behind building. It is the science behind not building.
I think the reason is that the swing is constructed of a rope (or chain) that is of finite thickness. By leaning forwards and backwards, the kid is actually creating higher tension in the fore, followed by the aft fibers of the member, and in so doing, he (or she) is able to get tiny oscillations started in the system, at which point it's off we go. In a perfect system where the attachment to the rigid structure is frictionless and meets at an infinitely small point, the kid goes nowhere.
It's the only thing that I can think of.
Engineering is not the science behind building. It is the science behind not building.
GregLocock
Automotive
may raise more questions than it answers
Cheers
Greg Locock
I rarely exceed 1.79 x 10^12 furlongs per fortnight
Thanks Greg, I'll get back to you when I've read it.
I may be some time
Doug Jenkins
Interactive Design Services
I may be some time
Doug Jenkins
Interactive Design Services
![[wink]](/data/assets/smilies/wink.gif "[wink] [wink]")
SomptingGuy
Automotive
"Kids, stop it! You are defying my limited understanding of the laws of physics."
- Steve
- Steve
GregLocock
Automotive
Here's a little model. It is a compound pendulum, with a motor at the joint. For the first 29 seconds it rocks its legs each way at the resonant frequency of the swing.
Then it has a little rest. The swing keeps swinging.
Then it uses a control algorithm - legs forward when moving forward.
There's no friction in the model. The chain on the swing is a rigid link. The seat/body is fastened to the 'chain' via a rotational spring, this doesn't seem to make much difference but stops some mathematical problems.
Why, since you ask, yes I did have the day off.
I wonder if replacing the rigid chain by a series of links makes a difference?
Cheers
Greg Locock
I rarely exceed 1.79 x 10^12 furlongs per fortnight
Then it has a little rest. The swing keeps swinging.
Then it uses a control algorithm - legs forward when moving forward.
There's no friction in the model. The chain on the swing is a rigid link. The seat/body is fastened to the 'chain' via a rotational spring, this doesn't seem to make much difference but stops some mathematical problems.
Why, since you ask, yes I did have the day off.
I wonder if replacing the rigid chain by a series of links makes a difference?
Cheers
Greg Locock
I rarely exceed 1.79 x 10^12 furlongs per fortnight
GregLocock
Automotive
Try again, it is a 3 meg file, unzips to about 10
Cheers
Greg Locock
I rarely exceed 1.79 x 10^12 furlongs per fortnight
Cheers
Greg Locock
I rarely exceed 1.79 x 10^12 furlongs per fortnight
SomptingGuy
Automotive
Nice model.
I recall force being applied where my hands held the chains. And motion too, therefore energy being shifted. Only on the forward part of the swing though. Tucking the knees under and leaning forward on the back swing I can't yet explain in simple terms.
- Steve
I recall force being applied where my hands held the chains. And motion too, therefore energy being shifted. Only on the forward part of the swing though. Tucking the knees under and leaning forward on the back swing I can't yet explain in simple terms.
- Steve
If you've ever tried to sit in a swing that was completely static, not even moving a mm, and tried to start it swinging with nothing but leg kicks, then you know it's a long process.
That's why most kids get a push, or push off with their feet on the ground to get it moving first.
IMO, all that's really going on is the kid grabs the chain or rope a couple feet above the seat. Holds that portion and what's above it in place, then kicks his feet out until his body is parallel to the ground or nearly upside down, this makes the portion of the chain below his hands start swinging a little. When you relax that grip, the whole chain/rope swings back rather than just the portion below your hands, then repeat that step and the swinging momentum gets larger and larger with each leg kick.
Cool question thuogh.
That's why most kids get a push, or push off with their feet on the ground to get it moving first.
IMO, all that's really going on is the kid grabs the chain or rope a couple feet above the seat. Holds that portion and what's above it in place, then kicks his feet out until his body is parallel to the ground or nearly upside down, this makes the portion of the chain below his hands start swinging a little. When you relax that grip, the whole chain/rope swings back rather than just the portion below your hands, then repeat that step and the swinging momentum gets larger and larger with each leg kick.
Cool question thuogh.
SomptingGuy
Automotive
... I suspect ADAMS. Greg is a well-known jockey. I divorced it a long time ago, but it it's really fun for non work related games (I did planetary motion models).
- Steve
- Steve
Thanks Steve.
I hear many good things about it. Think I will try to get my hands on a trial version.
I know that could be done in say Matlab by solving the DE and linking it with a real-time plot, but I suspect ADAMS makes this less time consuming. Not to mention its probable 3D multi-dynamics capabilities.
![[peace]](/data/assets/smilies/peace.gif "[peace] [peace]")
Fe
I hear many good things about it. Think I will try to get my hands on a trial version.
I know that could be done in say Matlab by solving the DE and linking it with a real-time plot, but I suspect ADAMS makes this less time consuming. Not to mention its probable 3D multi-dynamics capabilities.
Fe
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