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Will a Newtons Cradle work with rings instead of balls?


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#21 Silver_rose

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Posted 31 October 2014 - 05:18 PM

I'm quite sure with rings you will have a consistent loss of momentum


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#22 Mister Sympa

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Posted 01 November 2014 - 10:52 AM

It's trying to work.

 

And I'm fairly certain that with anything, there will be a consistant loss of momentum.

 


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#23 SpleenBeGone

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Posted 02 November 2014 - 04:57 PM

That's pretty much how things work, yes.

 

I'm hoping it will work with the aluminum though. I might need to find some pretty hard steel, which gets a lot harder to work with. 


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#24 No-Danico

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Posted 02 November 2014 - 11:22 PM

Umm, ball bearings. That's my suggestion. Use them somehow. I dunno.

 

I was going to say witchcraft, but I'm pretty sure that can get you burned at the stake in the south-west.


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#25 Silver_rose

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Posted 03 November 2014 - 09:12 AM

And I'm fairly certain that with anything, there will be a consistant loss of momentum.


That's pretty much how things work, yes.

I'm hoping it will work with the aluminum though. I might need to find some pretty hard steel, which gets a lot harder to work with.


Um no, that's only with a closed system... such as when you play pool and there's no force applied, other than the one you apply with the cue.

A newton's cradle has an external force applied to the system, gravity, making it an open system.

Look here's why the rings wont work, and especially with harder metals.
A Newton's cradle is a physical representation of conservation of energy and momentum. The balls require a material with a certain level of elasticity, so that the transfer of energy is easily facilitated amongst the seemingly still balls. They also must be balls because the kinetic energy needs to be transferred evenly, this doesn't occur with rings as efficiently, mostly because there's a hole in the middle, probably also because they're cylindrical.

Here's basically how it works - Ball one is lifted in the air, creating potential energy, as ball one is dropped this is converted into kinetic energy. Ball two accepts the kinetic energy transfer being indented slightly (by a few microns) and as transferring the energy, the ball moves back to shape. Ball two also accepts the vector quantity of the momentum in the direction in which ball one moved. This happens until ball n accepts the energy and momentum and because there is no other ball to pass it on to, it moves outwardly reaching the high point of the arc, creating potential energy again. The outside force, gravity, then acts upon it, converting it to kinetic energy again and the momentum vector points in the opposite direction.

It wont work with harder material and it wont work well with rings, there is consistent loss of energy and therefor momentum.

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#26 SpleenBeGone

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Posted 03 November 2014 - 09:51 AM

I think you're misinformed with some things. 

 

First of all, there is a constant loss of momentum because it is an open system, outside forces are constantly acting on it. It's going to lose momentum due to friction in the bearings and air resistance, as well as gravity to a lesser extent. 

 

More importantly, hard metals will actually work better for the transfer of energy. There's a reason that all of the current cradles are made from super hard chrome coated steel ball bearings or glass,  and not plastic or rubber. You want as little deformation as possible. There was a pretty good demonstration with this on mythbusters.(Relevant link) When they scaled up, they couldn't find steel that was hard enough not to just lose all the energy through vibrations and material deformation. This actually brings up the main point of why the rings may not work. They're simply going to vibrate a lot more, no matter what I make them out of, and that's a loss of kinetic energy. 


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#27 Silver_rose

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Posted 03 November 2014 - 10:56 AM

http://science.howst...tons-cradle.htm

 

You're right, I am a little misinformed. I only did like 15 minutes of research on this and I've done my physics studies in reverse. I'm doing Newtonian physics next semester.

 

But when I said elasticity, it relates to a metals ability to reform with as little loss of energy as possible.

 

Also I can't find any evidence which states that the harder metal is, the better it transfers energy... So if you could show me some that would be great.

 

You're right, there is a loss of momentum, at the moment when the outer ball moves outward and up to the point of potential energy, however the momentum returns when gravity is acted upon it and the vector is reversed. The point in which momentum is conserved is in the moment of the non-moving balls of energy and momentum transfer, and is also where it simulates a closed system.

Momentum = mass x velocity, where velocity is 9.8m/s (gravity)
 

With the rings it's not because they vibrate more, it's because they don't transfer the energy as well at the balls.
When the ball hits another ball, it vibrates, as everything vibrates, but because it's nature as a ball the energy is spread evenly over the ball and then concentrated to a point. This doesn't apply to a ring, it can vibrate evenly on the y-plane and concentrate to a point, but it can't on the x or the z plane and because the energy isn't transferred, momentum isn't transferred.


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#28 SpleenBeGone

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Posted 03 November 2014 - 01:14 PM

I misunderstood your use of elasticity, I was thinking in a non-physics way.

Your use of it is actually what makes harder metals transfer energy better though. (TIL) Here's a really long explanation, about half way down they talk about elasticity. https://www.lhup.edu...ario/cradle.htm

 

Ok, so coming up is a lot of stuff that I knew the basic principles of in practice, but I spent the last hour or more researching to give you a proper answer in this post. Everything might not be exactly right, but it should be enough to point you in the right direction should anything be wrong. 

 

Elasticity is measured on a stress:strain curve. The harder something is, the more it resists change in shape and the quicker it will return to it's original shape. (Baring a permanent change it shape due to the force.) There's a lot of information on google about how steel is more elastic than rubber because it resists change better. I can only assume it was a homework question with the amount of information in that specific scenario. >.>

Hook's law goes into some pretty detailed math about this that is honestly over my head on a scholarly level. http://en.wikipedia....iki/Hooke's_law

On a practical level however, a good example is that spring steel is very hard, Somewhere around 63 Rockwell, compared to say a good hammer at maybe 50-55 Rockwell. 

 

In an idea situation (that sadly can't exist) a perfectly elastic material would be 100% resistant to change. It wouldn't deform, it wouldn't vibrate because that's energy loss in movement, expressed by sound waves. You'd get what Captain America's shield is made out of. If you had something that hard, when a force is applied to it, there would be no way for the material to shed the energy itself and it would have to transfer 100% of the energy to something else. 


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#29 Mister Sympa

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Posted 04 November 2014 - 04:45 PM

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Arguing physics like gentlemen. I approve.


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#30 SpleenBeGone

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Posted 06 November 2014 - 06:17 PM

Got my taps in, penny for scale.

Time to do some science.

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