Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Duncan Amos on 11/10/2010 09:30:04
-
Duncan Amos asked the Naked Scientists:
Hi
Would a "Newton's Cradle" perform correctly if the balls were spherical magnets held apart by magnetic repulsion? i.e. view from the front - a five-ball cradle would have the spheres lined up as N-S, S-N, N-S, S-N, N-S and the pivot points of the arcs of swing are aligned above the neutral points so that everything is at rest. It might be that the falling ball would need to be outside the set in the rest state as it's removal might influence the rest state of the others, or not?
Hoping you can help calm an old man's puzzled brain!
Duncan Amos
What do you think?
-
A Newtons cradle works because when the falling ball strikes the first ball it compresses it and a wave of compression passes thru it and cause's the second ball to move and so on until it reaches the end and the process is repeated in the reverse order.
If the balls were magnetised the falling ball might strike the first ball if it was moving fast enough but would not impart sufficient energy to it to keep up the transmission of energy and the movements would quickly die out.
The situation could be simulated using a electrical delay line with the L C elements seperated by Zenner diodes which would emulate the repulsive forces.
-
Not wishing to appear ungrateful for your reply, I can draw a solid block of aluminium with a spherical hole in the middle in SketchUp but it doesn't mean that it can be made and I've seen PSpice simulations that say two transformers in parallel won't work - so I'm not always a believer in simulations...
If I lay a bar magnet on a surface and move another bar magnet towards it in 'repulsion mode' then the distance between the two magnets remains constant, once the first begins to move, as long as the sutface friction remains constant. I can see that there could be a 'squish effect' (and, presumably a rebound effect) but it still seems logical to me that the 'magnetic energy' would form part of the total kinetic energy in this instance and would therefor be part of the total of the force acting upon the bodies at rest.
I realise that as a 'lay person' I am restricted to a simplistic view as a result of lack of science background, but in your reply I can't see 'where the magnetism has gone'...
Has anybody actually tried the experiment?
-
I think the balls would be set in motion swinging back and forth in unison, the whole point of a Newtons cradle is that the balls must be in contact for the efficient transmission of energy.
PS I can think of many ways to engineer an aluminium block containing a spherical hole.
I see no need to construct a model any skilled programmer could produce a computer simulation, someone may well have done it already
-
There could be another problem. If it's constructed in the usual fashion, I would not be surprised if one of magnets does a 180 degree flip and tangles up the strings.
Might look more like Newton's underwear than his cradle.
-
Here is a link to some interesting simulations but no magneized Newtons cradle as yet.
Goto it programmers
www.upscale.utoronto.ca/PVB/Harrison/Flash
-
You could try putting pieces of sponge rubber between the balls that would have much the same effect.
-
Some interesting replies so far - thanks Guys (honestly, it's not a test!)
So are we saying that because it would probably only work with steel balls in direct contact, Newton's laws only apply to steel balls in this example because they can pass the shock wave without dispersion (as would not be the case with foam)? I'm happy to be corrected but I don't think that magnetism would equate to foam rubber - although I follow the thinking - and, to me, the magnetic component of a magnetic ball must figure somewhere within its total energy...
My 'Spherical hole in a block of Alluminium" was actually a simplification of the situation I was presented with by someone with a doctorate - he actually wanted a fully-populated printed circuit board in the centre of a solid block and also didn't want any connections passing through either so how the hell it was supposed to work was aparently 'my problem'... I am intrigued as to how a spherical space in the centre of a solid block could be created (we only had CNC milling) - please elaborate (maybe as a new topic?)...
"Newton's Nappy shows Newton's Motions - now there's a title for a Royal Institution Christmas Lecture!!!
-
I think Syhprum's foam rubber suggestion is a good idea.
Let's consider a really simple Newton's cradle which only has two spheres. What happens is that when the first sphere whacks the second sphere, the first sphere stops dead, and all (well, nearly all) its kinetic energy is transferred to the second sphere.
If you allow them to repel each other with a magnetic field, the first sphere will decelerate while the second sphere accelerates, so only some of the kinetic energy of the first sphere will transfer to the second. The first sphere will keep moving, so both the spheres will be moving at the same time.
You'll see the same effect if you put a compression spring between the spheres, or, a piece of foam rubber.
-
Ok, I'll buy that.
If we think of a two ball cradle, with the second ball fixed, then the first ball would be deccelerating (exponentiall?) as it approached the opposing magnetic field - is that correct? - and presumably would accelerate at the same rate if the poles of the fixed ball were reversed to give attraction? If the clamped ball was released at the exact moment of contact then how far would the two balls swing? Would the kinetic energy within the combined pair be equal to the kinetic energy of the lone ball at the moment of impact or what?
This has raised another thought in my mind - if one is over the North magnetic pole of the earth and drop a bar magnet north pole downwards, at what point in its fall will it flip - is it fixed or dependent upon the velovity of the falling magnet? - e.g. if dropped from a great height it would be moving faster than if from shoulder height so would, flip closer to the earth's surface?
Oh, the burden of an enquiring mind on others...
-
Ok, I'll buy that.
If we think of a two ball cradle, with the second ball fixed, then the first ball would be deccelerating (exponentiall?) as it approached the opposing magnetic field - is that correct? - and presumably would accelerate at the same rate if the poles of the fixed ball were reversed to give attraction? If the clamped ball was released at the exact moment of contact then how far would the two balls swing? Would the kinetic energy within the combined pair be equal to the kinetic energy of the lone ball at the moment of impact or what?
This has raised another thought in my mind - if one is over the North magnetic pole of the earth and drop a bar magnet north pole downwards, at what point in its fall will it flip - is it fixed or dependent upon the velovity of the falling magnet? - e.g. if dropped from a great height it would be moving faster than if from shoulder height so would, flip closer to the earth's surface?
Oh, the burden of an enquiring mind on others...
Tell you what - you do the experiments and let us know how it goes. I'll expect a comprehensive write-up on my desk first thing Monday morning.
-
Could avoid the flipping problem by replacing balls with ring magnets ...
[ Invalid Attachment ]
and replace straight pole with "U" shaped thing ...
[ Invalid Attachment ]
Here is a link to some interesting simulations but no magneized Newtons cradle as yet.
Goto it programmers
Possibly relevant animation here ... http://www.falstad.com/coupled/ (needs Java to run)
-
But wouldn't we have to rename it "Newton's woody?"
-
But wouldn't we have to rename it "Newton's woody?"
"abentabacus (http://en.wikipedia.org/wiki/Abacus)" more like.
-
Why would there be a difference between electromagnetic repulsion caused by magnets and electromagnetic repulsion caused by trying to shove lots of electrons into the same space? Or, to look at it the other way say we try the experiment with sponge rubber, then replace the rubber with steel coil springs, then replace the coil springs with bits of steel then round off those bits and smooth them into the balls. Haven't we got back to an ordinary cradle?
I don't see why it would matter if you used magnets or not (except that they might spin round and stick to eachother).
-
I don't see why it would matter if you used magnets or not (except that they might spin round and stick to eachother).
I believe it's because of the rate of transfer of energy. With springs (or magnets) some of the energy will be stored in the spring during the transfer process, so the "receiver" will start to accelerate while the "transmitter" is still decelerating. Consequently, the transmitter and receiver will both be moving at the same time, and that's not what a Newton's cradle is supposed to do.
EDIT: Of course, to some extent the same thing happens with "inelastic" steel balls, but it all happens so quickly, it's not very apparent.
-
it is a bit of a battle to get the "falstead" simulation to work but when you do you see all the balls moving in unison just as you would expect.
-
I can report that a magnetic Newton's Cradle works just fine [:)] -- exactly like the usual variety, but the swinging magnets never contact each other. Magnetic repulsion keeps them about 1-1/2 inches apart. I like to hold the two outside magnets up and drop them simultaneously, [;)]↔as the wild action is very entertaining. Note that this device uses stiff wires to suspend flat, plastic-encased magnets by two hinge points -- somewhat like a series of playground swings. There is no way for the swinging weights to tangle.
My magnetic Newton's Cradle was bought from the online MIT Museum Store, which does not seem to be in business at this time. [:(] [:(] I may upload a picture of it later.
John Blinke
Rochester, Michigan, USA
Duncan Amos asked the Naked Scientists:
Hi
Would a "Newton's Cradle" perform correctly if the balls were spherical magnets held apart by magnetic repulsion? i.e. view from the front - a five-ball cradle would have the spheres lined up as N-S, S-N, N-S, S-N, N-S and the pivot points of the arcs of swing are aligned above the neutral points so that everything is at rest. It might be that the falling ball would need to be outside the set in the rest state as it's removal might influence the rest state of the others, or not?
Hoping you can help calm an old man's puzzled brain!
Duncan Amos
What do you think?
-
Good Post - John. Nice to see a bit of empirical data on this one - a photo or a mobile phone video would be even better.
Isn't that a gold star to the OP and BC, and "needs to try harder" for the rest of the class.
-
Thank you John...
As the original poster (is that the OP referred to above?) I'm delighted to actually have an answer rather than a mountain of theory (which you seem to have disproved anyway!!!). Hats off to the guys at MIT for creating the test apparatus too (they seem to have a lot of 'sensible science' going on there).
I posed another question somewhere on this board about (essentially) "Where do photons go when they die and that hasn't produced a single reply whatsoever...
Funny things photons, from what I can make out... If e=mc², and a photon is a massless particle, then lasers simply cannot work as e equates to zero - which obviously explains my failed DVD player!!!
Thanks again
Duncan
P.S.
Don't let me get started on the practical aspects (or otherwise) of the Higgs Boson
-
I would like to see a series of experiments adjusting the strength of the magnets, the mass of the balls the length of the suspension and their spacing then we could get back to theory.
I think there is an Ignobel prize in this.
-
Duncan - you're right OP=Original Post/Poster
BTW
E2 = p2c2 + m2c4
also
E=hν
maybe try taking the food/dust/lego out of the dvd player - that's normally what screws mine up
General Question - do we have an h bar we can use (God the puns are gonna flow)? Also is there a native latex implementation (What have I said?!?!) other than pasting gif/png?
-
Ewoooo! We might be able to make billiard balls silent by embedding magnets in their outer surface.
-
Duncan - you're right OP=Original Post/Poster
General Question - do we have an h bar we can use?
We have a lot of fubars, but I haven't seen an h bar.
-
Interesting "toy".
Theoretically, it should transfer the same amount of energy. But, I'm going with energy dissipation.
You might need to attach with hinged wire to keep from twisting, but you could probably do it. Magnets of all shapes are available on E-Bay. Cylinder magnets might be as effective as spheres.
Anyway...
If you had 2 balls. Then Ball 1 might impart its energy to ball 2 with both the magnetic repulsion and the impact, although the question might be whether it would tend to be kicked back, or would stop "dead".
When you have 5 balls though, you would end up with gaps between all the balls.
Some of the energy would likely be transmitted into vibration/oscillations between the balls, rather than being transmitted from 1 to 2, then pushed all the way to the last ball. Also, think of when ball 1 strikes ball 2. It will then kick back a distance, and again likely oscillate.
Anything that could dissipate energy would be bad.
The question then would be what the natural oscillation frequency would be. Your attachment would have a pendulum effect, and would be essentially of a fixed frequency, at least for the first and last ball. So, your trick would be to set all of your frequencies to amplify rather than cancel.