Jeff Cook Paper

[tinu]

http://www.youtube.com/watch?v=C69a28Jpmec (made just now in response to Cook's claim of no back reaction)

Hi TK,

One question: are you absolutely sure that the back movement is not due to the potential energy stored by the big wood board due to its deformation?
(Of course I'm not expecting any violation of Newton 's principle but I'm just curious).

Best regards,
Tinu

[broli]

Nice experiment TK and nice point tinu. Experimentation is not about expecting something but discovering something.

[TinselKoala]

Hi TK,

One question: are you absolutely sure that the back movement is not due to the potential energy stored by the big wood board due to its deformation?
(Of course I'm not expecting any violation of Newton 's principle but I'm just curious).

Best regards,
Tinu

I did take some pains to prevent the gross motion of the board from influencing the recoil, and I illustrated the degree of recoil caused by a gross motion of the board, and it was different both in character and magnitude from the recoil in the experimental situation. However, if the board itself is compressing like a spring my experiment obviously didn't control for that. However, this mechanism for producing recoil is "inelastic", in that softer, more highly damped boards would produce less recoil, right?
I'd be happy to repeat the experiment with a more precise and rigid setup if anybody really thinks it's worthwhile. But I think the onus is on Cook to support his contention that there "isn't" a recoil. Perhaps he's carrying the railgun analogy too far.... but there is recoil in a railgun too, you just have to know where to look for it.
I suppose the test of whether the board compression is doing it would be to use a non-magnetic projectile and a physical constriction in the tube that would require the same pull on the string to pull the projectile past it. This would pull the ring against the board with the same force as in the experimental trial, and so should produce the same recoil contribution from the board. Except that with magnets it is the ring being pulled on, and in the non-magnet case the pull will be on the tube, which then pulls on the ring. Right now my tube is rather loose in the ring; I'll have to firm that connection up before I do another series of tests.

[TinselKoala]

Measure the amount of push (or pull) with a weight measure attached to both ends of the magnet, should be equal, no?

I'm not sure what you mean by this. If the pushes (or pulls) on both ends of a magnet are equal, the magnet won't accelerate, it will just sit there, or continue moving at a constant velocity. Likewise... if a magnet isn't moving, the forces on it are in balance.

There have been lots of studies examining whether or not "repulsion" is stronger than "attraction" using the N and S poles of magnets. If you draw out how field lines behave you will be able to see why some people think there could be a difference.

In the ring magnet - bar magnet case I show, there are several "stable" positions and behaviours, depending on the strengths of the magnets of course, but also sensitively depending on the length of the bar magnet (very short in Cook's case: a disk) and the thickness of the ring magnet, and their relative radii.

[TinselKoala]

Nice experiment TK and nice point tinu. Experimentation is not about expecting something but discovering something.

I'd go even further. It's ok to have expectations.... as long as you are trying to _disprove_ them with your experiment.

My experiment could have had a null result. That is, it could have shown NO recoil. That would have disproved my expectation, and would also have been fairly unambiguous, given the setup, using strong magnets and a bar instead of a disc for the projectile. If I saw no recoil, I'd repeat the test with a lighter, or perhaps mechanically biased, ring magnet, until there was no doubt at all that there was no recoil and my apparatus wasn't just hiding it by its lack of sensitivity.

But simply making an arrangement of parts that then appear to show no recoil, and then using that as "proof" that there isn't recoil .... that is not an experiment, yet I could have easily done that, by using heavier rings, shorter bars, pushing instead of pulling, etc.

The point I'm trying to emphasise is that your experiment must be capable of showing that your expectations are false, for it to be a real experiment. Many "experiments" I see are actually carefully constructed _demonstrations_ that don't even have the potential to "go wrong" and produce a result that does not agree with the experimenter's expectations or pet theory. These demonstrations "prove" nothing at all.