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Are there any interesting, important or (for the non physicist) astonishing examples where the recoil principle (as special case of conservation of linear momentum) is applied beside rockets and guns?

I am teaching physics in high school and was just wondering if there are any more interesting examples than the two textbook standard examples above.

I am not interested in examples which are not in the domain of macroscopic, classical physics like Mößbauer-effect or something like that.

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A helicopter effectively uses the recoil effect since blowing a column of air downward provides the lift. – FrankH Nov 6 '11 at 1:10
the necessity of tail rotor in chopper is also a nice example... – Vineet Menon Nov 8 '11 at 8:35

4 Answers 4

The most astonishing application for me is ablation pressure: when you shine a gas of high energy X-rays onto a material, the material will eject particles outward off the surface, and the outgoing ejected particles make a reaction pressure on the material, which compresses the remaining stuff. This ablation pressure is critical for getting implosion in the infamous Teller-Ulam design (see here:

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""e a gas of high energy X-rays "" What is that? – Georg Nov 6 '11 at 7:40
@Georg: A collection of thermal X-ray photons hitting a surface. It is best thought of as a gas, because the X-rays are far from a wave limit. It is also best to comment when downvoting, especially when this is for a spurious reason. – Ron Maimon Nov 6 '11 at 9:15
I did not downvote this answer! BTW wiki explains that ablation implosion as a simple vaporisation heated by Röntgen heating. – Georg Nov 6 '11 at 9:26
@Georg: Oh sorry... somebody is following me and downvoting everything. Any surface heating will work, but the x-rays are the ones relevant for Ulam-Teller, since the source is an atomic explosion. – Ron Maimon Nov 6 '11 at 9:38
@Ron: I sometimes suspect somebody's following me around and downvoting too. On the bright side, it means somebody's paying attention! – Mike Dunlavey Nov 6 '11 at 19:23

Super-simple rocket propulsion is easy to do:

  • stand on a red wagon and throw weights off it in one direction

  • put a garden hose with a nozzle against a scale and turn it on. It will generate thrust.

  • one I've wanted to try but haven't had the nerve: sit on a bicycle and use a hand-held leaf blower as propulsion.

  • you can buy rockets that consist of a water bottle and an air pump. The hard part is finding them afterward.

  • one of my favorites (though not really a rocket) is an Astro-Blaster. They're easy to make. Just stack up rubber balls in weight ratios 1/3, 2/4, 3/5, ..., as many as you want. If you stack up N of them and drop them (carefully, in a line) they will all stay on the surface, except the top one, which will bounce to $N^2$ the height you dropped them from (ideally). It really gets attention.

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Yarkovsky effect (almost like a photon rocket, I think).

Non-uniform thermal radiation (because 'day' is hotter than 'night') of a celestial object can give a net recoil. More spoiler:

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The sudden recoil of a DC motor when it starts rotating. See: It contains analysis of this and other examples.

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