# Applications of recoil principle in classical physics

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

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: http://en.wikipedia.org/wiki/Teller%E2%80%93Ulam_design).

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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:

http://en.wikipedia.org/wiki/Yarkovsky_effect

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The sudden recoil of a DC motor when it starts rotating. See: http://arxiv.org/abs/0710.2155 It contains analysis of this and other examples.

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