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This question already has an answer here:

Third law of motion - "For every action there is an equal and opposite reaction"

I was considering the situation, where I may be motionless in space with only a flashlight and no forces acting on me. I could throw the flashlight, and begin my very slow drift in whatever direction I please, but I was wondering, if I direct the flashlight in a certain direction and turn it on, would that also push me until the batteries ran out, even if it's a tiny amount? What if I were generating directed radio waves or similar?

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marked as duplicate by John Rennie newtonian-mechanics Nov 5 '15 at 17:43

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  • $\begingroup$ Yes, conservation of momentum works. This is a duplicate, but hard to look up on my phone... $\endgroup$ – Jon Custer Nov 5 '15 at 17:05
  • $\begingroup$ Related, but not duplicates: physics.stackexchange.com/q/57442, physics.stackexchange.com/q/2229 $\endgroup$ – DilithiumMatrix Nov 5 '15 at 17:09
  • $\begingroup$ @JonCuster while I do expect this to be a duplicate (I can't find one from a quick search, however), if you can't find the duplicate - then perhaps such a comment is not constructive. $\endgroup$ – DilithiumMatrix Nov 5 '15 at 17:11
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Yes. It is much easier to think of this in terms of conservation of momentum: Because light (and electromagnetic radiation in general) has momentum, you will have to gain momentum in the opposite direction to conserve total momentum --- just like if you were to throw the flashlight.

It is difficult to think of this in terms of forces because we tend to think of such dynamics problems in terms of 'forces of acceleration' --- which is ill-defined for photons.

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