If I were able to set a flashlight out in the middle of space and turned it on, would it move?
Asked by: David Buchwald


You set your flashlight is space to remove friction and all other external forces. In empty space, absolute motion can't be defined without a reference point but we can talk about the CHANGE of motion, or acceleration, of the flashlight.

The flashlight is emitting photons in one direction, and photons, even though they have a REST mass of zero, have momentum equal to hf/c where f is the photon frequency and h and c are constants. Conservation of momentum requires that the momentum of emitted photons in one direction be balanced by the momentum of the flashlight in the opposite direction. Since the flashlight's momentum is mv, where m = its mass, and v = its velocity, the flashlight WILL continue to change its velocity as photons are emitted. That means it will accelerate, just as a rocket accelerates opposite to the momentum of hot gases expelled.

Given the relatively small momentum of photons vs. an average flashlight's mass, however, the resulting acceleration of your flashlight rocket would be very small.
Answered by: Paul Walorski, B.A., Part-time Physics/Astronomy Instructor

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