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Since light beams carry momentum, why would a person holding a flashlight not feel a recoil similar to that of a rifle being fired?

David Z
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yankeejman1042
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1 Answers1

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One can do a simple calculation. Suppose you hold a $60\text{W}$ lightbulb and you turn it on. Each second it emits $60 \text{J}$ of energy. Using the energy-momentum relation for photons, $E=pc$, we get a momentum of $p = 60\text{J}/c = 2\cdot 10^{-6}\ \text{kg}\cdot\text{m}/\text{s}$, or a force $F = 2\cdot 10^{-6}\ \text{N}$. In contrast, a Wikipedia page about firearms says that each bullet of a .44 Remington Magnum comes out with a momentum of roughly $5\ \text{kg}\cdot\text{m}/\text{s}$. That's quite a bit more than the flashlight.

(I should make a disclaimer that I know nothing about guns, I just pulled the numbers from Wikipedia. When we have a difference of six orders of magnitude, you don't really care about these things.)

Javier
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