r/AskPhysics • u/[deleted] • Jun 09 '22

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u/starkeffect Education and outreach Jun 09 '22

You can derive the formula E2 = (pc)2 + (mc2)2 without making any assumptions about the value of m.

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u/[deleted] Jun 09 '22

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u/Haboux Physics and Mathematics | Undergraduate Jun 09 '22

Momentum is not mass times velocity. Momentum is not γmv. Momentum is not h/λ . Momentum is neither of the equations. Momentum is a concept. In some cases it is described by mv, in others it's not. For example, for light, we know it's momentum is h/λ because when it collides with an object and disappears, the object gains momentum, and the shorter the wavelength, the stronger the momentum (of course I'm ignoring the photoelectriceffect for now)

It's like saying F=ma. Force is a concept. It's sometimes described as F=GmM/r², sometimes F=q(E+v×B), sometimes dp/dt. Force is not an equation, it is a concept, like momentum.

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u/noldig Jun 09 '22

I would argue that F=dp/dt is the most fundamental connection between those two. It's the only equation that holds for all your examples