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Question on momentum in SR.

I'm trying to understand this holistically...

From what I understand... p=mv is an approximation.

when we look at the energy equation for a photon E = pc.

Since a photo is massless, this momentum p is an intrinsic feature of a particle apart from its mass...

So...

1. Why is it that we can use the mass of electrons/chairs/planets to calculate their momentum? Is there an a prior reason that the two are equivalent?

2. How does this momentum in SR relate to the operator momentum in QM?

Thank you!

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# Exactly what is momentum?

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