How do you find the momentum of an electron when it is moving with a speed of 0.75c?

Answer 1

The momentum in special relativity is given by the following formula:

p = gamma*m*c^2

where c is the speed of light, m is the mass of (in this case) the electron and gamma is the so-called gamma factor, which is:

gamma = 1/sqrt(1-(v/c)^2)

where v is the speed of the electron.

Using v = 0.75c we have for gamma:

gamma = 1/sqrt(1-0.75^2)

which is about 1.5.

Thus we have:

p = 1.5*m*c^2.