# Electrons - Relativistic velocity addition

## Homework Statement

an observer measures the velocity of two electrons, finding one with a speed c/2 in the x direction and the other c/2 in the y direction. what is the relative speed of the two electrons.

## The Attempt at a Solution

Im not sure if this is right it seemed to easy.

but suppose that electron 1 is called b and the other a

then v(b)= √(c2/4+c2/4)
since they are at right angles to each other. is this right?

The velocities are very high, you need to use special relativity here.

phinds
Gold Member

## Homework Statement

an observer measures the velocity of two electrons, finding one with a speed c/2 in the x direction and the other c/2 in the y direction. what is the relative speed of the two electrons.

## The Attempt at a Solution

Im not sure if this is right it seemed to easy.

but suppose that electron 1 is called b and the other a

then v(b)= √(c2/4+c2/4)
since they are at right angles to each other. is this right?

No. That is a non-relativistic velocity-addition and would give you the right answer for a speeding bullet on Earth but NOT for things traveling at relativistic speeds.

The velocities are very high, you need to use special relativity here.

No. That is a non-relativistic velocity-addition and would give you the right answer for a speeding bullet on Earth but NOT for things traveling at relativistic speeds.