# Relativity mass

1. Feb 26, 2012

### uebmeister

Hello, i have problem with an homework about relativity. i wonder if anyone here could help me solving this assignment below or help me to find any good formulas?

Two particles each with the mass m are moving with the speed v toward each other. They collide and coalesce to one bigger particle. The new mass of the particle is now 3,0m. Determine v.

Grateful for respons :)

2. Feb 26, 2012

### Staff: Mentor

Hi uebmeister, welcome to PF!

Have you learned about four-vectors, specifically the four-momentum? If not, what formulas have you learned that contain relativistic energy mass and velocity?

We aren't allowed to do the homework problems for you, but we can point you in the right direction. I just need to know what you have been taught so far.

3. Feb 26, 2012

### uebmeister

We have learnd about the four vectors for a time ago, but our teacher gave us this assignment and said that we were suppose to do this task by deriving formulas and thereby solve m. I have been sick for a while and therefore m i totally new for this so i donĀ“t even know where i m suppose to start. So do you have any clue?

The formulas i know are:

relativistic mass

The energy of moving

Momentum

Movement energy

I have the possibility to search for formulas in a formulary

4. Feb 26, 2012

### Staff: Mentor

Since the particles are each moving at the same velocity and their rest mass is the same you know that half of the total mass comes from each. So each particle has a relativistic mass of 1.5 m. So the easiest way is to solve the relativistic mass equation for v.

However, I dislike relativistic mass as a general concept, so the way I would solve it is to use the norm of the four-momentum:
$$mc=|(1.5 m c, 1.5 m v, 0, 0)|$$
and again solve for v.

Either way will work.

5. Feb 26, 2012