# Homework Help: Total energy, mass, and momentum.

1. May 30, 2005

### Jchem

An electron with speed 0.8c, has a rest energy of .51MeV.

What are its mass, its momentum, and its enegy (including rest energy)?

----------------------------------------------------------------------

Ok, isnt it possible to get the three answers without using the .5MeV ?

I have 3 formulas for mass, momentum and total energy that dont use the .5Mev

but then I feel like I might be doing something wrong since they gave me the value.

Im not sure how to post the forumlas, but they are on this page

http://galileo.phys.virginia.edu/classes/252/energy_p_reln.html

thanks for any help

2. May 30, 2005

### vsage

The rest energy is the $$m_0c^2$$ part of the formulas found on that page. Also, hyperphysics ( http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html ) is the #1 most helpful site I have found when I need to reference something. You will probably understand the formulas a little better if you look at the sections dedicated to relativistic energy there.

Last edited by a moderator: May 30, 2005
3. May 30, 2005

### OlderDan

The only way to avoid using the given information is if you know the rest mass of the electron. Look at the E equation in the limit of zero velocity. What is it telling you?

OOPs too late again. It's still the right question, and now you have the answer

4. May 30, 2005

### dextercioby

You don't need that 0.51 MeV.Just that

m_{el}~9.1*10^{-31}Kg.

Daniel.