# Homework Help: Relativity : relativistic energy

1. Jan 27, 2007

### Delzac

1. The problem statement, all variables and given/known data
Find the speed of a particle whose total energy is 3 times its rest energy.

2. Relevant equations
$$KE = \gamma mc^2 - mc^2$$

3. The attempt at a solution
i let total energy = 3mc^2 and then :

$$\gamma mc^2 = KE + mc^2$$
$$3mc^2 = KE + mc^2$$
$$v = \frac{\sqrt{3}}{2} c$$

Is this correct? or should i let $$\gamma mc^2 = 3mc^2$$ and work it out immediately?

Any help will be appreciated.

2. Jan 27, 2007

### chanvincent

$$3mc^2 = KE + mc^2$$ is correct, but I dont see how is this connected to $$v = \frac{\sqrt{3}}{2} c$$, So I can't point out which part you did it wrong...
yes

3. Jan 27, 2007

### Meir Achuz

Use 1/sqrt{1-v^2/c^2}=3,and solve for v.

4. Jan 28, 2007

### Delzac

yeah, got it thanks, English problem. bah. :P