# Sum of the energies of the two photons

1. Jun 13, 2008

1. The problem statement, all variables and given/known data
A positron has the same mass as an electron but is of opposite sign (9e-31 kg).
What is the sum of the energies of the two photons which are produced when the positron and the electron annihilate?
Assume the speed of the positron is .93c (c is the speed of light) with an electron at the same speed.

2. Relevant equations

I truly don't know of any equations except e- + e+ => $$\gamma$$ + $$\gamma$$

$$c\ =\ 2.99792458\ \times\ 10^{8}\ m\ s^{-1}$$

3. The attempt at a solution
I squared the velocity .93c^2 and multiplied it by the mass (9e-31 kg) to find Joules (since the answer must be in Joules).
I'm don't know what else one could do.

Even a hint as to which equations to use would be great.

Last edited: Jun 13, 2008
2. Jun 13, 2008

### Reshma

Use the law of energy conservation. The minimum energy an electron can have is its rest energy i. e. $m_ec^2$. The total energy is the sum of the rest energy and the Kinetic energy. $E = m_ec^2 + K$. You have been given the speed of the electron and the positron. Can you calculate their total energy?