### E=mc2

Probably the most famous equation in physics, Albert Einstein’s equation shows that energy and mass are just different forms of the same thing.

#### Amazing But True

- Mass-energy equivalence is the physics concept that the mass of an object is the measure of its energy content. Another way of saying this is that there exists a constant, proportional relationship between the mass in a system and the energy in a system. This relationship is given by

\begin{align*}E=mc^2\end{align*}

where \begin{align*}E\end{align*} is the energy, \begin{align*}m\end{align*} is the mass and \begin{align*}c\end{align*} \begin{align*}(3\times 10^8 m/s)\end{align*} is the speed of light.

- The above equation is also known as the rest energy and \begin{align*}m\end{align*}, sometimes written as \begin{align*}m_0\end{align*}, is known as the rest mass. When an object has momentum, the above equation is still a really good approximation until the speed approaches a fraction of the speed of light. When an object is moving with a velocity that is at a fraction of the speed of light, the energy-mass equivalence equation must be rewritten to include the momentum

\begin{align*}E=(m_0c^2)^2 + (pc)^2\end{align*}

where \begin{align*}m_0\end{align*} is the rest mass and \begin{align*}p\end{align*} is the momentum.

- View One Form of the Derivation of The Mass-Energy Equation at the video below:

http://www.youtube.com/watch?v=hW7DW9NIO9M

- See How The Full Equation Comes Into Being at the video below:

http://www.youtube.com/watch?v=NnMIhxWRGNw

#### Show What You Know

Using the information provided above, answer the following questions.

- If a classmate tells you that a photon breaks the mass energy equation, how you would explain to them that they were wrong?
- How is it that no matter how hard you try, any object with mass will never reach the speed of light \begin{align*}c\end{align*}?
- Are there any cases where light can travel faster than \begin{align*}c\end{align*}?