# B Speed,mass,energy and a question.

1. Feb 20, 2017

### shihab-kol

Suppose instead of moving at the speed of light , someone moves at a speed of 2*10^8 m/s .Then will his mas and energy exist simultaneously ? I mean mostly energy and some mass ?

2. Feb 20, 2017

### Drakkith

Staff Emeritus
At that velocity the person's mass will remain unchanged but their kinetic energy will be immense.

3. Feb 20, 2017

### Janus

Staff Emeritus
The way your question is phrased leads me to wonder if you are laboring under a misconception; one I have come across before.

Are you under the impression that the equation E=mc2 means that a mass moving at the speed of light has "turned into" energy. If so, that is not the meaning of this equation. While the equation contains a speed "c", this does not imply any movement on the part of the mass. The c2 part of the equation just acts as a conversion factor. The equation gives the energy equivalent of M, or the energy you would get if the mass were converted to energy (such as in the combination of two antiparticles with a combined mass of m).

In fact, this particular equation only works when you assume that mass m is at rest with respect to you. If it is in motion at velocity v, then you use
$$E = \frac{mc^2}{\sqrt{1-\frac{v^2}{c^2}}}$$

Which, if you subtract mc^2 from it, will give you the kinetic energy of of mass m at velocity v.

As Drakkith has already pointed out, the mass of m will not change as v increases. However E will approach infinity as v approaches c.