In a particle accelerator, you accelerate an electron. Afterwards, you measure it's energy to be 350 keV. Tell my why you can't find the speed from this energy using your knowledge of classical mechanics.

In order to see what the problem is, I will attempt to find this speed. 350keV=5.6110^-14 J E_k=1/2 * mv² =>  v=sqrt(2E_k/m) By plugging in numbers, we find that the velocity of the electron, v=3.5110⁸. This velocity is bigger then the speed of light c=310⁸, and it is a known fact that nothing can travel faster than light. This means that relativistic effects must've taken place, which prevented the particle from reaching the speed of light, but allowed it to have such a high energy.