4. Why is energy conserved?

            The energy of a particle is defined as the magnitude of its mass times the square of the magnitude of its velocity. The kinetic energy of a particle is equal to the amount of work required to accelerate a particle from rest to a velocity “v” and it is equal to .

            The sum of the energy of two particles involved in a collision is given before collision by  and after collision by , where the x-axis is taken to pass through the line of centers at impact. Here we see that the energy before collision has the same magnitude as after the collision. Further, the energy of a particle which does not collide is invariant. It thus follows that the energy of any system of isolated particles does not change.

            No process can take place in the universe which violates any of the three laws at any time: mass conservation, linear momentum conservation, and energy conservation.