Question

How do you measure the kinetic energy of a colliding particle if you only know its rest energy? I'm looking at the law of conservation of energy in particle collisions.

Answer 1

Well remember that while energy is conserved, mechanical energy--either potential or kinetic--need not be conserved. Mechanical energy can be converted into or be converted from other types of energy, such as electromagnetic or chemical.

If mechanical energy is conserved in a collision, it's a perfectly elastic collision and you can use conservation of energy to deduce conservation of mechanical energy.

But many (and maybe most) physical interactions are not perfectly elastic and in that case you should you conservation of momentum, which always holds, whether the collision was elastic or not.

Answer 2

Thanks for the reply. Now it makes sense!