Law of conservation of energy? Please help.?

Question

Why does the law of conservation of energy must also include changes in mass?

Answer 1

Because matter is also energy.  You can calculate the amount of energy in a lump of matter using Einstein's famous E=mc2.  C is the speed of light and m is mass.

This is important in the nuclear reactions of fusion and fission.  These reactions provide energy by converting some of the mass of the nuclei of the atoms involved into energy.

In non-nuclear reactions, and indeed in the majority of situations, we do not need to look for mass into energy conversions.  For example with two physics trolleys colliding, there is no need to take this into account.  The only mass loss we then need to be aware of is loss of bits of the trolleys (if the break off), which will affect the system.