Question

Can someone explain the law of conservation of energy?

Answer 1

The law of conservation of energy is:

-- Energy can't be created or destroyed.

-- Energy can't just appear out of nowhere.  If you suddenly have
more energy, then the 'extra' energy had to come from somewhere.

-- Energy can't just disappear.  If you suddenly have less energy,
then the 'missing' energy had to go somewhere.
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There are also conservation laws for mass and electric charge.
They say exactly the same thing.  Just write 'mass' or 'charge'
in the sentences up above, in place of the word 'energy'.
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And now I can tell you that the conservation laws for energy and mass
are actually one single law ... the conservation of mass/energy.  That's
because we discovered about 100 years ago that mass can convert
into energy, and energy can convert into mass, and it's the total of BOTH
of them that gets conserved (can't be created or destroyed).

How much mass makes how much energy ?

The answer is        E = m c²  .