The subscript after the element indicates the number of atoms of that element in the molecule. So, in H20, the subscript after the H, which stands for hydrogen, is 2. This means that there are 2 hydrogen atoms in a water molecule.
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The book that weighs less
By the work energy theorem, the total work done on the stone is given by its change in kinetic energy,

We have


Then the total work is

The law of conservation of energy states that in any reaction, energy cannot be created or destroyed.
In other words, energy has to be conserved in every reaction.