Answer:
The mechanical energy of a body is the sum of the potential energy and the kinetic energy
[tex]E_m=E_p+E_k[/tex]
The potential energy has the following formula
[tex]E_p=m.g.h[/tex]
Gravity is a constant as is the mass, the only thing that makes the potential nerve change is the height of our reference system zero
As the rock is in the ground, it is at zero level, therefore it has no height. Its potential energy is zero
kinetic energy has the following formula
[tex]E_k=\frac{1}{2}m.V^2[/tex]
Mass is a constant value so the kinetic energy changes when we vary its speed
The moment we push the rock, it acquires a nonzero speed and we have a certain amount of kinetic energy
As we know, mechanical energy is the sum of these two energies and as we have seen, its potential energy is always zero, at the moment when the rock acquires speed (therefore it has kinetic energy) at that point our effort to push the rock change the mechanical energy
