Question

Imagine that you push on a large rock. At what point does your effort change the rock’s mechanical energy?

Answer 1

It changes when you have started pushing the rock because at that moment it has started moving

Answer 2

The mechanical energy of a body is the sum of the potential energy and the kinetic energy

`E_m=E_p+E_k`

The potential energy has the following formula

`E_p=m.g.h`

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

`E_k=(1)/(2)m.V^2`

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