What is the central idea for stacking rocks vertically to test the effects of forces

You can see how forces can stack on one another by comparing the normal force of the bottom most rock to the normal force of the upper most rock.

Explanation:

Say you have 3 rocks all with mass m=3kg

These 3 rocks will all have the gravitational force of -29.4N (multiply 3kg by -9.8m/s^2)

If none of the rocks are stacked, then they all have the same normal force as well (29.4N)

However, stack them, and see how their normal forces change.

m1 will continue to have a normal force of 29.4N

m2 will now have a normal force of 58.8

m3 will now have a normal force of 88.2

Though they all retain the same gravitational force, their normal force changes depending on the forces acting down on them.

Mass 1 have no forces acting downwards other than gravity.

Mass 2 has the force of mass 1 and gravity acting down on it.

Mass 3 has the force of mass 1, mass 2, and gravity acting down on it.

an energy transformation flow diagram is shown

where is it ?