Potential energy

Potential energy is stored in how a system is arranged, where its parts sit relative to each other, and it is always measured from a chosen zero: U = mgh near the ground, or U = -GMm/r for two masses with the zero placed at infinity. With that choice a bound system has negative potential energy, and only the change from one place to another is physical; the bare number depends on where you set the zero.

Potential energy invites three errors. The first treats it as an absolute number and drops the minus sign on U = -GMm/r, when the value depends on the chosen zero and a bound system is negative. The second misreads the force from the energy graph: the force is the negative slope, F = -dU/dx, so it points downhill toward lower U and vanishes where the slope is flat, not where U is zero (in more than one dimension it points the way U falls fastest, the direction of steepest decrease). The third hands a potential energy to every force, when only conservative forces like gravity and ideal springs have one and friction and drag do not.