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> 5. Point out that PE involves an interaction between two objects:
ball and earth for gravity, or two masses for a diatomic spring. But
the PE doesn't belong to either. It belongs to the system of two
objects (plus their field, if you have introduced that concept
already in class).
1) You are correct. But most textbooks say that an elevated
object "has a potential energy =m*g*h." Can this be
tolerated in the very first encounter with PE?
John D already spoke to part of this, namely the idea that it is only
an approximation good for small h (compared to earth's radius).
The other part is the question of which object "owns" the energy.
It is okay to say one object owns it entirely provided the other
object is so massive that it doesn't change in KE. In that case, the
dynamics of this massive object are of no real interest.
So PEg = mgh belongs to the ball when the other object is the earth.
PEs = kx^2/2 belongs to the mass when the other end of the spring is
anchored to earth.