Let's consider a block on a frictionless table. The block is connected to a fixed support on the table via a massless spring. Suppose the block is pulled aside by a distance x and then released. The potential energy of the "block+spring" system at the instant of release is taken to be 1/2 kx2 (provided potential energy is zero at x=0).
I am aware of the following.
If external forces do no work on the system and internal forces are conservative, the mechanical energy of the system remains constant.
The difference in potential energy corresponding to a conservative force is defined as negative of the work done by the force.
Now, the forces internal to the "spring+block" system are:
- Force by the spring on the block
- Force by the block on the spring
Why isn't the potential energy corresponding to the force 2 is not taken into account while computing the potential energy of the entire "system"? Is is that force 2 is nonconservative?
My question may seem wierd. But when the gravitational potential energy of a body+earth system is defined, both works (by the gravitational force of the body and by the gravitational force of the earth) are taken into account. Work due to gravitational force of body is dismissed only because of its small value.