Conservative and non-conservative forces

It should be clear at this point that potential energy can be a very useful concept, allowing one to solve quite complicated problems with easy. However, not all forces have associated potential energies. For example,

gravity DOES have a potential energy
a spring DOES have a potential energy
friction DOES NOT have a potential energy

We call forces which do have associated potential energies conservative forces. One way you can recognize a conservative force is to take an object on a round trip and see what happens to it.

If the net work done by the force during the round trip is ZERO, then the force is conservative
If the net work done by the force during the round trip is NOT zero, then the force is non-conservative



                         Conservative        Non-conservative
----------------------------------------------------------------------

  Examples                  gravity            friction
                            spring force       air resistance
                            electricity        water resistance


  Work done by
  force during
  a round trip              zero               negative
  (closed path)



  Does it have
  a potential energy?        yes                 no

----------------------------------------------------------------------