Take two quarters and lay them on a table. Press down on one quarter so it cannot move. Then, starting at the 12:00 position, roll the other quarter along the edge of the stationary quarter, as the drawing suggests. How many revolutions does the rolling quarter make when it travels once around the circumference of the stationary quarter? Surprisingly, the answer is not one revolution.

Conservative energy - The work it does is stored in the form of energy that can be released at a later time - Gravity - Spring force Non conservative energy - Work done cannot be recovered later as kinetic energy; instead, it is converted to other forms of energy - Friction - Tension in a rope Conservative force - A force that does zero total work on any closed path - Work done by friction on a closed path is not zero o = ∙ = - Work done by gravity on a closed path is zero o = ∙ = - The force is conservative if the work done by a force in going from an arbitrary point A to an arbitrary point B is inde