<P> Sometimes there is not enough information about the forces acting on a body to determine if it is in equilibrium or not . This makes it a statically indeterminate system . </P> <P> A stationary object (or set of objects) is in "static equilibrium," which is a special case of mechanical equilibrium . A paperweight on a desk is an example of static equilibrium . Other examples include a rock balance sculpture, or a stack of blocks in the game of Jenga, so long as the sculpture or stack of blocks is not in the state of collapsing . </P> <P> Objects in motion can also be in equilibrium . A child sliding down a slide at constant speed would be in mechanical equilibrium, but not in static equilibrium (in the reference frame of the earth or slide). </P> <P> Another example of mechanical equilibrium is a person pressing a spring to a defined point . He or she can push it to an arbitrary point and hold it there, at which point the compressive load and the spring reaction are equal . In this state the system is in mechanical equilibrium . When the compressive force is removed the spring returns to its original state . </P>

Minima of potential energy and points of stable equilibrium