Inclassical mechanics, aparticle is inmechanical equilibrium if thenet force on that particle is zero.[1]: 39 By extension, aphysical system made up of many parts is in mechanical equilibrium if thenet force on each of its individual parts is zero.[1]: 45–46 [2]
In addition to defining mechanical equilibrium in terms of force, there are many alternative definitions for mechanical equilibrium which are all mathematically equivalent.
More generally inconservative systems, equilibrium is established at a point inconfiguration space where thegradient of thepotential energy with respect to thegeneralized coordinates is zero.
If a particle in equilibrium has zero velocity, that particle is instatic equilibrium.[3][4] Since all particles in equilibrium have constant velocity, it is always possible to find aninertial reference frame in which the particle isstationary with respect to the frame.
An important property of systems at mechanical equilibrium is theirstability.
In a function which describes the system's potential energy, the system's equilibria can be determined usingcalculus. A system is in mechanical equilibrium at thecritical points of the function describing the system'spotential energy. These points can be located using the fact that thederivative of the function is zero at these points. To determine whether or not the system is stable or unstable, thesecond derivative test is applied. With denoting the staticequation of motion of a system with a singledegree of freedom the following calculations can be performed:
When considering more than one dimension, it is possible to get different results in different directions, for example stability with respect to displacements in thex-direction but instability in they-direction, a case known as asaddle point. Generally an equilibrium is only referred to as stable if it is stable in all directions.
Sometimes theequilibrium equations – force and moment equilibrium conditions – are insufficient to determine the forces andreactions. Such a situation is described asstatically indeterminate.
Statically indeterminate situations can often be solved by using information from outside the standard equilibrium equations.
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 arock balance sculpture, or a stack of blocks in the game ofJenga, so long as the sculpture or stack of blocks is not in the state ofcollapsing.
Objects in motion can also be in equilibrium. A child sliding down aslide at constant speed would be in mechanical equilibrium, but not in static equilibrium (in the reference frame of the earth or slide).
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.
The minimal number of static equilibria of homogeneous, convex bodies (when resting under gravity on a horizontal surface) is of special interest. In the planar case, the minimal number is 4, while in three dimensions one can build an object with just one stable and one unstable balance point.[5] Such an object is called agömböc.
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