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| Equilibrium of Rigid Bodies |
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| A rigid body is said to be in equilibrium if, both the linear and angular momentum of a rigid body have a constant value. For the equilibrium of a rigid body, the body need not be at rest. However, if it is at rest, it is called the static equilibrium. |
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| Conditions for the rigid body to be at equilibrium |
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 The vector sum of all the external forces acting on the rigid body must be zero. |
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 The vector sum of all the external torque acting on the rigid body must be zero. |
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| The figure shows dishes kept at different orientation, on each of which a marble is placed. If the marble in figure (a) is displaced slightly, it would rattle about for a while and come to rest in its original place. |
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| This is an example of stable equilibrium. A small disturbance to a system in such a state does not produce any drastic change and the system eventually returns to its original configuration. On the other hand, if the delicately poised marble in figure (b) is displaced even slightly it would roll over and not get back. This is an example of unstable equilibrium. |
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| The marble in figure (c) will roll over only if given a push. This is an example of metastable equilibrium. The marble on the flat plate is equally comfortable wherever it is and is said to be in neutral equilibrium. |
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