Can an object be in mechanical equilibrium when only a single force acts on it?
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No. An object can be in mechanical equilibrium only when there are no forces acting on it, or when the resultant of multiple forces acting on it is zero. When only one force is acting on a object the resultant force is exactly same as the single force acting on it, which causes object to move under the action of the force.
Frequently it appears that an object is in state of mechanical equilibrium even when a single force is acting on it because the object does not move. This happens because when we try to move a heavy object with a force insufficient to overcome the frictional resistance, the force applied results in an equal and opposite force in form of friction. Thus in reality there are two equal and opposite forces acting on the object, rather than only one.
A single force can not bring mehanical equilibrium.Taking the context of the 3rd law of Newton which quite crisp and carefully worded: To every action, there is always an equal and opposite reaction.The 3rd law means by action, the force. So the forces exist in pairs. Moreover, by equiibrium, we mean either the body is at rest or in uniform motion (both translational or rotational) as aresult of no external force or net external force is zero by the first law of motion. Hence No mechanical equilibrium condition is obtained by one single force. Even if the force is insufficient it gets an equal and opposite reaction which contradict the concept of single force. Therefore the mechanical equilibrium is a condtion of the net resultant force being zero. If only two forces are there,they should be equal and opposite to have the equilibrium condition.
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