Equilibrium

When two or more than two forces act on a body in such a way that there is no change its translational or rotational motion then the body is said to be in equilibrium. OR An object is said to be in equilibrium if it has zero acceleration.

Kinds of Equilibrium 
1-Static Equilibrium

Everybody is in static equilibrium if all the forces acting on it cancel the effect of each other i.e. when there is no unbalanced force acting on the body then body is said to be in “Static Equilibrium”
 

Examples

            A book lying on a table is in static equilibrium because its weight is cancelled by the normal reaction of table.

                If the two forces and are equal in magnitude and they are acting on a ring in opposite direction. The ring will remain at rest.

2-Dynamic Equilibrium

If the two forces act on a body maintains its state of uniform motion under the influence of the acting forces then this type of equilibrium is called “Dynamic Equilibrium”.

Examples

           A car moving with uniform velocity is the example of Dynamic Equilibrium. The force of engine acts in forward direction while the force of friction between road and tires acts backward. These two forces, being equal and opposite cancel the effect of each other and the car moves with uniform velocity.

        Jumping out of a paratrooper from an aeroplane is another example of dynamic equilibrium. At a particular velocity the reaction of air on parachute becomes equal to the weight of the weight of the paratrooper. At this stage, both the forces cancel effect of each other and the parachute falls down with a uniform velocity.

Torque (Moment of Force)

The object which can rotate about an axis will start rotating under the influence of a suitable force; the turning effect of a force is called “Torque or Moment of force”. Torque may rotate an object in clockwise or anticlockwise direction.

                                               OR

          The moment of a force about a point is the product of force and perpendicular distance of its line of action from the fixed point.

          Torque is represented by "" (Tau) and it is the product of force (F) and moment arm (d).

                 Torque ( )= Force x Perpendicular Distance

                                                      OR  

                                                           

Factors on which Torque ( ) depends:

The torque depends upon the following two factors:

         1-The magnitude of the applied force (F).

         2-The moment arm (d). The perpendicular distance between the axis of rotation of the body and line of action of the force is called the “moment arm”.