Rotational Motion

 

Card # 8-Rotational motion Rigid body:

The bodies, which do not change their shapes or volume by application of force, are called rigid bodies. When a force or torque is applied on a rigid body, then it revolves around its own axis. This motion of the body is called rotational motion. The axis about which the body rotates is called axis of rotation.

Couple-:

When two equal & opposite forces act on a body at different points then the pair of such forces is called couple. Couple has the tendency to rotate the body. In the diagram given below the body is rotating about XY axis in anticlockwise direction.

Moment of couple It is the measurement of couple of force. Moment of couple = Force x perpendicular distance between the force = Force x moment of arm Τ=Fxd S.I. unit of Τ= N x m

TORQUE A force in transnational motion produces linear acceleration. Similarly in rotational motion torque produce angular acceleration. The turning effect of force is called torque. A point near to axis turning effect is less. It increases with increase of moment of arm. Τ=Fxd

Vector form of torque Let a body is rotating about an axis XY, force F if applied at point A, distance r. OB = perpendicular distance between O & F Therefore Τ = F x OB or Sin θ = OB/OA Τ = F x r sin θ 1) If r=0, Τ= 0 2) θ = 0˚, 180˚, Τ = 0 3) Sin θ = 1 or θ =90˚ 4) F=0 Τ=0 Direction: - Direction of Τ is perpendicular to plane consisting of r and F.