Applied Physics I: Chapter 4: Oscillations and Waves

Moment, Couple and Torque

Definitionm, Explanation, Formula - Moment, Couple and Torque.

MOMENT, COUPLE AND TORQUE

 

(i) Moment of a force: The moment of a force about a point is defined as the product of the magnitude of the force and the perpendicular distance from the point to the line of action of force.


Explanation: Let 'F' be the force acting on a body, at A as shown into Fig 4.3. Then the moment of force 'F' about 'O' is M0 = F×d

Where 'd' is the perpendicular distance from the point 'O' to the line of action of force F.

 

(ii) Couple: A couple constitutes a pair of two equal and opposite forces acting on a body, in such a way that the lines of action of the two forces are not in the same straight line.


Explanation: Let 'P' and 'Q' be the two equal and opposite forces acting on the body AB as shown in Fig.4.4 then these two forces form a couple and the moment of the couple about A is MA and about B is MB, then we can write

Couple = MA = MB = P×d

 

(iii) Torque: Torque is the rotating force and is equal to the moment of the couple. Torque is the product of one of the forces forming couple and the perpendicular distance between the two opposite forces.


Explanation: Let us consider two equal and opposite forces F and ‒F acting tangentially on the rim of a circular disc from the extremities of diameter as shown in Fig 4.5.

It forms a couple. If the couple rotates through a small angle 'θ'. Then,

The distance moved by the force F in rotating the body through an angle 'θ' = Length of the arc AB.

Here the length of the arc AB = гθ

Work done by a single force = Frθ

Work done by two forces constituting a couple = 2Frθ.

Here 2Fr is the moment of the couple (or) torque (τ)

Work done by the torque = τθ

 

Applied Physics I: Chapter 4: Oscillations and Waves : Tag: Applied Physics : - Moment, Couple and Torque


Applied Physics I: Chapter 4: Oscillations and Waves



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