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Moment of a force is a measure of the turning effect produced by the force on the rigid body. This force causes rotation about the pivot and so must include the direction of rotation as clockwise or anti-clockwise.
The moment of a force F about a point P is the product of the magnitude of F and the perpendicular distance from pivot P to the line of action of the force. It is measured in newton-metres, Nm, and the direction of rotation should be mentioned as clockwise or anti-clockwise.
Moment, M, of force F about point P=|F|×d
Vectors: Moment is cross product of force and distance
Particle and Rigid Body
Particle is an object whose dimensions can be neglected as they are very small when compared with other objects in the system. This implies that:
• mass of a particle is concentrated at a single point.
• Rotational effect of any force is zero
• Effects due to air resistance is ignored.
Rigid Body
• Rigid body is a solid body in which deformation is neglected as it is almost zero.
• Rotational effect of a force cannot be neglected. They cause turning effect called moment.
Rod is a rigid body with one dimension (length but no thickness).
Lamina is rigid body with two dimensions (thickness is neglected, like paper).
Uniform body: Mass is distributed uniformly over the entire volume and can be considered to be concentrated at the centre of the body.
Light object: Mass of the object can be neglected.
