The principle of moments states that, when an object is in equilibrium, the sum of anticlockwise moments about any point equals the sum of clockwise moments about the same point.

The moment of a force is the product of the force and the perpendicular distance between the axis of rotation and the line of action of the force.

  • $\tau = r \times F$
  • Unit of moment: Nm



A couple is a pair of forces, equal in magnitude but opposite in direction, whose lines of motion do not coincide.

  • Will still rotate as there is a net moment
  • As forces are equal and opposite, resultant force is zero and so there is no linear acceleration
  • $\tau = r \times F$


Torque(Moment of a couple) is the product of one of the forces and the perpendicular distance between their lines of action of the forces.

  • Can take moment from any point


Next: Equilibrium

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