• Force is that which alters the state of a body either at rest or in motion.
  • Moment is turning force. It is combined effect of force and distance.
  • Moment of force – is the effect of turning of a force or is the turning effect of force.
  • It depends on;
  1. The force applied.
  2. The distance from the turning.

              Moment of force = Force x Perpendicular distance from the turning point

                                             F x D                           SI UNITS = NM

  • It implies that force increases while distance decreases or the force applied is inversely proportional to the distance of its line of action. That is a very large turning effect can be produced with a comparatively small force provided distance from the turning point is large.

            Principles of moment.

  • It states that when a body/system is at equilibrium, the sum of clockwise moment about a point is equal to the sum of anticlockwise moment about the same point.

                        Clockwise moment = F3D3 +F4D4

                        Anticlockwise moment = F1DI + F2D2

            At equilibrium, clockwise moment = anticlockwise moment

                                    F3D3 +F4D4 = F1DI + F2D2

            Application of moment.

  1. A beam balance.
  2. A hammer when removing a nail.
  3. Top opener.
  4. See- saw.
  5. Wheelbarrow.

            Parallel forces.

  • When a system is at equilibrium and acted upon by forces, then the sum of the vertically upward forces is equal to the sum of vertically downward forces.


  1. A ladder of length 16 cm weighs 420 N and has the center of gravity 7 cm from one end. It is carried horizontally by two men each carrying it 1 cm from the end. Calculate the force exerted by each man.
  2. A uniform wooden rod of length 6m is carried by two boys each holding it at the ends. If the rod weighs 750N, find the force exerted by each boy.
  3. A uniform metal rod of length 80 cm and mass 3.2 kg is supported horizontally by two vertical spring balances C and D. Balance C is 20 cm from one end while balance D is 30 cm from the other end. Find the reading of each balance.


  • It is used to describe any systems or forces which will turn an object without making it accelerate in any direction.
  • It is a system of forces in which the forces balances but their turning effect will not.
  • It is formed by two equal but opposite forces whose line of action do not coincide.
  • The two forces cannot be replaced by one force. They do not have resultant force.


  • Is the turning effect of a couple.

                        Torque = one force x perpendicular distance

                        Torque = F1D = F2D

Application of torque.

  1. Turning a bicycle.
  2. Tightening or untightening a nut using a wheel spanner.
  3. Forces on water sprinkler or water tap.
  4. Forces applied on steering wheel of a car when going round a bend.



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