Moments

• Moment


  The turning effect of a force
• Moment of a force

  1. Force (N) x Distance (m)

  2. Perpendicular distance from the pivot to the line of action of the force
• Moment of a force

  • Force on a spanner causes a turning effect or moment on the nut (which acts as pivot)
  • A larger force or a longer distance (spanner) would mean a larger moment
• To get the maximum moment (or turning effect)


  • Push at right angles (perpendicular) to the spanner
  • Pushing at any other angle means a smaller distance, and so a smaller moment
• If the total anticlockwise moment equals the total clockwise moment about a pivot


  The object is balanced and won't turn
• Example


  • A 6 m long steel girder weighing 1000 N rests horizontally on a pole 1 m from one end
  • Tension in a supporting cable attached vertically to the other end is 400 N
• Calculating the tension in the supporting cable

  1. Calculate the total anticlockwise moment

  2. Write an expression for the total clockwise moment

  3. Set the moments equal to each other and rearrange for the tension