When an object is modelled as a?particle?we do?not?need to consider the?rotation?of the object due to forces. When we model an object as a?rigid body??then we need to consider the rotation of the object.

What is a moment?

• The?moment?of a force is the measure of its ability to cause a body to?rotate?about a specific point (usually called the?pivot)
• The?units?for moments are?newton metres?(N m)
• A force can cause a body to rotate?clockwise?or?anti-clockwise
• The moment of a force about a point is calculated by?multiplying?the magnitude of the?force?by the?perpendicular distance?from the line of action of the force to the point
  • We can choose the?positive direction?to be?clockwise?or?anti-clockwise?and the?negative direction?is the?opposite
  • A?zero moment?will not cause a body to rotate
• The?moment?of a force about a point is?zero?if the line of action of the force goes through the point

How do you decide if a moment is clockwise or anti-clockwise?

• ?A clockwise moment will cause a body to rotate in the clockwise direction
• To visualise this:
  • Imagine that the perpendicular line from the point to the force is the radius of a circle
  • The direction of the force tells you which way the circle goes round

What is the resultant moment?

• The?resultant?moment is the?sum?of?all?the moments acting on a body (both positive and negative)
• To find the resultant moment:
  • Define the positive direction (clockwise or anti-clockwise)
  • Add together?all the moments in?that direction
  • Subtract all?the moments in the?opposite direction
  • The overall value is the resultant moment
• The?sign?of the?resultant?moment indicates which direction the body will rotate

Worked Example

Exam Tip