Forces and Moments: Part 4

 

Moment of a couple:

 

A couple is defined as:

  • Two parallel forces
  • Same Magnitude
  • Opposite direction
  • Separated by a perpendicular distance d

 

F
 

A moment produced by a couple is called a Couple Moment. In the figure below let M be the couple moment

 

 

So a couple moment is a free vector which can act at any point and depends only on r, not on rA and rB.

 

Remember:

 

 

Scalar Formulation:

Magnitude: M = Fd

Direction and sense using right-hand rule

 

 

Vector Formulation:

Magnitude: M = r x F

 

Note: The moment of a couple does not depend on the point one takes the moment about. In other words, a moment of a couple is the same about all points in space.