04 3-1 (d) Equivalent couples
Definition
- Two couples in the same plane are equivalent if their moments are equal (in size and in sense).
- Equivalent couples have the same effect on a rigid body even if their forces and arms are different.
✍️ Worked examples
Example 8:
- is a rectangle with cm and cm.
- Equal forces of 96 N act along and (opposite directions).
- Find the equal forces acting at and parallel to whose couple is equivalent to this one.
Solution:
- First couple: arm cm, moment N·cm (clockwise).
- cm.
- The distance of from is cm, and the same for on the other side, so the arm of the second couple is cm.
- Equal moments: , so N, acting clockwise.
Example 9:
- A square has side 10 cm.
- Forces of 60 N act along and .
- Find the equal forces at and parallel to forming an equivalent couple.
Solution:
- First couple: arm , moment N·cm.
- Diagonal ; the distance from to is , so the arm of the new couple is .
- , so N.