08 3-2 (d) The resultant couple
Definition
- The sum of two or more coplanar couples is a single couple whose moment is the algebraic sum of the moments:
- It is called the resultant couple. If the couples are in equilibrium.
✍️ Worked examples
Example 17:
- Three couples act on a plate: 200 N forces with arm 0.8 m (anticlockwise), 500 N forces with arm 2.3 m (clockwise), and 100 N forces with arm 4 cm (clockwise).
- Find the resultant couple.
Solution:
- N·m.
- N·m.
- N·m.
- N·m: a clockwise couple of magnitude 994 N·m.
Example 18:
- A square of side 10 cm is acted on by 40 kg·wt forces along and (a couple) and 70 kg·wt forces along and (a second couple, opposite in sense).
- Find the resultant couple.
Solution:
- kg·wt·cm.
- kg·wt·cm.
- kg·wt·cm (clockwise, magnitude 300).
Example 19: What force applied in the opposite sense with arm 12 cm balances a resultant couple of 300 kg·wt·cm?
Solution:
- The balancing couple must have moment , so and kg·wt for each force.