Multiple choiceQuestion 1 · Recall
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By the end you can say what a couple is and explain why its only effect is to make something rotate.
Turn a steering wheel with both hands: one pushes up, the other pulls down. The wheel spins, but it doesn't try to leave the dashboard.
That pair of forces is a couple. It's the cleanest way there is to make something turn.
Couple
A couple is a pair of forces that are equal in magnitude, opposite in direction and parallel, but whose lines of action are separated (not the same line).
Its only effect is to produce rotation.
Check the two effects a set of forces can have:
Zero resultant force plus a turning effect means rotation and nothing else.
| Forces on a free object | Resultant force | Turning effect | What happens |
|---|---|---|---|
| one force, not through G | yes | moves and turns | |
| equal and opposite, same line | 0 | none | no overall motion (the rod may deform) |
| a couple | 0 | yes | turns only |