Two ways to turn
A moment has not only a size but a direction — though not left or right, but the way round it turns.
When a force acts on a body, the direction of its moment is decided by the way that force turns the body. There are only two ways to turn.
Turning the same way the hands of a clock go.
Turning against the hands of a clock.
Figure to be added
Two forces on one strip
Consider a wooden strip pivoted at O as in figure 11.6. The force F1 turns it one way; the force F2 turns it the other.
clockwise moment = F1 × d1
anticlockwise moment = F2 × d2
When both forces act at once they work against each other, so what is left is the difference between them.
resultant moment = F1 × d1 − F2 × d2
Here the clockwise moment has been taken as positive.
Which one counts as positive?
Taking clockwise as positive is a convention, not a law. Take anticlockwise as positive instead and only the sign of the answer changes; the way the body turns is the same. Keep to one convention throughout a single problem.
And what if the two moments happen to be equal? That is the next page.