Turning needs a moment
To move a body forward what is needed is a force. To turn it, what is needed is not a force but a moment of a force.
- The body travels from one place to another
- What it takes is a force
- Unit: N
- The body turns about an axis
- What it takes is a moment
- Unit: N m
The turning effect
Turning effect of a force
The tendency of a body to rotate because of a force applied to it is called the turning effect of that force.
The two terms sit close enough together to be easy to confuse. The difference is simple: the turning effect is what happens; the moment is how you measure it.
| Term | What it means | Does it have a unit? |
|---|---|---|
| Turning effect | The tendency of a force to make a body rotate | No — it is an effect |
| Moment | The size of that tendency | Yes — N m |
A turning effect is produced on a body by applying a force at some distance from the axis of rotation. If that distance is zero the moment is zero too, and so there is no turning effect at all.
A force does not always turn something
That a force is present does not mean turning is present. Turning happens only if the line of action of the force lies away from the axis.