Torque
A torque is when we have a couple acting on an object, to rotate it.
It basically measures the magnitude of the turning effect (moment) caused by a couple.
Because it’s a rotation, it must have a moment!
Calculating torque
The turning effect is the sum of both moments in the couple.
- For example, if we have a couple with two forces:
- A
5N force2m away from the pivot, clockwise - A
5N force3m away from the pivot, clockwise
- A
- …then we know that the first moment must be
5\times2=10Nm and the second moment must be5\times3=15Nm . - Adding them together, we get
10Nm+15Nm=25Nm , so our torque is 25Nm.
We could also have done this by simply multiplying the force by the distance between the couple’s forces:
2m+3m=5m M=5N\times5m=25Nm - That gives us the same torque of
25Nm .
Torque formula
We can use that to write a formula for torque, which looks very familiar:
Where:
\text{torque} is the combined moment of the forcesF is the magnitude of a single force (one of the components of the couple)d is the distance between the couple’s forces
Units of torque
Torque is just the combined moment, so it’s measured in the same way: using newton-metres.
Calculations
Two forces of 8N act clockwise on a metal bar. The forces are 9m away from each other. Find the turning effect.
M=Fd M=8N\times9m M=72Nm
The torque on a stick is 7Nm. The perpendicular distance between the couple is 3.5m. Find the sum of the magnitudes of the forces acting.
M=Fd F=\frac Md F=\frac7{3.5} F=2N - The force of one is 2N
- So the combined force of both components of the couple must be
2\times2=4N - Answer:
4N