Couple
Forces in Action - OCR A-Level Physics
Key Definition
Couple
A pair of equal and opposite parallel forces whose lines of action do not coincide. A couple produces pure rotation with no translational motion.
A pair of equal and opposite parallel forces whose lines of action do not coincide. A couple produces pure rotation with no translational motion.
$$\tau = Fd$$
$$\tau = Fd$$
- A coupleA pair of equal, opposite, parallel forces whose lines of action do not coincide, producing a pure turning effect (torque) with no resultant force. has zero resultant force (the two forces cancel) but a non-zero resultant moment (the torque).
- The torque of a couple is the same about any point, so you do not need to specify a pivot when calculating it.
- Everyday examples: turning a steering wheel with two hands, opening a jar lid, the magnetic torque on a compass needle.
- $F$ is the size of one of the forces and $d$ is the perpendicular distance between the two lines of action.
Common Mistake
LOW
Students often: double the torque by writing $\tau = 2Fd$, treating each force as contributing $Fd$ separately about the centre.
Instead: use $\tau = Fd$ where $d$ is the FULL perpendicular distance between the two forces, not half of it.
Instead: use $\tau = Fd$ where $d$ is the FULL perpendicular distance between the two forces, not half of it.
Examiner Tips and Tricks
- When asked to identify a couple, state both conditions: equal and opposite forces AND their lines of action do not coincide.
- If two forces share the same line of action, they cancel completely; that is NOT a couple.