Torque due to a Couple
Two equal forces pushing in opposite directions cancel out — yet the object still spins. This simulation shows why a couple produces pure rotation with no net force, and why its turning effect does not depend on where you measure it from.
How to use this simulation
- Change the size of the forces and their separation
- Move the pivot point anywhere on the body
- Display the resultant force and the total moment
What to look for
- The resultant force is always zero, so the centre of mass does not accelerate
- The turning moment is identical about every point you choose
- Widening the separation increases the moment even with the same forces
The physics behind it
A couple is a pair of equal, opposite, parallel forces whose lines of action do not coincide. The resultant force is zero, so there is no acceleration of the centre of mass, but the moment is Fd, where d is the perpendicular distance between the lines. That moment is the same about every point, which is why a couple produces rotation without translation.