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Moment and torque

A torque $$(\vecphy{\tau})$$ or moment $$(\vecphy{M})$$ is the tendency of a force to twist or rotate an object about an axis. The concept of moment can be confused with the momentum $$(p=mv).$$

The magnitude of $$\tau$$ is given by the product of the force $$F$$ and the perpendicular distance $$r$$ between the point at which the force is applied and the axis of turning: $$$\tau=M=rF$$$

The SI unit of torque is the Newton metre $$(N m)$$. It is dimensionally equivalent to the unit of work (i.e. $$1\text{ J}=1\text{ N m}$$) but describes a different quantity.

When you hold a pen lying on the table at its middle and push it at its tip, the pen will turn around its middle.

$$F_1$$ on the left acts alone and will change the overall direction of the object as well as creating rotational acceleration. The couple of $$F_2$$ and $$F_3$$ creates a moment but no net force.
$$F_1$$ on the left acts alone and will change the overall direction of the object as well as creating rotational acceleration. The couple of $$F_2$$ and $$F_3$$ creates a moment but no net force.