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Lever arm and moment
Beginner
Torque depends on perpendicular distance from the axis.
The lever arm (moment arm) is the perpendicular distance from the axis to the line of action of the force. Torque = force × lever arm.
Door handle far from the hinge: easier to open.
Pushing along the line to the hinge gives zero torque.
In more detail
The lever arm is the perpendicular distance from the axis to the line of action of a force. Torque depends on the lever arm, so forces applied far from the axis are most effective.
Real-world uses
- Door handles are placed far from the hinge, so they are easy to turn.
- Crowbars give mechanical advantage through a long lever arm.
- Pushing a door near the hinge needs much more force than pushing at the handle, because the lever arm is shorter.
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Frequently asked questions
What if the force is not perpendicular?
Use only the perpendicular component, or the sine of the angle in the torque formula.
Why is a door hard to open near the hinge?
Because the lever arm is small, so the torque for a given push is small.
What is the lever arm?
The perpendicular distance from the axis of rotation to the line of action of the force.
Related topics in Rotational Motion
Relation between linear and angular quantities · Rotational kinematics · Torque · Equilibrium of a rigid body · Centre of mass of shapes · Centre of gravity