Torques and levers

Torque, moment arm and the three classes of levers in the human body.

The outline, topic by topic

Each topic is weighted the way the exam weighs it.

Torque: what is it?
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A force's turning effect, equal to the force times its moment arm, in N·m.
Pushing a door far from the hinges is easier.
In a curl, the weight feels heaviest with the forearm horizontal. Why?
- The load's moment arm is longest there
- The weight is heavier there
- The biceps' moment arm is shortest there
- The elbow is fully extended there
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The load's moment arm is longest there
The load's weight does not change, but its torque peaks when its moment arm does, with the forearm horizontal.
Moment arm: what is it?
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The perpendicular distance between a force's line of action and the axis of rotation.
It changes with the joint angle.
Why does the body mostly use third-class levers, though they are weak for force?
- They reduce the load on the joints
- They give much speed and range at the end of limbs
- They save the muscles' energy
- They protect the tendons from stretching
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They give much speed and range at the end of limbs
The muscle attaches close to the axis: a small shortening gives a large movement at the end of the limb. Force is traded for speed.