Biomechanics

These four quantities are all vectors. Vectors need a direction as well as a magnitude. Mass is not a vector.

The patient’s base of support (their area of contact with the bed) is not changed by a slide sheet.

For example, the atlas vertebra is the fulcrum for the lever action of nodding yes. The sternocleidomastoid and the trapezius muscles provide the effort forces.

Changing the direction of motion by turning a corner (even if the speed does not change) requires an unbalanced force. Being still or moving at constant speed indicates that there are no unbalanced forces acting.

Newton’s second law, F = m × a putting in the numbers gives:
F = 3 × 10 = 30 N (a = 9.8 ms−² but ten is close enough).

Consider flexing the forearm using the biceps brachii. Its tendon is inserted on the radius bone very close to the elbow joint, while the load (the forearm) is further away. Such an arrangement allows a short contraction of the biceps brachii to be converted to a large sweep of motion of the hand.

When the imaginary line joining the Earth’s centre with the person’s centre of gravity passes through the area of a person’s base of support, they are stable. That is, they won’t overbalance and tip over.

An unbalanced force will result in some movement. The movement of the limb indicates that an unbalanced (muscle) force was acting.

The moveable pulley provides a mechanical advantage. A fixed pulley merely redirects the direction of traction without altering its magnitude.

Pulleys change the direction of the force provided by the hanging masses and (if the pulley is a moveable one) provide a mechanical advantage.