Kinetic Concepts + Newton's laws

Kinetics

Study of forces that cause or change motion in the body. Includes forces, Newton’s laws, and mechanical loads.

What is force?

A push, pull, friction (rub), or impact (blow). Defined by strength, direction, and point of application. Causes motion or shape change.

Unit of force?

Newton (N) = force required to accelerate 1 kg mass at 1 m/s². Formula: F = m × a.

What is net force?

The single resultant force of all acting forces. Determines overall effect. Net force = 0 → no acceleration (motionless or constant velocity).

Newton’s 1st Law (Inertia)

A body remains at rest or constant velocity unless acted on by external force. Inertia ∝ mass. Larger mass = harder to move/accelerate.

Newton’s 2nd Law (Acceleration)

Force causes acceleration proportional to force, inversely to mass. Formula: a = F/m. Larger mass = more force needed.

Newton’s 3rd Law (Reaction)

For every action, there is an equal and opposite reaction.

Compressive load

Pressing/squeezing force along body’s longitudinal axis.

Tensile load

Pulling/stretching force along body’s longitudinal axis.

Shear load

Force directed parallel to surface; causes sliding between parts. Example: ACL tear.

Bending load

Asymmetrical load → compression on one side, tension on other. Combination load, non-axial.

What is stress?

Force distribution within a body when external force acts. Stress = Force ÷ Area. Stress ↑ when area ↓.

Repetitive vs Acute loading

Repetitive (microtrauma): repeated small forces → chronic stress injuries. Acute (macrotrauma): single large force → sudden injury.