Linear Kinematic Forces

Biomechanics 201

Kinetics

“The relationship between forces acting on a system and the motion of the system.” - Simple: The study of the causes of movement.

Rigid Body

An object of a finite size with no deformation.

(no deformation means that internal distances between parts of an object remain the same).

Mass (m)

Quantity of matter that composes an object in kg.

Measure of inertia for linear motion and gives rise to gravitational attraction.

Mass, Density and Volume

m = pV

Volume (V)

How much space something takes up in m²

Density (p)

How much matter (mass) is present per unit of volume. kg / m³

e.g: how tightly is mass packed in?

Centre of Mass (COM)

Geometric point where every particle of an objects mass is equally distributed.

Changes with changes in object configuration. May or may not lie within an individuals volume (dependant on body part arrangement).

Inertia

Resistance of any physical object to any change in its state of motion. Or the tendency of objects to keep moving in a straight line at a constant velocity.

Property of mass (kg).

Concussions

Sudden movement where the brain (a soft, squishy membrane), hits the skull. Skull is moving backwards while the brain is moving forward. Membranes die, releasing toxins intot he brain which kills more membranes.

Force (newtons - N)

Push or pull on an object that causes it to accelerate.

Is a vector with a direction and magnitude, causing point of application to be important.

Internal and external forces

Internal forces exist within an object. Whereas, external forces act from outside the object.

Important to define the object you are analysing.

Free-body diagrams

Only show external forces acting on an object.

Weight

F_W = mg

Acts on the COM. Fw = force due to gravity acting on the mass of the object.

Stress

Stress: σ = F / A

F - force; A - area it is applied over.

Pressure

Stress due to a compressive or pushing force.

Two types of pressure.

Concentrated force - pressure

A force acting or considered to act at a single point.

Distributed force - pressure

Force applied over some extended area.

If object is not deformable (doesn’t change shape), then it can be approximated by a concentrated force acting at the centre-of-pressure.

Lines of action

Imaginary lines that extends from the force vector in both directions.

A force can move along this line without changing its effect.

Net force

Is the vector sum (resultant) of all forces acting on an object.

Individual forces on an object can be replaced by the net force which will have the same effect.

Equilibrium

When two forces of equal magnitude acting in opposite directions along the same line of action cancel each other out.

Newtons First Law

Inertia: “A body will maintain a state of rest or constant (linear) velocity unless acted upon by an external force.

Resistance to a change in motion = inertia (kg).

Newtons 2nd Law

Law of acceleration.

“A body of mass (m) will move with an acceleration (a) if the sum of all external forces acting on it is greater than 0.

Newtons 3rd law

The law of action and reaction.

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

Ground reaction force

The force applied upwards onto an object from the ground.

Normal

Perpendicular (right angles or orthogonal) to a surface. Trigonometry

Compression

Pressing or squeezing force directed normal (perpendicular) to a surface.

Tension

Pulling or stretching force directed to a normal surface.

Shear

Sliding or tearing force directed parallel to a surface.

Trigonometry

Angle of the surface

Resistive Force

Force acting on a resting or sliding object from the surface that is equal in magnitude and opposite in direction to the sum of other normal forces acting on the object.

This force changes based on other normal forces acting on the object.