Cornell University Ergonomics Web

1. Definitions of terms and processes

Scalar quantity - magnitude of a variable (e.g. mass, temperature, distance, time)

Vector quantity - magnitude + direction of a variable. The magnitude of a vector is a scalar quantity, but this also has a direction (e.g. force)

Vector quantities can be represented by an arrow (A), where the length of the arrow represents the magnitude of the vector, from a point of application in the direction of the line of action as follows:

A. Vector addition - vectors add together to produce a resultant vector (e.g. two forces acting in different directions will produce a resultant force in neither of the original directions but in a new, resultant direction). Consider the two vectors A and B shown below:

B. Vector addition (> 2 vectors) - more than 2 vectors can be add together to produce a resultant vector. Consider the two vectors A and B shown below:

Using the tail-to-tip method these three vectors, A , B , and C can be added to give the resultant vector D as follows:

C. Vector subtraction - vectors can be subtracted to produce a resultant. Consider the two vectors A and B shown below:

Using the tail-to-tip method these two vectors, A and B , are subtracted to give the resultant vector C as follows:

D. Coplanar vectors- vectors operating in the same plane.

FORCE AND MOTION

Force - mechanical disturbance or load which is translated into torque (e.g. when we push, pull, hold, lift, kick etc.).

Newton's 1^st Law of Motion - a body that is originally at rest will remain at rest, or a body moving with constant velocity in a straight line will maintain its motion until anexternal resultant force is applied.

Inertia - tendency for a body to maintain its state of rest or of uniform motion in a straight line. For a stationary object, inertia must be overcome before motion can begin (e.g. pushing a heavy cart). The more inertia an object has the harder it is to start motion.

Newton's 2^nd Law of Motion - acceleration of a body is directly proportional to the net force acting on the body and inversely proportional to its mass.

F = ma where F = force, m = mass and a = acceleration.

Newton's 3^rd Law of Motion - for every action there is an equal and opposite reaction.

Definitions of Terms

Velocity = Position/Time

Acceleration = Velocity/Time

Units of Force

System in the US = Pound (lb)

SI system = Newton (N)

c.g.s system = dyne (dyn)

Note: 1N = 10⁵ dyn = 0.225 lb

Classification of Forces

Force system - 2 or more forces acting on a body.

External forces - forces acting outside of the human body.

Internal forces - forces acting inide of the human body on musculoskeletal structures.

Normal force - force applied perpendicular to a surface (e.g. book on a table) .

Tangential force - force applied on a surface in a direction parallel to that surface (e.g. pushing a cart).

Tensile force - force that causes stretching/elongation of a body. Muscles produce tensile forces, especially in the tendons.

Compressive force - force that causes shrinkage of a body in the direction that it is applied (e.g. compression of the intervetebral discs when we stand).

Coplanar forces - all forces acting on a 2-D surface.

Collinear forces - all forces with a common line of action (e.g. force on a rope in a tug-of-war).

Concurent forces - lines of action of forces have a common point of intersection (e.g. ball of foot when walking).

Parallel forces - lines of action of force that are parallel to each other.

Frictional forces - surface forces that resist the sliding of bodies in contact with each other.

Gravitational force - force of gravity that acts on a mass to give it weight.

Moments - force applied to an object that translate/deform or rotate the object. Torque is the rotational force.

Pressure - force distributed by a load over an area of a surface (referred to as an equivalent force or concentrated load).

Units of Pressure

System in the US = Pounds per square inch (psi)

SI system = Pascal (Pa)

c.g.s system = dyne/cm²