## Characteristics Of Forces And Force Systems (Engineering Science)

## What Is A Force?

A **force** is either a push or a pull.

- Force is a vector quantity since it has both magnitude and direction.
- The SI unit of force is the newton (N).

The fact that force is a vector quantity means that forces can be added or subtracted geometrically according to the rules of vector algebra. The graphical vector representation of a force shows a force as a straight line with an arrow, in which the length of the line is proportional to the magnitude of the force and the arrowhead indicates its direction.

### Effects of a force

A force cannot be seen but can be detected by its effects. Effects of a force include:

- changing the state of motion of an object.
- changing the shape of an object.
- changing the size of an object.

#### Effect of force on the state of motion of an object

A force can be described as something that changes the state of motion of an object. That is because a force can cause:

- a stationery object to move.
- a moving object to increase or decrease its velocity.
- a moving object to stop.

## Classification Of Forces

Forces can be classified in many ways according to their properties and effects. Forces can be classified according to their effect on the body, for example:

**External force**– This is a type of force that is applied externally to a body.**Internal force**– This a resistance to deformation by the material that makes up a body.**Active force**– This is a type of force which causes a body to move or change its shape.**Passive force**– This is a type of force which opposes motion, a tendency to move or deformation of a body.

## Forces systems

A **Force system** is a collection of forces acting on a body. Force systems can be classified according to the relative positions of the forces, for example:

- Concurrent forces
- Coplanar forces
- Parallel forces
- Collinear forces

### 1. Concurrent forces

**Concurrent forces** are a collection of point forces whose lines of action of intersect at a single point.

Since these forces all act at a single point, there is no resultant moment about this point due to the forces. Lack of moments about a body under concurrent forces allows us to treat the body as a particle.

### 2. Parallel forces

**Parallel forces** are forces that lie in the same plane but have lines of action that never intersect each other. Since they never intersect each other, parallel forces are **non-concurrent**.

Parallel forces can either be in the same direction or in opposite directions.

### 3. Collinear forces

**Collinear forces** are forces that have the same line of action.

This means that collinear forces are forces on the same straight line.

### 4. Coplanar forces

**Coplanar forces** are a collection of forces whose lines of the action lie in the same plane.

This means that, if the forces in a system all lie in the same plane, it is known as a coplanar force system. A force system can be coplanar and also concurrent, parallel, non-concurrent or non-parallel at the same time. For example:

#### Coplanar non-concurrent parallel force system

Coplanar non-concurrent parallel force system is a system of forces whose lines of action of lie in the same plane and are parallel to each other but may not have the same direction.

#### Coplanar concurrent collinear force system

Coplanar concurrent collinear force system is a collection of forces in the same plane that lie along the same straight line.

#### Coplanar concurrent non-parallel force system

Coplanar concurrent non-parallel force system is a collection of forces whose lines of action pass through a common point and have different directions but in the same plane.

#### Coplanar non-concurrent non-parallel force system

Coplanar non-concurrent non-parallel force system is a collection of forces whose lines of action lie in the same plane but do not pass through a common point.