 ## 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:

1. Concurrent forces
2. Coplanar forces
3. Parallel forces
4. 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. 