# Introduction to Forces

## What is a force?

A force is anything that can cause a change to objects. Forces can do things like:

• change the shape of an object,
• accelerate or stop an object, and
• change the direction of a moving object.

A force can be classified as either a contact force or a non-contact force.

A contact force must touch or be in contact with an object to cause a change. Examples of contact forces are:

• the force that is used to push or pull things, like on a door to open or close it
• the force that a sculptor uses to turn clay into a pot
• the force of the wind to turn a windmill

Contact forces

Contact forces

A non-contact force does not have to touch an object to cause a change. Examples of non-contact forces are the forces due to:

• gravity, like the Earth pulling the Moon towards itself;
• electricity, like a proton and an electron attracting each other; and
• magnetism, like a magnet pulling a paper clip towards itself.

Non-contact forces

Non-contact forces

The unit of force in the international system of units (S.I. units) is the newton (symbol N). This unit is named after Sir Isaac Newton who first defined force. Force is a vector quantity and so it has a magnitude and a direction. We use the symbol $$\overrightarrow{F}$$ for force.

This chapter will often refer to the resultant force acting on an object. The resultant force is simply the vector sum of all the forces acting on the object. It is very important to remember that all the forces must be acting on the same object. The resultant force is the force that has the same effect as all the other forces added together.