**Pre-reqs:** None.

I intend to talk about forces and force diagrams, but there is a more fundamental question to address first. What is a force? Most texts define it as a push or a pull. That really isn’t a bad definition. Maybe a better (or maybe worse) definition would be “forces are things that change the motion of an object” (change being the key word). If I had to choose one definition of force, it would be something like this:

**Force:** *A force is an interaction between two objects. There are 4 known forces:*

• Gravitational force: An attractive long range force between objects with mass
• Electromagnetic force: An attractive or repulsive long range force between two objects with charge
• Strong Nuclear force: An attractive short range force between particles like protons and neutrons
• Weak Nuclear force: A short range force responsible for beta decay. *Yes, I know that is a confusing force – for introductory physics, you won’t use this force*

All forces are some form of the above forces.

**Important properties of forces**

• Forces are an interaction between TWO objects. It is not possible to have a force on an object and not have another object involved.
• Forces are vectors. They have magnitude and direction
• The unit for force is the Newton. If you do a whole bunch of cool stuff, they will name something after you also.
• Forces are NOT properties of an object like mass or speed or color. They are properties of an interaction between two objects. Yes, I already said that, but it is important.

There are some more things about force you will need to know. For now, this should be enough.