History

Newtons 1st Law

Newtons 2nd Law

Newton's 3rd Law

Sources

Newton's Third Law of Motion

For every action, there is an equal and opposite reaction.
            -Newton

A force is a push or pull upon an object that results from its interaction with another object.  Forces result from interactions.  These interactions can be contact interactions such as friction or applied forces.  As well as distant interactions such as gravitational or magnetic forces.  Newton said that whenever two objects interact with each other they exert forces upon each other. 

Example:  When you sit in a chair, your body exerts a downward force on the chair and the chair exerts a force upward on your body.  This is also known as action and reaction forces. 

In this picture you can see how the person and the wall have a force on each other.  The skateboard and the person would also have a force.  There is a downward force of the person on the skateboard and the skateboard gives an upward force to the person.