It is surprisingly hard to say what a force really is but on a simple level we say that forces are either pushes or pulls. It would be better to label “pulls” as forces of attraction and “pushes” as forces of repulsion.
Some forces have their effect when objects are in contact whereas others allow objects to affect each other from a distance (non-contact). There is lots of really interesting physics in non-contact forces but for now it’s enough to be able to give some examples and to know a few basic facts.
The common non-contact forces are gravitational force, magnetic force and electrostatic force. (Electrostatics aren’t in the AQA Trilogy course but you should be able to recognise the force that arises after two different materials have been rubbed together – such as hair or small pieces of paper being attracted to a plastic comb.)
Magnetic and electrostatic forces can be either attractive or repulsive but gravitational forces always attract. In all cases, the forces are vector quantities (they have both magnitude and direction) and the strength of the force varies inversely with the distance between the affected objects. This means objects that are closer together experience a stronger force than objects that are further apart.
Weight is a non-contact force but if an obstacle prevents the motion that it would normally create (“downwards”) then it turns into a contact force. So if a book rests on a table, its weight is a downward force that presses on the table. If the table doesn’t collapse, it must have enough structural strength to resist the weight of the book. This resistance is equal and opposite to the weight. The resistance is known as a normal contact force.
The book’s weight and the table’s normal contact force are equal and opposite so the book remains stationary on the table. It is not possible for the normal contact force to be greater than the weight, so you will never see a table throw a book up into the air by applying a force that is greater than the book’s weight!
Note that all forces (both contact and non-contact) are vectors so they are represented in diagrams using arrows. The length of the arrow is proportional to the magnitude of the force and the direction of the arrow indicates the direction of the force. And because vectors act in a specific direction, the arrows that are used to represent them must be completely straight!
To remind yourself about vectors, and non-vectors (known as scalars) read this article.
Other examples of contact forces include tension and all forms of friction. Both air resistance and water resistance are examples of friction, so they are contact forces. It’s easy to overlook the fact that air resistance is caused by contact with air particles – unless you are trying to walk or cycle against a strong wind, when the reality of air contact becomes very obvious.
Mr Tarrant's physbang 'blog 