Forces in Static Situations/Lesson Document

Forces in Static Situations

What are forces? A force on an object is a push or a pull on that object. It is best to always talk about the forces acting on a particular object i.e. specify the object upon which the pushes or pulls are acting. A force can rise in two ways: (a) Through contact with another object. Examples: File:FFS1

Note that, on perfectly level ground two motionless objects can lay side-by-side in contact but will not be exerting forces on each other. This is rather like placing two balls side-by-side in contact on a snooker table. (b) ‘Mysterious’ forces These are non-contact forces that occur when objects interact gravitationally or magnetically. There are also ‘mysterious’ electrical no-contact forces. File:FFS2

It is quite legitimate to refer to these ‘non-contact’ forces as ‘mysterious’, since even scientists find it difficult to explain how they arise. The size (or strength) of the ‘mysterious’ non-contact forces decreases as the distance between the interacting objects increases. "'How do you describe a force?"' A force is described in terms of two qualities: its size and its direction. Scientists often use an arrow to indicate the size and direction of a force: File:FSS3

Where do forces act? File:FSS4

The force of reaction acts over a whole surface of contact – it is usually drawn at the centre of the surface, slightly displaced from other ‘force arrows’ for clarity. The two surfaces are also drawn slightly apart for the same reason.

File:FSS5 Balanced forces A stationary object remains still because all the forces acting upon it are ‘balanced’, i.e. the effect of each force acting on a body is cancelled out by an equal and opposite force. When the forces on an object are balanced, a scientist would say there is no net force or no resultant force acting on it i.e. all the forces cancel each other out. File:FSS6