What is a force?
A force can be a push or a pull. It is not something you can see or touch but can see it in action. Forces can be measured using a device called a force meter. The unit of force is called the Newton. It is represented by the symbol N. A force of 2N is smaller than 7N.
A force usually results from an interaction. The interaction can be a physical one or a non-physical one. Forces resulting from physical interaction are called Contact Forces, and examples include Frictional, Tension, Air Resistance, and Spring force.
A force resulting from non-physical interaction is called Action-at-a-distance force and examples include gravitational, electrical, or magnetic force.
Force meters (left illustration) contain a spring connected to a metal hook. The spring stretches when a force is applied to the hook. The bigger the force applied, the longer the spring stretches and the bigger the reading.
A Force diagram
A force diagram is used to show the forces acting on an object. An arrow, with a name, length and direction is used to represent a force.
In a force diagram, the longer the arrow, the bigger the force. See this illustration below: