3.1.1 Force Diagrams

Why do we need force diagrams?

• Force diagrams are used to help understand a given scenario and show which forces are acting on which particles and in which direction they are acting
• In diagrams an arrow is used to represent a force acting on a particle which shows the direction in which the force is acting
• The magnitude of the force is normally written next to its arrow

What types of forces could be involved?

• Specific types of force encountered (which may not be mentioned in the question nor labelled on a given diagram) are: weight (W N), tension (T N), thrust (T N), friction (F N) and normal reaction (R N)

• Remember that the diagram is drawn to help understand the scenario - cars, blocks, etc are modelled as particles occupying a single point in space and so all forces acting on the car, block, etc act at that same single point
• The main forces that you will see are:
  • tension (a “pulling” force) acts away from a particle,

    thrust (a “pushing” force) acts towards it

  • weight is W = mg where m kg  is the mass of the particle and g is the acceleration due to gravity – In the CIE A Level Mechanics modules the value of is approximated to , unless told otherwise
  • friction acts parallel to the surface in the opposite direction to motion
  • the normal reaction acts perpendicular to the surface (and friction)

Worked Example

Exam Tip