Force Diagrams (AQA A Level Maths: Mechanics)

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 (RN)

• 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 – usually g = 9.8 m s^-2
    • If you are told, or are using g = 9.8 m s^-2  ensure you round your final answer to two significant figures (as 9.8 is rounded to 2 s. f.)
    • If you are told to use g = 10 m s^-2 ensure you round your final answer to one significant figure (as 10 is rounded to 1 s. f.)
  • friction acts parallel to the surface in the opposite direction to motion
  • the normal reaction acts perpendicular to the surface (and friction)