Force diagrams and free body diagrams

1. Understanding Force Diagrams

• Force diagrams and free body diagrams help visualise the forces acting on an object.
• Key points to consider when analysing forces:
  • Identify and name the forces.
  • Specify which object exerts a force on which object.
  • Indicate the direction of each force.

2. Force Diagrams vs. Free Body Diagrams

Force Diagram

• The object is drawn as a block, with all acting forces represented as arrows.
• Arrows start at the point of application.

Free Body Diagram

• The object is drawn as a dot, and forces acting on it are arrows pointing away.
• Useful for analysing forces individually.
• For objects on an inclined plane, the gravitational force is broken into two components:
  • Parallel component $(Fg​sinθ):$ Acts down the slope.
  • Perpendicular component $(Fg​cosθ):$ Acts into the surface.

3. Force Symbols in Diagrams

Symbol	Force Type	Description
$F_g$​ $or$ $w$	Gravitational force (weight)	Acts downwards towards Earth's centre.
$F_N$	Normal force	Acts perpendicular to a surface.
$F_f$	Frictional force	Opposes motion between surfaces.
$F_T$ ​ $or$ $T$	Tension	Force in a rope, cable, or string.
$F_{app}$	Applied force	Exerted by a person or another object.

4. Steps to Draw Force and Free Body Diagrams

Example: A person pushes a car along a rough, flat road.

1. Draw the object (block for force diagram, dot for free body diagram).
2. Identify forces acting on the object.
3. Draw an arrow for weight $(F_g​)$, acting downwards.
4. Draw an arrow for normal force $(F_N​)$, acting upwards.
5. Draw an arrow for applied force $(F_{app}​)$, in the direction of motion.
6. Draw an arrow for friction $(F_f​)$, opposite to motion.
7. If using a free body diagram, erase the surface and leave only the force arrows.

6. Key Takeaways

• Free body diagrams simplify force analysis.
• Inclined plane problems require breaking weight into components.
• Direction of friction is always opposite to motion.