Free Body Diagrams
Higher Tier Only
- Free body diagrams are useful for modelling the forces that are acting on an object
- Each force is represented as a vector arrow, where each arrow:
- Is scaled to the magnitude of the force it represents
- Points in the direction that the force acts
- Is labelled with the name of the force it represents
- Free body diagrams can be used:
- To identify which forces act in which plane
- To resolve the net force in a particular direction
Free body diagrams can be used to show the various forces acting on objects
Worked example
Draw free-body diagrams for the following scenarios:
a) A picture frame hanging from a nail
b) A box sliding down a slope
c) A man fishing in a stationary boat
d) A car accelerating along a road
Part (a)
- The size of the arrows should be such that the 3 forces would make a closed triangle as they are balanced
Part (b)
-
- There are three forces acting on the box
- The normal contact force, R, acts perpendicular to the slope
- Friction, F, acts parallel to the slope and in the opposite direction to the direction of motion
- Weight, W, acts down towards the Earth
Part (c)
- As the boat is not moving, the size of both arrows must be the same
Part (d)
- As the car is accelerating, the size of the thrust must be larger than the size of the friction force
- As in part (c), the upwards and downwards forces must be equal
