Free-Body Diagrams
A diagram isolating one body and showing every external force acting on it, used to write the equations of motion.
What is Free-Body Diagrams?
A diagram isolating one body and showing every external force acting on it, used to write the equations of motion.
Key formula / rule: One body per diagram; only external forces appear.
Key points
- Construct free-body diagrams for bodies on planes, inclines and in lifts.
- Analyse the normal reaction in accelerating systems.
Common exam trap
Drawing action-reaction pairs on the same body.
Definitions
- Term
Free-Body Diagrams
- Meaning
A diagram isolating one body and showing every external force acting on it, used to write the equations of motion.
- Term
Free-Body Diagrams — explanation
- Meaning
A disciplined free-body diagram is the single highest-return habit in mechanics. Each body gets its own diagram, only external forces appear, and the axes are chosen along the acceleration.
Learning objectives
Construct free-body diagrams for bodies on planes, inclines and in lifts.
Analyse the normal reaction in accelerating systems.
Formulae
- Key point
One body per diagram; only external forces appear.
- Key point
Choose axes along the acceleration to reduce components.
- Key point
Normal reaction is not always equal to mg.
Prerequisites
PHY-U1-NLM-T1-S1-C2
PHY-U1-KIN-T2-S1-C2
Common mistakes
Drawing action-reaction pairs on the same body.
Assuming N = mg on inclines or in accelerating lifts.
Keywords
Free-Body
Diagrams
Practice preview
Which of the following forces should NOT be included in a Free-Body Diagram for a single object?…
easy
A block is placed on a frictionless inclined plane making an angle theta with the horizontal. Which forces are correctly shown in its Free-Body Diagram?…
medium
What is the primary purpose of drawing a Free-Body Diagram (FBD) in mechanics?…
easy