The FBD is the single most critical skill in mechanics. Every JEE problem should begin with an FBD.
Step 1: Isolate the body. Mentally separate it from surroundings.
Step 2: Draw ALL forces ON the body:
- Weight (mg): always vertically downward
- Normal (N): perpendicular to contact surface, away from surface
- Tension (T): along the string, pulling away from body
- Friction (f): along the surface, opposing relative motion or tendency
- Applied force: as specified in the problem
Step 3: Choose coordinate axes. For inclines, choose along and perpendicular to the incline. For horizontal surfaces, choose horizontal and vertical.
Step 4: Resolve and apply F = ma along each axis independently.
Common FBD Errors:
- Including forces exerted BY the body (only forces ON it)
- Drawing "ma" as a force (it's the NET result)
- Missing the normal force
- Wrong direction of friction
- Not resolving forces along incline axes
- Forgetting that N != mg on an incline or with angular applied forces
Pro tip: For multi-body systems, draw SEPARATE FBDs for each body. Use constraint equations to relate their accelerations. Then solve the simultaneous equations.