Free Body Diagram – Complete Notes, Revision, Important Questions & Downloads
A Free Body Diagram (FBD) is the foundational problem-solving tool in Newton's Laws of Motion. The object of interest is isolated from its surroundings, and every force acting on it — gravity, normal reaction, tension, friction, applied force — is represented as an arrow at its point of application. NEET tests FBDs through multi-body problems, inclined plane force analysis, and Atwood machine setups where correct force identification on each body determines whether the equation of motion is set up correctly. The core procedure: isolate the body, identify all contact and field forces, draw arrows, choose axes, write F_net = ma (or ΣF = 0 for equilibrium).
NEET Weightage — Free Body Diagram
Newton's Laws of Motion (Chapter 4)| NEET Year | Questions from this Topic | Bar | Marks |
|---|---|---|---|
| 2024 | 1 | 4 | |
| 2023 | 1 | 4 | |
| 2022 | 0 | 0 | |
| 2021 | 1 | 4 | |
| 2020 | 0 | 0 | |
| 2019 | 1 | 4 | |
| 6-Year Total (2019–2024) | 2–4 | 8–16 |
FBD procedure: (1) Identify the body of interest. (2) Isolate it — mentally remove all contacts and replace with force arrows. (3) Draw all external forces: gravity (mg downward), normal reaction (perpendicular to surface), tension along string, friction parallel to surface. (4) Choose a coordinate system. (5) Write ΣF = ma along each axis.
Critical rule: Include only forces ON the body. Newton's third law pairs (the forces the body exerts on others) must NOT appear in the FBD of the body. Including reaction forces on the same body is the most common FBD error.
How to Prepare Free Body Diagram for NEET
Master the force identification hierarchy For any body in NEET Physics, systematically check for: (1) Gravitational force — always present, mg downward through the center. (2) Normal force — present at every solid surface contact, perpendicular to the surface and away from it. (3) Tension — along the string, toward the pulley or connecting point. (4) Friction — at surface contacts where relative motion (or tendency) exists, parallel to surface and opposite to motion (kinetic) or tendency (static). (5) Applied external force — direction and magnitude given in the problem. (6) Buoyancy — only in fluid problems. Never skip step (1) and step (2). Normal force direction is NOT always vertical — it is always perpendicular to the contact surface.
Practice FBDs for the five canonical NEET configurations Five configurations appear in virtually every Newton's Laws NEET question: A) Block on a horizontal surface with applied force at an angle. B) Block on a smooth or rough inclined plane. C) Two blocks in contact on a surface. D) Two blocks connected by a string (horizontal). E) Atwood machine (masses over a pulley). For each, draw the FBD of EVERY body separately. Write equations: ΣF_x = ma_x, ΣF_y = ma_y. Solve for the unknown. Verify: acceleration should be in the direction of net unbalanced force. If all forces balance, a = 0 (equilibrium).
Sign convention discipline across multiple bodies For multi-body problems: choose a consistent positive direction for the entire system (usually the direction of acceleration). For a two-block system pulled by force F, if both blocks accelerate rightward at 'a', write F − T = m₁a for block 1 and T = m₂a for block 2. Tension T appears as a forward force on the back block and a backward force on the front block — consistent with Newton's third law. Never mix sign conventions within the same problem. For Atwood machines: one block's upward direction = positive for that body, and the other block's downward direction = positive for that body, so both equations have the same 'a'.
Study Materials — Free Body Diagram
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Rapid Revision — Free Body Diagram
Concept → Trap → Example1) Definition and Core Procedure
CoreFBD: The object of interest is isolated from its surroundings and the interactions between the object and the surroundings are represented in terms of forces. Choose the axes and write equation of motion. Five-step procedure: (1) Choose the body. (2) Isolate it. (3) Draw ALL external forces on it (not the forces it exerts on others). (4) Set up coordinate axes (usually along and perpendicular to acceleration direction). (5) Write F_net = ma for each axis.
- Only include forces ACTING ON the body. The force the body exerts on another object (Newton's third law pair) must appear in that other object's FBD, not in this one. Including both action and reaction on the same body is the single most common FBD error in NEET problems.
- Normal force is ALWAYS perpendicular to the contact surface. On a horizontal surface, it's vertical. On an inclined plane at angle θ, it acts at angle θ from the vertical (i.e., perpendicular to the slope). On a wedge, the normal from the wedge on the block is perpendicular to the wedge surface.
- NEET application: A block on a horizontal surface with an applied force F at angle θ upward. FBD of block: (1) mg downward. (2) Normal N upward (perpendicular to surface = vertical here). (3) Applied force F at angle θ (components: F cosθ horizontal, F sinθ upward). (4) Friction f horizontal (opposing motion). Equation along vertical: N + F sinθ − mg = 0 → N = mg − F sinθ. Along horizontal: F cosθ − f = ma.
2) FBD for Multi-Body and Pulley Systems
High PriorityFor multi-body problems: draw a separate FBD for EACH body. Connect the bodies through Newton's laws: the tension in a string is equal and opposite on the two bodies it connects. Acceleration of bodies connected by an inextensible string over a frictionless pulley is the same in magnitude (constraint equation). For Atwood machine: draw FBD for m₁ (lighter, going up): T − m₁g = m₁a. FBD for m₂ (heavier, going down): m₂g − T = m₂a. Solve simultaneously.
- For two blocks A and B on a surface connected by a string, pulled by force F on B: FBD of B: F (forward) − T (backward, string pulls B) = m_B × a. FBD of A: T (forward, string pulls A) = m_A × a. Solving: a = F/(m_A + m_B); T = m_A × F/(m_A + m_B). The string tension is always LESS than the applied force when masses are positive.
- For three blocks in contact (no string): the mutual contact force between blocks 1 and 2 equals m_contact × a = (m₂ + m₃) × F/(m₁ + m₂ + m₃). The contact force between the front block and the middle block equals m₃ × F/(m₁ + m₂ + m₃). These contact forces must appear in BOTH FBDs (as an equal and opposite pair).
- NEET trap for Atwood machine: when the pulley is inside a lift accelerating at 'a', replace g with g_eff = g ± a in all Atwood machine formulas. This is because the FBD of each mass in the lift frame includes a pseudo force if analyzed in the non-inertial frame, or the weight term changes in the inertial frame analysis.
3) FBD on Inclined Plane and Non-Inertial Frames
ApplicationOn an inclined plane at angle θ: align ONE axis along the incline (positive direction = down the incline or up, depending on motion) and the other perpendicular to it. Forces: mg along (mg sinθ down the slope) and perpendicular (mg cosθ into surface). Normal N = mg cosθ. Acceleration along slope = g sinθ (frictionless). For non-inertial frame (lift or accelerating surface): add a pseudo force = ma_frame opposite to the frame's acceleration direction.
- Inclined plane FBD axis choice: always resolve gravity into components along and perpendicular to the incline. Along incline: mg sinθ (down the slope). Perpendicular to incline: mg cosθ (into the slope). Normal force N = mg cosθ (perpendicular equilibrium). Net force along incline = mg sinθ (frictionless) → a = g sinθ.
- Block on an accelerating inclined plane (incline has horizontal acceleration b): in the lab frame, draw FBD with N (perpendicular to incline), mg (downward). The block's actual acceleration has components both along and perpendicular to the incline. Along incline: m × a_along = mg sinθ − mb cosθ → a_along = g sinθ − b cosθ. Condition for block at rest on incline: a_along = 0 → b = g tanθ.
- Non-inertial frame (accelerating lift): draw FBD of block inside lift. If the lift accelerates upward at 'b', add pseudo force mb downward on the block (opposite to lift's acceleration). This transforms the non-inertial problem into an effective-g problem where g_eff = g + b. For an inclined plane inside such a lift: effective component along incline = (g + b) sinθ.
US Curriculum Gaps — Free Body Diagram
Topics in this section are tested in NEET but organised differently in standard US physics courses.FBD in Non-Inertial Frames with Pseudo Force (AP Physics C Gap)
AP Physics 1 covers FBDs in inertial frames. NEET regularly requires FBDs drawn in non-inertial frames (accelerating lifts, accelerating inclined planes, rotating frames) where a pseudo force (= mass × frame acceleration, opposite to frame's acceleration) must be included. AP Physics C: Mechanics covers non-inertial frames briefly, but AP Physics 1 does not. In NEET, questions like 'a block is on an incline inside a lift accelerating upward at a' require the student to either shift to the lab frame or add a pseudo force in the lift frame — a technique not systematically taught in AP Physics 1.
- NEET: block inside lift accelerating upward → pseudo force mb downward → effective g_eff = g + b
- NEET: block on accelerating incline → a_along = g sinθ − b cosθ (lab frame)
- AP Physics 1: FBDs in inertial frames only; non-inertial frames introduced only in AP Physics C
Systematic Force Identification for Multi-Body Contact Problems (AP Gap)
NEET problems routinely involve three or more bodies in contact (blocks stacked, blocks in a line with applied forces) requiring a separate FBD for each body and tracking the contact forces. AP Physics 1 tests this concept but NEET problems have higher complexity: three blocks in a row (A-B-C), force on A, asked for contact force between B and C. Students must write three FBDs and three equations of motion simultaneously. The systematic tabular approach to FBDs for three bodies in contact is emphasized in NEET preparation but not at the same depth in AP Physics 1.
- NEET: three blocks A-B-C on surface, force F on A, find contact force between B and C
- NEET: contact force = (mass of C) × a = m_C × F/(m_A + m_B + m_C)
- AP Physics 1: two-body systems emphasized; three-body FBDs less common
NEET-Style Practice Questions — Free Body Diagram
4 QuestionsPractice Problems — Free Body Diagram
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Physics — Newton's Laws of Motion Revision Checklist
Use this section for quick chapter tracking before mocks, part tests, and final NEET revision.
FAQ — Free Body Diagram
Notes · Downloads · Revision · Important QuestionsWhat is a Free Body Diagram?
What forces should I include in a FBD?
Why should Newton's third law pair forces not both appear in the same FBD?
How is normal force direction determined?
How do I handle FBDs for connected-body problems?
How is FBD used for bodies in equilibrium?
What is the correct approach to FBD on an inclined plane?
When is a pseudo force added to a FBD?
What are the most common FBD errors in NEET?
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