Free Body Diagram

Forces
representation

Also known as: FBD, force diagram

Grade 9-12

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A simplified diagram that isolates a single object and represents all external forces acting on it as labelled arrows originating from the object's centre of. Drawing a free body diagram is the essential first step in solving any force or motion problem.

Definition

A simplified diagram that isolates a single object and represents all external forces acting on it as labelled arrows originating from the object's centre of.

💡 Intuition

A simplified picture that shows every push and pull acting on one isolated object.

🎯 Core Idea

Isolate one object and show all external forces acting on it.

Example

A box on a ramp: draw arrows for weight (down), normal (perpendicular to ramp), friction (up the ramp).

Notation

Each force arrow is labelled (e.g., \vec{W} for weight, \vec{N} for normal force, \vec{f} for friction, \vec{T} for tension). Arrow length represents relative magnitude. A coordinate system is typically chosen to simplify calculations.

🌟 Why It Matters

Drawing a free body diagram is the essential first step in solving any force or motion problem. Without one, students commonly miss forces or include incorrect ones, leading to wrong answers in Newton's law calculations.

💭 Hint When Stuck

When drawing a free-body diagram, first identify the object of interest and draw it as a simple dot or box. Then systematically add every force acting on it: weight (always down), normal force (perpendicular to surface), friction (opposing motion), tension (along the rope), and any applied forces. Finally, label each arrow with its name and magnitude if known.

Formal View

A free-body diagram represents a body as a particle with all external forces \vec{F}_1, \vec{F}_2, \ldots, \vec{F}_n drawn as vectors from the body. Newton's second law is then applied: \sum_{i=1}^{n} \vec{F}_i = m\vec{a}.

🚧 Common Stuck Point

Only draw forces ON the object, not forces the object exerts on others.

⚠️ Common Mistakes

  • Including forces the object exerts on other objects — a free-body diagram shows only forces acting ON the chosen object, not forces it exerts on others.
  • Forgetting the normal force or friction when an object is on a surface — both are almost always present for objects in contact with surfaces.
  • Drawing net force as a separate arrow alongside individual forces — the net force is the result of adding all forces, not an additional independent force.

Frequently Asked Questions

What is Free Body Diagram in Physics?

A simplified diagram that isolates a single object and represents all external forces acting on it as labelled arrows originating from the object's centre of.

When do you use Free Body Diagram?

When drawing a free-body diagram, first identify the object of interest and draw it as a simple dot or box. Then systematically add every force acting on it: weight (always down), normal force (perpendicular to surface), friction (opposing motion), tension (along the rope), and any applied forces. Finally, label each arrow with its name and magnitude if known.

What do students usually get wrong about Free Body Diagram?

Only draw forces ON the object, not forces the object exerts on others.

Prerequisites

forcevectors

How Free Body Diagram Connects to Other Ideas

To understand free body diagram, you should first be comfortable with force and vectors. Once you have a solid grasp of free body diagram, you can move on to newtons second law and equilibrium.

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