A Free-Body Diagram (FBD) is a visual tool used in physics and engineering to analyze the forces acting on a single object. It simplifies a complex physical situation by isolating the object—”freeing” it from its surroundings—and representing all external forces as vectors (arrows) acting on it. This video provides an introduction to free body diagrams: Introduction to Free Body Diagrams Flipping Physics YouTube · Nov 13, 2014 Key Features and Components
The Object: The object of interest is usually represented by a simple dot or box, often called the “body” or center of mass.
Forces as Vectors: Arrows are drawn originating from the center of the object to represent each force.
Direction: The arrow points in the direction the force acts.
Magnitude: The relative length of the arrow represents the strength of the force.
Labels: Each force arrow is labeled to indicate its type (e.g., Fgravcap F sub grav end-sub for gravity, Fnormcap F sub norm end-sub for normal force, Ffrictcap F sub frict end-sub for friction, Ftenscap F sub tens end-sub for tension). Common Forces Included Gravity ( Fgravcap F sub grav end-sub
or mg): Always acts downward toward the center of the Earth. Normal Force ( Fnormcap F sub norm end-sub
): Acts perpendicular to the surface an object is resting on. Tension ( Ftenscap F sub tens end-sub or T): A pulling force exerted by a string, rope, or cable. Friction ( Ffrictcap F sub frict end-sub ): Acts parallel to the surface and opposes motion.
This video explains how to draw a free body diagram with examples: Drawing Free-Body Diagrams With Examples The Physics Classroom YouTube · Nov 2, 2019 Purpose and Benefits
Visualization: Helps clarify which forces are acting on an object and in which direction. Problem Solving: Used to calculate net forces (
) and analyze equilibrium (where forces balance) or acceleration.
Simplification: Allows focusing on a single object at a time rather than complex, interacting systems. Example: A Box Resting on a Table Isolate the object: Draw a box (the book). Identify Vertical Forces: The force of gravity ( Fgravcap F sub grav end-sub ) pulls it down. The table pushes up with a normal force ( Fnormcap F sub norm end-sub
Draw the Diagram: A dot in the middle, an arrow pointing down labeled Fgravcap F sub grav end-sub , and an equal-length arrow pointing up labeled Fnormcap F sub norm end-sub
Result: The arrows are the same length because the book is not accelerating, meaning forces are balanced.
If you’re interested, I can walk through an example of a more complex problem, like an object on an inclined plane or with friction. Introduction to Free Body Diagrams