What Is a Free Body Diagram?
A simplified drawing that shows a single object and all the forces acting on it, represented as arrows pointing in the directions the forces act.
A free body diagram (often shortened to FBD) is one of the most powerful tools in physics. When an object is moving — or not moving — multiple forces are often acting on it at the same time. Drawing these forces clearly helps us understand why an object moves the way it does.
The "free" in free body diagram means we isolate the object from its surroundings — we only care about the forces acting on that object, not the forces it exerts on other things.
Think of a free body diagram like a character profile in a game. You zoom in on just one character and list all the effects (forces) acting on them — gravity pulling down, the ground pushing up, friction slowing them down — ignoring everything else in the world for a moment.
Free body diagrams are used by engineers, physicists, and designers to analyse everything from bridges to rockets to the human body.
The Language of Forces: Arrows and Vectors
Forces are vectors — they have both a magnitude (size) and a direction. In a free body diagram, we represent each force as an arrow:
- The direction of the arrow shows which way the force acts
- The length of the arrow represents the relative magnitude (a longer arrow = a stronger force)
- The tail of the arrow starts at the object (or at the point where the force acts)
The size or strength of a force, measured in Newtons (N).
A quantity that has both magnitude and direction. Forces, velocity, and acceleration are all vectors.
In MYP Physics, forces are measured in Newtons (N), named after Sir Isaac Newton. A force of 1 N is roughly the weight of a small apple!
When drawing arrows, consistency matters. If you label one arrow as 10 N and draw it 1 cm long, then a 20 N force should be drawn 2 cm long. This way, your diagram visually communicates the relative sizes of forces.
11 more sections in this topic
Pick this up in your Library: it holds the whole topic, notes, cheatsheet and questions.