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Physics
Free Body Diagrams
Forces, Normal Force, Friction, and Tension
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A free body diagram (FBD) is a simple sketch that shows a single object with arrows representing every force acting on it. It is one of the most powerful tools in physics because it forces you to identify all forces before writing equations. Without an accurate FBD, even experienced physicists make errors in multi-step problems.
The key rule is to include only forces acting on the object - not forces the object exerts on other things. Each arrow should point in the direction the force acts and be labeled clearly. Once drawn, the diagram tells you directly which direction the net force points, which is the direction of acceleration.
Key Facts
- Weight (gravity) always points straight down:
- Normal force is perpendicular to the contact surface - not always vertical.
- Friction opposes the direction of motion (kinetic friction) or impending motion (static friction).
- Tension in a rope acts along the rope, away from the object.
- For equilibrium (constant velocity or at rest): of forces in every direction .
- On an incline, decompose weight into components parallel and perpendicular to the slope.
Vocabulary
- Free body diagram
- A diagram showing only the object of interest and all external forces acting on it as arrows.
- Normal force
- The contact force perpendicular to a surface that prevents objects from passing through each other.
- Friction force
- A contact force parallel to a surface that opposes relative sliding motion.
- Tension
- A pulling force transmitted through a string, rope, or cable.
- Equilibrium
- A state in which the net force on an object is zero, so it moves at constant velocity or stays at rest.
Common Mistakes to Avoid
- Including forces the object exerts on other things. Only include forces acting on the object being analyzed.
- Drawing the normal force as always vertical. On an inclined surface the normal force is perpendicular to that surface.
- Leaving out friction when an object is sliding. Kinetic friction always opposes the direction of motion.
- Using weight and normal force interchangeably. They are equal in magnitude only on flat, horizontal surfaces with no vertical acceleration.
Practice Questions
- 1 Draw a free body diagram for a 10 kg box sitting on a horizontal table. Label all forces and calculate the normal force.
- 2 A 5 kg block is on a frictionless 30° incline. Draw the FBD and find the acceleration down the slope.
- 3 A picture hangs from two wires that each make a 40° angle with the ceiling. Draw the FBD and write equations for equilibrium.