How Do You Draw and Interpret a Free-Body Diagram?

SL and HL: To draw a free-body diagram, isolate one body and represent every external force acting on it as a labelled arrow. To interpret it, add the force vectors to determine the resultant force, then apply Newton's second law, $\sum \vec{F}=m\vec{a}$.

Drawing the Diagram

1. Choose the body or system. Represent it as a dot or simple box. Do not include surrounding objects unless they are part of the chosen system.
2. Identify every external force acting on that body. Each arrow begins at the body and points in the force's direction.
3. Label forces using standard symbols, such as weight $F_g=mg$, normal force $F_N$, tension , friction , or drag .

Interpreting the Diagram

Resolve angled forces into components, then calculate the net force along each axis. For example, if a $5.0\,\text{kg}$ box experiences $20\,\text{N}$ rightward and $8.0\,\text{N}$ leftward, then

$$F_{\text{resultant}}=20-8.0=12\,\text{N}$$

so

$$a=\frac{F_{\text{resultant}}}{m}=\frac{12}{5.0}=2.4\,\text{m s}^{-2}$$

rightward. If the resultant force is zero, acceleration is zero; the body may be stationary or moving with constant velocity.

A common misconception is that a free-body diagram shows motion. It shows forces, not velocity, and arrow lengths should indicate relative force magnitudes where possible. Newton's third-law pair forces must not appear on the same diagram because they act on different bodies.

Exam Technique

For an IB Draw question, include only forces acting on the selected body and label each arrow clearly. For Determine or Explain, state the force balance, use signs consistently, and connect the resultant force to acceleration through Newton's second law.