How to draw a free-body diagram

Choose one object.

Imagine a box pulled horizontally across a level floor. Choose the box as your object. Draw a dot or a simple box, then draw an arrow for each external force acting on it. Do not include forces the box exerts on other objects.

• Weight: the force of gravity, downward.
• Normal force: the floor’s contact force, upward.
• Applied pull: in the direction of the horizontal pull.
• Kinetic friction: opposite the box’s sliding motion relative to the floor.

Velocity and acceleration are not additional forces. They can be noted separately, but do not add them to the force arrows.

Choose axes and write the force equations.

Take right as positive x and up as positive y. For this example, the pull is entirely horizontal and the box has no vertical acceleration.

Horizontal: F_pull − f_k = ma_x
Vertical: N − mg = 0

The vertical equation gives N = mg here. That equality is a result of these assumptions; it is not true for every contact situation. An angled pull, for example, changes the vertical balance.

A worked example.

A 5 kg box slides right while being pulled with a horizontal force of 20 N. Kinetic friction is 5 N to the left. The floor is level, the box stays on it, and there are no other external forces. Use g = 9.8 m/s².

1. Find the weight. mg = 5 × 9.8 = 49 N downward.
2. Find the normal force. With zero vertical acceleration, N = 49 N upward.
3. Find the net horizontal force. 20 − 5 = 15 N to the right.
4. Use Newton’s second law. a = 15 / 5 = 3 m/s² to the right.

The units check: a newton is a kg·m/s², so dividing newtons by kilograms gives m/s².

Two common mistakes.

Adding an “ma force.” The quantity ma equals the net force; it is not another force to draw. Adding it as an extra arrow counts the effect twice.

Always drawing friction opposite the pull. Kinetic friction opposes relative sliding. Static friction opposes the tendency to slip and adjusts up to its limiting value; it is not automatically equal to its maximum.

Try a change.

Keep the 5 kg mass, 5 N kinetic friction, and all the other assumptions, but increase the horizontal pull to 30 N. What is the horizontal acceleration? Does the normal force change?

Apply the diagram to your course problem.

Before calculating, check the chosen object, each force’s source, and your sign convention. If the model is unclear, one-on-one physics tutoring gives you time to work through those decisions.

Further reading: OpenStax, University Physics Volume 1, §6.1: Solving Problems with Newton’s Laws. The example on this page was prepared for this guide.