Free-Body Diagrams
A free-body diagram shows every force acting on one object.
About this topic
A free-body diagram isolates one object and shows every force on it as an arrow from its centre, labelled with the type and size: weight pointing down, normal force perpendicular to a surface, tension, friction, applied force and air resistance.
Do not include forces the object exerts on others or forces that are not actually present, such as a mysterious force of motion. Draw arrows with lengths roughly proportional to the sizes. Then choose axes, often along a slope, resolve forces into components, and apply F = ma in each direction. If the forces are balanced the object is in equilibrium.
Example: a box sliding down a ramp has weight pointing straight down, a normal force perpendicular to the ramp surface, and kinetic friction pointing up the ramp. A book resting on a table has just two arrows, weight down and normal force up, equal in size. Engineers use such diagrams for bridges, cranes and aircraft.
Tip: the most frequent error is drawing the force of the push that started the motion after it has ended. Only forces acting right now count. Another is adding arrows for forces on other objects in a pair.
Key ideas
- Draw the object as a dot
- Arrows show force direction and size
- Sum components to find the net force
Quick check
Use cases: where Free-Body Diagrams helps
- Homework: review the key idea behind Free-Body Diagrams before you start a problem set.
- Exam revision: use the quick check to test whether you can recall Free-Body Diagrams without notes.
- Visual learning: play with the interactive lab to see how Free-Body Diagrams changes when you adjust its inputs.
- Teaching a friend: explain Free-Body Diagrams in your own words, then compare your version with the key ideas.
The same laws describe car crashes, sports, satellites and everyday machines.
Frequently asked questions
What is Free-Body Diagrams?
A free-body diagram shows every force acting on one object.
What will I learn in Free-Body Diagrams?
Draw the object as a dot. Arrows show force direction and size. Sum components to find the net force.
Is Free-Body Diagrams part of Physics?
Yes. It is one of 150 Physics topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the Free-Body Diagrams lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study Free-Body Diagrams?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
Do I need an account, and is my progress saved?
No account is needed. Marking a topic as learned is saved in your own browser, so it stays on that device and is not sent to a server.