BeginnerTopic 4 of 106Engineering Mechanics
Free-body diagrams
Isolate a body and draw every external force and moment acting on it before writing equations.
In depth
An FBD isolates a body and shows all external forces and moments, with axes and dimensions. Replace supports by their reactions and show weights at centres of gravity. Equilibrium or motion equations are written from the diagram.
Key points
- Draw the body alone
- Include every contact and body force
- Label knowns and unknowns
ExampleFor a ladder against a wall draw weight, wall normal, floor normal and friction.
What is Free-body diagrams?
Isolate a body and draw every external force and moment acting on it before writing equations.
Why is it important to understand free-body diagrams?
A free-body diagram is the most important habit in mechanics, because a wrong diagram gives a wrong answer however good the algebra is.
What should I include in a free-body diagram?
Every external force, including weight, reactions, friction and applied loads, drawn at its point of action. Internal forces are excluded when you isolate the body.
Can I draw the free-body diagram of a whole system?
Yes, and it is often the fastest way to find external reactions before you isolate individual parts.
How is Free-body diagrams used in practice?
Typical applications include analysing the forces on a ladder leaning against a wall, checking the reactions on a bracket bolted to a wall and finding the tension and friction on a block on an incline.
What should I learn before Free-body diagrams?
Study "Mass vs weight" first (topic 3 of 106 in Engineering Mechanics). It covers the groundwork this topic builds on.
What are the key points of Free-body diagrams?
Draw the body alone. Include every contact and body force. Label knowns and unknowns.
Can you give an example of Free-body diagrams?
For a ladder against a wall draw weight, wall normal, floor normal and friction.
What comes after Free-body diagrams?
The next topic is "Resolving forces into components", which builds directly on this one.
How difficult is Free-body diagrams?
It is classed as Beginner in Engineering Mechanics (topic 4 of 106). It is an entry-level topic. It relies only on basic school mathematics and the ideas introduced earlier in this discipline.