Free-Body Diagrams and Common Forces
Seeing the Forces
What is a free-body diagram?
A free-body diagram is a sketch of one object showing only the external forces acting on it. It helps you focus on what matters for Newton’s laws.
Common forces
Common forces include weight , normal force, tension, friction, and spring force. Weight acts downward near Earth’s surface.
Common forces and directions
| Force | Typical direction | Notes |
|---|---|---|
| Weight | Downward | Magnitude |
| Normal force | Perpendicular to surface | Acts from a surface |
| Tension | Along a rope or string | Pulls away from the object |
| Friction | Along surface | Opposes relative motion or attempted motion |
Do not include non-forces
Do not draw velocity or acceleration as forces. Only actual interactions belong on a free-body diagram.
Friction basics
Static friction can adjust up to a maximum value, while kinetic friction often has a roughly constant magnitude. Friction usually acts opposite the direction of slipping or attempted slipping.
Which force is usually perpendicular to a flat surface?
The normal force acts perpendicular to the surface.
Correct answer: Normal force
What is the weight of an object of mass near Earth’s surface?
Weight equals mass times gravitational field strength.
Correct answer: mg
Drawing a free-body diagram
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1
Step 1: Isolate the object.
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2
Step 2: Identify every external interaction.
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3
Step 3: Draw arrows for each force with correct direction.
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Step 4: Label each force clearly.