Newton's laws and free-body diagrams that make sense
Nearly every force problem is won or lost at the free-body diagram. Noeta insists on the diagram before the algebra.
What this covers
- The three laws of motion
- Free-body diagrams
- Friction, normal force, tension
- Inclined planes
- Connected bodies and pulleys
Worked example
Problem
A 5 kg block on a frictionless table is pulled with 20 N. Find its acceleration.
- 1Draw the forcesWeight down, normal force up (they cancel), applied force 20 N horizontally.
- 2Net forceOnly the 20 N is unbalanced, so F_net = 20 N.
- 3Apply F = maa = F/m = 20/5.
Answer: 4 m/s² in the direction of the pull
Mistakes that cost marks
- Adding a phantom 'force of motion' in the direction of travel.
- Assuming the normal force always equals mg, even on an incline.
- Mixing up action-reaction pairs that act on different bodies.
Questions students ask
Why is my free-body diagram always wrong?
Usually because a force with no physical source is drawn. Every arrow needs a named agent touching or attracting the object.
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