Free-Body Diagrams

Lesson 6 of 11 · 7 min · Free, no account needed

What you will learn

A free-body diagram shows one object alone, with an arrow for every force on it and nothing else.
Every force needs a source: something touching the object (a push, the normal force from a surface, tension in a rope) or gravity.
If the forces cancel, the object is in equilibrium: it stays still or keeps a steady speed in a straight line.

A car cruises at a steady 100 km/h. Is the engine pushing harder than the air, or exactly the same?
A 1.5 kg book lies on a table. Drag the table's arrow until the book stops shaking.
The grey arrow is the book's weight, 15 N (rounding g to 10 m/s² in this lesson).
The driver sets cruise control. Press Play and watch both arrows.
One object, every force, nothing else: a free-body diagram.
A free-body diagram: draw the object alone and one arrow for each force on it. Longer arrow, bigger force.
A surface pushes out of itself: the normal force. A rope or wire pulls along itself: tension. Gravity pulls down: weight.
If the arrows cancel, the object is in equilibrium: at rest or at a steady speed in a straight line. Simplified: we draw every force from the object's centre.
Build the diagram for a lamp hanging still from its wire.
Game: force detective. Five diagrams, each with fake forces. Tap the fakes.
Three spare taps for the whole game.
Code: a game engine keeps lists of forces for each sprite, across (x) and up-down (y), and adds them every frame. Sort the forces.
A lift rides up at a steady speed. Drag the cable arrow to the right size.
The lift and its passengers weigh 6000 N.
Back to the hook. Your bet against the real cruise. Press Play.

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