Forces and Free-Body Diagrams
▶︎ Watch it animatedinteractive step-through · ~3 min · optionalA free-body diagram turns a physical situation into Newton's second law. You shrink the object to a single dot and draw one arrow for every force acting on it, each arrow starting at the dot. Get it right and the equations almost write themselves; get it wrong and every later step carries the mistake. The Physics C convention names each force by what acts on what — force on A by B — so every arrow says what is pushing or pulling.
Most broken diagrams fail in one of three ways. A real force gets left off — usually the normal force, friction, or the tension on the far side of a pulley. A force with no source gets added, most often a "force of motion" aimed the way the object travels. Or a single net-force arrow stands in for the separate forces, swapping the answer in for the inputs. The cure is one discipline: name what exerts each force, draw it from the dot, and never draw the net force as one of the arrows.
The work
3 ways in · any order
Lesson
Forces and Free-Body Diagrams
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How to turn a physical setup into a correct free-body diagram. Builds the force-naming convention (force on A by B), the rule that every arrow starts at the dot, and the standard setups for inclines, pulleys, and surfaces in contact. Covers the normal force, friction, and tension, and the difference between a single force and the net force. Closes with a ten-scenario skill check on building and reading free-body diagrams.
Diagnostic
10-item topic check
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Ten items covering the three main mistakes for Topic 2.2: forces left off the diagram, phantom forces added with no source, and one net-force arrow drawn in place of the separate forces. A few items also probe two related traps — that motion needs a force to keep going, and that the net force points the way the object moves. Take it cold to see what is still shaky, or after the lesson to confirm it is not.
Targeted Practice
Drill a single misconception
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Pick one of the mistakes you've missed and drill it on its own. The round is adaptive: two correct in a row clears it and you move on.