Forces and Free-Body Diagrams
A free-body diagram isolates one object and shows every external force acting on it as an arrow. Each arrow has to come from somewhere you can point to: Earth, a surface, a rope, a hand. If you can't name the source, the arrow doesn't belong on the diagram.
The trap is adding a force just because something is moving: a "force of the throw," a "force of motion," an inertia force pushing the object along. None of those have an external agent, so none of them belong on the diagram. If an arrow cannot trace back to something touching the object, it is not real.
The work
3 ways in · any order
Lesson
Forces and Free-Body Diagrams
›
Build a free-body diagram for each situation by toggling the forces you think are present. The ten-scenario applet runs each prediction against actual physics and tells you which arrows are real, which are missing, and which are phantoms.
Diagnostic
10-item topic check
›
Ten multiple-choice items that probe how you read and build force diagrams: identifying the agent behind each arrow, distinguishing real forces from "force of motion" thinking, and reasoning from a diagram to the net force.
Targeted Practice
Drill a single misconception
›
Pick one of the failure modes you've missed and grind it on its own. The round is adaptive: two correct in a row clears the misconception and you move on.