Free body diagram builder

Worked examples hand you the diagram already drawn. Here you draw it: pick a scenario, add the force arrows you believe act on the object, aim them, size them, and press check. The feedback names the exact force you missed, invented, or misdirected, and tells you the rule that fixes it.

Scenario:

Book at rest on a table. The book just sits there. Draw every force acting on the book, and only forces acting on the book.

Add force:
Add a force from the palette, or select an arrow to set its direction and magnitude. Arrows snap to the compass directions.
net Fx 0.00net Fy 0.00unbalanced

The method the checker is grading

A free body diagram is a contract with yourself: one object, every force acting on it, nothing else. Every dynamics problem on the AP exam starts here, and most lost points trace back to a bad diagram rather than bad algebra. The routine:

  1. Isolate the object. Strip away the table, the rope, the floor, and replace each one with the force it exerts on your object. The diagram shows the object alone with arrows.
  2. Draw only forces acting ON the object. The box pushes down on the table too, but that force acts on the table, so it does not belong in the box's diagram. Forces the object exerts on other things never appear.
  3. Every force is a real interaction with another object. Before you draw an arrow, name what is doing the pushing or pulling: Earth for gravity, a surface for normal force and friction, a rope for tension, a hand for an applied force. If you cannot name the other object, the force does not exist. There is no "force of motion" and no leftover force that keeps things moving.
  4. On an incline, the normal force is perpendicular to the surface. Not straight up. The surface pushes back at 90 degrees to itself, so on a 30 degree ramp the normal force leans 30 degrees away from vertical, and its magnitude drops to Fg cos(30). Gravity, meanwhile, still points straight down; slopes never tilt gravity.

Once the diagram is right, the physics is bookkeeping: add the arrows as vectors and the net force tells you the acceleration through Newton's second law. An object at rest or moving at constant velocity has a diagram whose arrows cancel exactly; an accelerating object has a leftover arrow pointing the same way as its acceleration. The elevator and incline scenarios above are the two places students most often see that leftover arrow for the first time.

Keep going: the free body diagram guide walks the method with worked solutions, the net force guide covers the vector addition the readout above is doing, and the inclined plane simulator lets you tilt the ramp yourself and watch the components trade off.

Embed this simulation on your class site, free

Paste this into any web page, LMS, or class site. It is responsive, works on phones, and the small credit link stays on.