Rowan draws the free-body diagram of a book on a table. He is tempted to add one more arrow: the book's downward force on the table.
The diagram is for the book. Does the arrow belong?
No. A free-body diagram carries only the forces ON its object. Interacting objects therefore get one diagram each, and each interaction force lands on exactly one of them.
Here is the book's diagram: the Earth's downward gravitational force on the book, and the table's upward push on the book.
And here is the table's diagram: the book's downward push on the table, the table's own weight, and the floor's upward normal force on its legs.
The table's upward push appears on the book's diagram; the book's downward push appears on the table's.
That is the placement rule: each force appears on one diagram only, the diagram of the object it acts on.
A mallet drives a tent stake. On which object's free-body diagram does the force 'the mallet's downward force on the stake' appear?
The two forces of a third-law pair therefore never share a diagram. One lands on each object's diagram.
A quick check for any arrow: name the force's acted-on object. If that object is not the diagram's object, the arrow is on the wrong diagram.
A heavy toolbox rests on a workbench. Which force belongs on the WORKBENCH's free-body diagram?
Tilda draws separate diagrams for a trailer and the car towing it. She puts the car's forward force on the trailer's diagram, and the trailer's backward force on the car's diagram. Is her placement correct?
A lamp hangs from a light cord fixed to the ceiling. Draw up the force lists for the lamp's diagram and the cord's diagram.
The lamp's diagram lists the forces ON the lamp: the Earth's downward gravitational force on the lamp, and the cord's upward tension force on the lamp.
The cord's diagram lists the forces ON the cord: the lamp's downward force on the cord, and the ceiling's upward force on the cord (the cord's own weight is small enough to ignore here).
The lamp-and-cord interaction contributes one force to each diagram, never two to one.
Lamp's diagram: gravitational force down, cord's tension up. Cord's diagram: lamp's force down, ceiling's force up.
On which object's diagram does the force 'the nail's upward force on the hammer' belong?
Name the acted-on object: the hammer.
A force lands on the diagram of the object it acts on.
On the hammer's diagram.
A potted plant rests on a stool. A student's diagram of the STOOL shows four arrows: (1) the plant's downward force on the stool, (2) the stool's weight, (3) the floor's upward normal force on the stool, (4) the stool's upward force on the plant. Which arrow does not belong?
A horseshoe magnet holds a steel ball beneath it, and the ball hangs still in the air. Which set of forces is the ball's complete free-body diagram?
Which statement about the two forces of a third-law pair on free-body diagrams is correct?