Clara sketches the forces on a book lying on a table. She draws the table legs, the wood grain, the book's cover. The forces get lost in the picture.
Physicists strip such a picture down to one standard drawing. Here it is.
The book becomes a plain box. A dot would do just as well. The table is not drawn at all.
Only the forces acting on the book are drawn: its weight, straight down, and the normal force from the table, straight up.
Each force is one arrow. The arrow's tail sits at the box, and the arrow points away from the box, in the force's direction.
Each arrow carries a label naming its force.
A drawing like this is called a free-body diagram: one body, freed from its scene, with only the forces on it.
Demi's mug rests on a desk. Which forces belong on the mug's free-body diagram?
On a free-body diagram of a bicycle, how is the bicycle itself drawn?
One rule needs care. The book also presses on the table. But that force acts on the table, not on the book.
So it never appears on the book's free-body diagram. A force the object exerts belongs on the other object's diagram.
Now consider Yusuf pressing a box against a wall. The box presses on the wall, and the wall presses back on the box.
The box's own diagram carries the force from the wall on the box. The force from the box on the wall belongs on the wall's diagram.
A soup can rests on a pantry shelf. Which forces belong on the can's free-body diagram?
Ask one question of every force: does it act on the can?
The can's weight acts on the can, so it is drawn.
The normal force from the shelf acts on the can, so it is drawn.
The can also presses on the shelf. But that force acts on the shelf, so it is not drawn.
Two arrows: the weight down, and the normal force from the shelf up.
Yusuf holds a box pressed against a wall. One force in the scene does NOT belong on the box's diagram. Which one, and where does it belong?
The box presses on the wall.
That force acts on the wall, not on the box.
So it belongs on the wall's free-body diagram.
The force from the box on the wall; it goes on the wall's diagram.
Yusuf pushes a stalled car from behind. Which force belongs on the car's own free-body diagram?
On any free-body diagram, how is each force drawn?
Demi leans a surfboard against a fence, and the surfboard presses on the fence. On which free-body diagram does the force from the surfboard on the fence appear?
Type the name of the standard drawing that shows an object as a dot or box, with a labeled arrow for each force acting on it.