Yusuf sets a toolbox on a workbench. It sits still.
Time to build its free-body diagram: which arrows, which directions, what lengths?
Start with the object: the toolbox becomes a plain box on the page.
Now ask what acts on the toolbox. Earth does: the toolbox's weight, one arrow, straight down, tail at the box.
The bench touches the toolbox. So the bench exerts a normal force on it: one arrow, straight up, tail at the box.
Nothing else touches the toolbox. So there are no other arrows.
A cereal bowl rests on a counter. Which forces appear on its free-body diagram?
Now the lengths. The toolbox stays at rest, so the two forces balance.
Balanced forces are drawn balanced: the two arrows get equal lengths.
The bowl stays at rest on the counter. How should its two arrows compare in length?
That is the whole diagram: weight down, normal force up, equal arrows.
Any object at rest on a horizontal surface gets this same two-arrow diagram.
Which directions do the resting bowl's two arrows point?
A car is parked on a flat driveway. Build its free-body diagram.
The car becomes a box.
Its weight: one arrow, straight down.
The driveway's normal force: one arrow, straight up.
The car is at rest, so the two arrows are drawn equal.
Two arrows: weight down and normal force up, equal in length.
A bag of flour rests on a pantry shelf. Build its free-body diagram.
The bag becomes a box.
Its weight: one arrow, straight down.
The shelf's normal force: one arrow, straight up.
The bag is at rest, so the two arrows are drawn equal.
Two arrows: weight down and normal force up, equal in length.
Hollis builds the free-body diagram of a flowerpot resting on a shelf. Which drawing is correct?
A flowerpot rests on a shelf, and its two arrows are drawn equal in length. Why are they equal?
Demi starts a diagram of a watermelon resting on a picnic table: the box and the weight arrow are drawn. Which one arrow completes the diagram?