Working out a pressure
Because the definition is also a formula, the pressure can be worked out whenever the force and the area are both known.
The problem
A cubical box of weight 400 N is placed on a table. If the area of the bottom of the box is 0.2 m2, find the pressure on the surface of the table beneath the box.
The solution
Normal force on the table = weight of the box = 400 N
Area the force acts on = area of the base = 0.2 m2
pressure = 400 N ÷ 0.2 m2 = 2000 Pa
Why the weight is the force here
The box rests on the table without moving, so the force it presses down on the table with is equal to its weight. Weight acts vertically downwards and the table top is horizontal — so that force is normal to the surface.
A common mistake
Take the area of the face the force acts on, not the whole outside area of the box. Here that is only the base, the face touching the table.
Stand the same box on its smallest face and the area falls, so the pressure rises — even though the weight has not changed at all.