At sea level atmospheric pressure is:

1 atm = 1.013 x 10⁵ Pa

This is an average value, and the pressure fluctuates somewhat. In good weather the pressure is higher than average and in bad weather it is lower than average.

Every square meter feels a force of over 100,000 N from the weight of all the air above it. This is a huge force, so why don't things (including ourselves) collapse from the force?

1. There's usually air on all sides of an object, so for the most part the forces balance.
2. The force just comes from air, which has such a small density it has very little effect.

The first answer is correct - the air generally applies forces from all sides and, for the most part, they cancel one another. As we will soon see there is actually a net upward force exerted on an object but, when the fluid is air, this force is usually very small.

Its low density notwithstanding, air does apply a force of over 100,000 N on each square meter of area. We can see the effect of this force by removing the air from inside a hollow object - then there's no outward force from air on the inside to balance the inward force from the air on the outside. This can have a dramatic, and rather damaging, effect on an object.

A container, closed on the right side but open to the atmosphere on the left, is almost completely filled with water, as shown. Three points are marked in the container. Rank these according to the pressure at the points, from highest pressure to lowest.

1. A = B > C
2. B > A > C
3. B > A = C
4. C > B > A
5. some other order

A static fluid is one that is at rest. Two very important things to remember about pressure in a static fluid are:

1. Pressure increases with depth.
2. Two points at the same vertical position experience the same pressure, no matter what the shape of the container.

How does pressure depend on depth? If point 2 is a vertical distance h below point 1, and the pressure at point 1 is P₁, the pressure at point 2 is:

P₂ = P₁ + ρgh

Point 2 does not have to be directly below point 1 - all that matters is the vertical distance. Any horizontal displacement is irrelevant.

Click and drag the sensors to measure pressure. The red sensor measures correctly only in air; the purple one measures correctly only in the water.