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Pressure

A.  Pressure is related to the word press and refers to a force pushing on a surface.

B.  Force and pressure are related, but they are not the same thing.

C.  Pressure is equal to the force exerted on a surface divided by the total area over which the force is exerted.

1.  P = F(N) / A(m2)

2.  The SI unit of pressure is called the pascal (Pa).
3.  One newton of force on one square meter is one pascal of pressure (1 N/ 1 m2  = 1 Pa)

D.  Pressure in fluids

1.  A fluid is a substance that can easily flow.

2.  Fluids can change shape

3.  Liquids and gases are both fluids.

4.  Fluid pressure is the force exerted by the fluid divided by the area over which the force is exerted

5.  Fluids exert pressure against the surfaces they touch.

a.  Fluids are made up of tiny particles called molecules.

b.  In fluids, molecules are constantly moving in all directions.

c.  As each molecule collides with a surface, it exerts a force on the surface.

d.  All the forces exerted by the individual molecules add together to make up the pressure exerted by the fluid.

6.  Air in Earth’s atmosphere is also a fluid.

a.  The pressure exerted by this gas is called air pressure or atmospheric pressure.

b.  Each cubic meter of air around you has a mass of about 1 kilogram.

c.  Air pressure is great because the atmosphere is over 100 km high.

d.  In a fluid that is not moving, pressure at any point is exerted equally in all directions.

e.  Equal and opposite pressures balance each other.

f.  You are not crushed by the weight of the atmosphere because it pushes against you equally in all directions, inside and out.

E.  Pressure changes as altitude changes.

1.  Air pressure decreases as elevation increases.

2.  Air pressure increases as elevation decreases.

F.  Pressure changes as depth changes

1.  Water pressure increases as depth increases.

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