PRESSURE

Depends on Force and Area

Pressure

• Describes how a force is spread over an area.

• It is a measure of how much force is acting on a certain area.

Pressure (cont.)

• When a cat lies down on your lap, the force of the cat’s weight is spread out over a large area of your lap.

• If the cat stands up, all of the force from the cat’s weight is concentrated into its paws.

Increasing Pressure

You can increase pressure in two ways:1.Increase the force

2.Decrease the area of the force

The Model

Pressure = Force / Area

P = F / A

Units: pascal (Pa)

** MATH TIP **

When you see this

symbol / you

always divide!

** MATH TIP **

When you see this

symbol / you

always divide!

Pressure acts in all directions in fluids• Fluids are made of loosely connected particles

that are too small to see.• Their motion is random; they crash into one

another and into anything around.

• As they collide with an object in the fluid, they apply constant pressure to the surfaces of the object.

Pressure in Air

• At sea level the pressure of the air in the atmosphere exerts 2.2 lbs. of pressure on every square centimeter of your body.

• You don’t notice it, though, because your body provides an equal reaction force!

Pressure in Air (cont.)

• Changing Elevation- As you go up, air pressure decreases.

• Changing Density- Air at lower elevations is denser.

As you go up the air pressure on you decreases. Your ears don’t adjust to the pressure change very quickly. You may feel pain or experience “popping” until your eardrum adjusts.

Pressure in Water• Water exerts more pressure on you than air

because it is denser. • As you go deeper, the pressure increases. • At 3300 ft. the pressure of the water would

collapse your lungs if you weren’t wearing special deep sea diving equipment.

Summary• Pressure is a measure of how much force is acting on

a certain area. • You can increase pressure in two ways:

1. Increase the force2. Decrease the area of the force

• P = F / A• Pressure acts in all directions in fluids.• As elevation increases air pressure decreases.• As depth in water decreases pressure increases.