Chapter 3: Forces in Fluids

Pressure

What is Pressure?

Pressure refers to a force pushing on a surface.

What are some examples of pressure?

1. Leaning against a wall

2. Standing on the ground

Forces and Pressure

Why does the woman wearing snow shoes stay on top of the snow and the two girls sink into the snow?

Forces and Pressure

The downward force, your weight, exerted on the snow does not change, so what is the difference?

The difference is the size of the area over which your weight is distributed.

When your weight is distributed over the larger area of the snowshoes, there is less downward pressure on the snow under the shoes.

Calculating Pressure

Pressure = Force Area

Force is measured inArea is measured inPressure is measured in

The unit for pressure is also called a Pascal1 N/m2 = 1 Pa

Newtonsm2

N/m2

Fluid Pressure

A fluid is a substance that can easily flow.

A fluid is able to change shape, therefore liquids and gases are both fluids.

Fluids

Fluids exert pressure against the surfaces they touch. In fluids, molecules are constantly moving in all directions and colliding with each other and any surface they meet.

Fluid Pressure

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

All of the forces exerted by the individual molecules in a fluid add together to make up the pressure exerted by the fluid.

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

Fluid Pressure All Around

Pressure exerted by the air is called air pressure or atmospheric pressure.

Air exerts pressure because it has mass. (About 1 kg for every 1 m3)

The force of gravity on the mass of air produces an average air pressure (at sea level) of 10.13 N/cm2.

Balanced Pressures

Why doesn’t the pressure of the air around you crush you?

The reason is because the pressure inside your body balances the pressure outside of your body.

Balanced Pressures

Some parts of your body, such as your lungs, sinus cavities, and inner ear, contain air.

Other parts of your body, such as your cells and blood vessels, contain liquids.

Variations in Fluid Pressure

Have you ever had your ears “pop” when you drive up the side of a mountain? Why?

Air pressure decreases as elevation increases.

Variations in Fluid Pressure

When the air pressure outside of your body changes, the air pressure inside will adjust too, but more slowly. For a moment, the air pressure behind your eardrums is greater than it is outside. Your body releases this pressure with a pop so that the pressures are once again balanced.

Variations in Fluid Pressure

What do you feel when you swim down to the bottom of a deep pool or lake? Why?

Water pressure increases as depth increases.

Variations in Fluid Pressure

The water pressure increases as you go down because there is more water above you. In addition, air in the atmosphere is pushing down on the water. The total pressure below the water results from the weight of the water and air above you.