Chapter 11 Forces in Fluids

 Why don’t you sink into the snow when you wear snow shoes?

Pressure` Pressure is the force pushing on a surface.

Because the size of the area over which the force is distributed has changed.

 Pressure = Force/Area  Pressure is equal to the force exerted on a surface divided by the total area over which the force is exerted.

Units for Pressure     Force is measured in Newton’s (N).

Area is measured in square meters.

SI unit for pressure is N/m 2 which is also called the Pascal (Pa).

1 Pa = 1N/m 2

Fluid Pressure   A fluid is a substance that can easily flow.

Fluids exert pressures on on surfaces they touch.

Fluids are made of molecules.

  Molecules are constantly moving.

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 =  Total force exerted by the fluid divided by the area over which the force is exerted.

Fluid Pressure all around.

 Air Pressure or atmospheric pressure is the pressure exerted on us by the air.

How can we support the weight of the atmosphere?

   Each cubic meter of air has a mass of 1 kg.

How can we support this and not feel a thing?

Because in a fluid that is not moving, pressure at a given point is exerted equally in all directions.

Balanced Pressures As the atmosphere is pushing down on your hand – there is also atmospheric pressure pushing up on our hand.

Remember the can crush lab?

Variations in fluid pressure  Air pressure decreases as elevation increases. Remember air pressure at a certain point results from the weight of air above that point.The higher you go the less weight above you.

Why do your ears pop?

 As the air pressure outside your body adjusts the air pressure inside your body has to adjust too.

 The pressure inside is greater than outside so pressure is released and all is balanced again.

Pressure and depth  Pressure increases as you descend – because there is more water above you and therefore more weight above you.