Fluids 101 Chapter 10

Fluids Any material that flows and offers little resistance to changing its shape. –Liquids –Gases –Plasma?

Density An important property of matter is density. Density is defined as mass per unit volume. The symbol is .

Density of water 1.0 g/ml 1.0 g/cm 3 1,000 kg/m 3

Pressure Force per unit area.

Pressure Measured in atm or Pascals ( Pa )

Pressure Standard atmospheric pressure = force per 1 m 2 of air at sea level. 1.01x10 5 Pa or 101 kPa

Pressure Under Water The pressure due to fresh water increases about 1 atm or 100 kPa for every 10 m.

Liquid Pressure Pressure of a liquid at rest depends only on density and depth. Pressure = density x depth

Water pressure Water pressure is greater in the deeper lake.

Pascal’s Principle The force exerted by a fluid on the walls of its container always acts perpendicular to the walls.

Pascal’s Principle When you are under water the water’s pressure pushes in on you from all sides. The force is perpendicular to your body.

Pascal’s Principle

Pascal Vases The pressure of a liquid is the same at any depth regardless of the shape of its container.

Pressure in a Column The pressure in a fluid column can be found using this equation:

Pressure in a Column AP test version of the same equation: - this stands for the initial pressure.

Problem What is the pressure exerted by water at a depth of 45.0 m?

Solution P = (1000 kg/m 3 )(9.8 m/s 2 )(45 m) 441 kPa P = 4.41 x10 5 Pa or 441 kPa

Buoyancy Buoyancy is an upward force that a fluid exerts on an object that is immersed in it. This is called the buoyant force.

Buoyant Forces

Archimedes Principle An object is buoyed up by a force equal to the weight of the fluid it displaces

A brick in water weighs less than when in air. The difference (buoyant force) is equal to the weight of the water displaced. Archimedes’ Principle

Apparent Weight The apparent weight ( F A ) is the difference between its actual weight and the buoyant force.

Problem A cube of steel that measures 5.0 cm on each side is immersed in fresh water. The density of steel is 9.0 x 10 3 kg/m 3. What is the buoyant force acting on the cube? What is the apparent weight of the steel?

Answer pt 1 F B = (1000 kg/m 3 )(0.05 m) 3 (9.8m/s 2 ) F B = 1.23 N

Answer Answer pt.2 F W = (9.0 x 10 3 kg/m 3 )(0.05 m) 3 (9.8m/s 2 ) F A = W – F B 9.8 N F A = N – 1.23 N = 9.8 N  m = ρV

“Bob, do you think I’m sinking? Be honest”