Liquids

Chapter 19

Liquids

• While in the liquid phase, a substance will take the shape of their container, but will not expand to fill the container.

• In the liquid state, molecules can flow. They freely move from place to place by sliding over one another.

Pressure in Liquids

• Any contained fluid exerts a force on the walls of its container. The ratio of this force to the surface area of the container is called pressure.

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• The unit for pressure is the Newton per meter squared, or the Pascal.

Pressure in Liquids

• In a fluid, pressure is exerted equally in all directions.

• Therefore, the pressure always acts perpendicular to every surface.

Pressure in Liquids

• For a liquid of uniform density, the pressure is proportional to the depth of measurement:

Pressure in Liquids

• For a liquid of uniform density, the pressure is proportional to the depth of measurement:

Pressure = weight density x depth

Pressure = density x g x depth

Pressure in Liquids

• For a liquid of uniform density, the pressure is proportional to the depth of measurement:

Pressure = density x g x depth

Pressure in Liquids

• “Pascal’s Vases” demonstrate that the pressure of the liquid depends on the depth alone, and not the volume or shape of the fluid.

Buoyancy

• When an object is placed in water, it weighs less than it did in air. This is due to an upward force exerted on the object by the water.

• The upward force is called the buoyant force.

Buoyancy

• When an object is placed in water, it weighs less than it did in air. This is due to an upward force exerted on the object by the water.

• The upward force is called the buoyant force.

Buoyancy

• If the buoyant force on an object is greater than the weight, the object will float.

Archimedes’ Principle

• When an object is submerged in a liquid, some of that liquid is displaced.

• The volume of liquid displaced is equal to the volume of the immersed object.

Archimedes’ Principle

• The Greek philosopher Archimedes determined the relationship between buoyancy and the amount of liquid displaced.

• An immersed object is buoyed up by a force equal to the weight of the fluid it displaces.

Archimedes’ Principle

Archimedes’ Principle

• The buoyant force depends on the volume of the object that is being submerged in the fluid – nothing else matters.

• For every liter of volume (or 1000 cm3), there is a 10-newton buoyant force.

Archimedes’ Principle

• For example, what is the buoyant force on:

- a half liter block of wood?

- a 100 milliliter brass weight?

- a 2 liter bottle of ginger ale?

- a 2 liter bottle half full of ginger ale?

- a 2 liter bottle full of air?

Archimedes’ Principle

• The buoyant force acting on an object does not depend on the depth of the water.

Density and Submersion

• Whether an object will sink or float in a liquid has to do with how great the buoyant force is compared to the object’s weight.

• This can be summed up with three simple rules:

1) If an object is denser than the fluid in which it is immersed, it will sink.

2) If an object is less dense than the fluid in which it is immersed, it will float.

3) If an object has a density equal to the density of the fluid in which it is immersed, it will neither sink nor float.

Flotation

• How can you make iron float?

• If the iron can be made less dense than water, it will float. This can be done by filling the piece of iron with pockets of air.

• This can be done by making bubbles in the iron as it cools, or by making a hollow object of iron.

Flotation

Flotation

The Principle of Flotation:

• A floating object displaces a weight of fluid equal to its own weight.

• For example, the aircraft carrier USS Enterprise weighs 90,000 tons.

• It displaces 90,000 tons of water.

Flotation

• To see how a submarine is able to make use of this principle to submerge and then re-surface, take a look at my website.

Pascal’s Principle

• Changes in pressure at any point in an enclosed fluid at rest are transmitted undiminished to all points in the fluid and act in all directions.

Homework

• Read ch 19 and do problems #1-20 on pages 280 and 281.

• Due Monday, 3/6