# Properties of Fluids Fluids Unit

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Properties of FluidsFluids Unit

Properties of FluidsDuring the Fluids Unit, we will be learning about the following properties of fluids:–Density –Buoyancy–Displacement (Archimedes’ Principle)–Pressure (Pascal’s Principle)–Viscosity

1. Density• The mass of a substance (number of particles

of substance) per unit volume.

• Density = _Mass_ Volume

• Lab Activity: Density Column• After pouring together five different liquids in a

beaker, the maple syrup layered at the very bottom (most dense), and the isopropyl alcohol layered a the very top (least dense).

2. Buoyancy

• Buoyancy is a force exerted by a fluid that opposes an object's weight

• It is the upward force on an object submersed in a fluid

• Whether an object floats or sinks depends on the force of buoyancy in relation to the object’s weight and gravitational pull

• If the buoyant push upward is greater than the gravitational pull downward, then the object will float, and vice versa.

The Forces at Work in Buoyancy

• Gravitational force “pulling” downward

• Buoyant force “pushing” upward

Three Types of Buoyancy• Negative Buoyancy: Object sinks

(gravitational “pull” force is greater than buoyant “push” force)

• Neutral Buoyancy: Object is suspended in middle (gravitational and buoyant forces are equal)

• Positive Buoyancy: Object floats at the top of fluid (buoyant force is greater than gravitational force)

Altering Buoyancy• Buoyancy can be changed if necessary. Some

examples include:- Fish can expand and contract their swim bladders in

order to “float” or “sink” or maintain neutral buoyancy. When swim bladder fills with air, it becomes less dense than water and the fish can float, and vice versa.

- Scuba divers wear weight belts to give them more density and less buoyancy, which allows them to submerse (sink) in water.

- Changing the amount of water in the ballast tank of a submarine allows a submarine sink or rise.

Boat Building Challenge

3. Displacement: Archimedes’ Principle

• The buoyant force on a submerged object is equal to the weight of the fluid that is displaced by the object.

• Archimedes (Greek mathematician and physician) sat in bathtub and realized that the level of the water in the tub rose as he got in; this lead him to run out of the bathtub screaming “EUREKA” when he discovered that he could use the principle of displacement to figure out whether the king’s crown was made of pure gold or not.

Displacement Demonstration

4. Pressure: Pascal’s Principle

• Any change in pressure applied to a fluid in a confined space is transferred unchanged throughout that object; In other words, a fluid within a confined vessel exerts an equal amount of force into all directions.

• Demonstration: Cartesian Diver• An everyday life example of Pascal’s Principle can

be observed in hydraulics

Cartesian Diver

Originally, the cartesian diver is neutrally boyant (suspended in the middle of the bottle).

When the bottle is squeezed, the cartesian diver becomes negativey buoyant and sinks to the bottom of the bottle.

When the bottle is released, the cartesian diver becomes neutrally buoyant again and rises to the middle of the bottle.

Cartesian Diver• There is just enough air in the diver to make it positively

buoyant. Therefore, the diver floats at the water's surface. As a result of Pascal's law, when the pressure is increased by squeezing the container, the pressure on the water increases the pressure on the air bubble in the diver. The air compresses and reduces in volume, permitting more water to enter the diver. The diver now displaces a lesser weight of water than its own weight so becomes negatively buoyant and sinks, according to Archimedes’ principle. When the pressure on the container is released, the air expands again, increasing the weight of water displaced and the diver again becomes positively buoyant and floats.

http://en.wikipedia.org/wiki/Cartesian_diver

Example of Pascal’s Principle: Hydraulics

5. Viscosity• The physical property of a fluid that limits its

ability to flow; Viscosity is the resistance to flow.• Both liquids and gases have viscosity (not solids)• The higher a fluid’s viscosity, the slower it flows.

The lower a fluid’s viscosity, the easier/faster it flows.

• A fluid’s viscosity depends on its internal friction• If the bonds between a fluid’s particles are strong,

then viscosity is high (does not flow easily). If the bonds between particles are weak, then viscosity is low (flows easily).

Viscosity and Temperature• How might the viscosity of a fluid be affected by

temperature?

• Viscosity Lab Activity