Density, Viscosity and Buoyancy

Objective 2

TEK 7 The student knows relationships exist between properties of matter and its components.

(A)Investigate and identify properties of fluids (gases and liquids) including density, viscosity, and buoyancy.

If you’ve ever carried bags of groceries, you know that some bags have greater mass than others, even though the volumes of the bags are the same. Mass and volume are general properties of all matter. Density is the ratio of mass to volume. The density of a specific kind of matter helps to identify it and to distinguish it from other kinds of matter. The unit for density of a liquids is grams per milliliter (g/mL).

• The density of a piece of matter is a combination of how compressed the material is and how much its atoms or molecules mass per unit volume. Water has a greater density than steam because the atoms in a liquid are closer together than those in a gas. But iron is denser than ice, because the iron atoms are heavier than the water molecules, even though they both are solids.

• Density is defined as the mass of a substance divided by its volume. The equation that is used for determining the density, D, of a substance is D = m/V, where m is the mass of the object, and V is its volume. More dense liquids will sink in the presence of less dense liquids.

• As temperature is increased (or decreased) there is no change in the mass of an object. However, since substances generally expand as they are heated, the volume will increase with an increase in temperature. This increase in temperature will cause the density to decrease.

Density is the same on the Moon

• Since density is related to the mass of an object, the density of a given volume of lead would be the same on both the Earth and the Moon, although the lead would weigh less on the Moon because of the lower gravity there.

• Typically, when you freeze a liquid, its atoms or molecules get closer together, such that the solid is more dense than the liquid. An exception to this is when you freeze water. When ice is formed, it actually expands, thus making it less dense than water.

Viscosity is a property that is often called on when liquids need to be classified. Each liquid has a different viscosity, or resistance to flow. Peanut butter has a much higher resistance to flow than milk, and thus it has a much higher viscosity than milk. We like ketchup that has a high viscosity. Likewise, motor oil is more viscous than gasoline.

The viscosity of motor oil is what allows the motor oil to stick to the metal parts of the engine. The weights given on motors oils ,S.A.E. values, correspond to "real" viscosity, taken at specific temperatures. Oils that fall into a certain range are designated 5, 10, 20, 30, 40, 50 by the S.A.E. The W means the oil is suitable for Winter use.

Viscosity is a measure of the ease with which molecules move past one another.

Viscosity depends on the attractive force between the molecules.

Viscosity decreases with increasing temperature - the increasing kinetic energy overcomes the attractive forces and molecules can more easily move past each other.

Viscosity also affects the shapes of lava flows and the mountains they erupt from. The more viscous the magma, the fatter the lava flow. Also, the more viscous the magma erupted, the steeper the volcano.

The viscosity of a given liquid is due to:

•the shapes of the molecules•the attractive forces present between the molecules.

•Substances with a high viscosity typically have both of these characteristics. The polar ends of the molecules attract each other, while the long structures of the molecules "get tangled" with other molecules. A viscous liquid is somewhat like a tank of snakes with each snake biting another snake’s tail, and each snake’s body wrapped around another snake’s body.

You may have heard that 90% of an iceberg lies below the water. Why is that? What determines whether something sinks or floats, and, if it floats, how much of it remains above the surface?

Buoyancy is the tendency of an object to float in a fluid. It is controlled by

differences in density between the object and the fluid. Buoyancy is summed up by Archimedes Principle: Any object, wholly or partly immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object.

People or objects that float are positively buoyant. Things that are too heavy and sink, they are negatively buoyant. Neutral buoyancy occurs when the force of gravity is balanced by the buoyant force. Divers want neutral buoyancy, a scuba diver does not want to float to the surface or sink to the bottom.

Two important forces are in play when Surfing: Buoyancy and Gravity:The water exerts its force on every part of the board that it touches. The weight of the surfer and board give a downward force due to gravity.

Three cubes of equal size are put in a fish tank. The left cube is made of stone, the middle cube is made of a plastic with equal density as water, the right cube is made of wood

Force of Gravity

Buoyant force

Negatively buoyant

Positively buoyantNeutrally

buoyant

Whether an object floats or sinks, is based onnot only its weight, but also the amount ofwater it displaces. That is why a very heavyocean liner can float. It displaces a largeamount of water.

• When an object displaces a fluid it is basically pushing the fluid out of the way. Objects with a large volume displace more fluids that those with small volumes. The more fluid an object displaces the more buoyant that object will be.

A block of wood made of oak, for example, sits deeper in the water (and therefore displaces more of the water) than does a block of pine. The reason is that it's heavier for its size, or denser -- in this case, the molecules that make it up are more closely packed together than the molecules that make up the pine.

We put a toy boat and a stone in a fish tank. The stone is lying on the bottom, whereas the boat floats at the surface. What will happen to the water level if we take the stone from the bottom and put it into the boat?

When the stone is in the boat, its entire weight is compensated by Buoyant force. For this, the boat has to lie deeper in the water when the stone is on board. More water is displaced in this situation. Displacement of water is upward (the only way the water can go). Hence the water level will rise.

                                                                                                 

                                                                                                                               

A standard hot air balloon has a volume of 1590 cubic meters (56,000 cubic feet) which is larger than the average house. How can it fly?

• Gases are fluids, therefore an object in air also experiences an upward force equal to the amount of weight of air displaced. If the upward force exceeds the weight of the balloon, the balloon and its contents it will rise.

• Pressure is defined as the force per unit area.

• Pressure = Force Area

Pressure

• A fluid exerts a pressure on all bodies immersed in it. A swimmer diving down in a lake can easily observe an increase in pressure with depth. Since a liquid is nearly incompressible, its density does not change significantly with increasing depth. Therefore, the increase in pressure is caused solely by the increase in depth.

Gases have low densities and variations in pressure can be observed. The air pressure in a typical room is the same everywhere, but it is noticeably lower at the top of a mountain than at sea level. Because air is a gas, it is compressible. Its density decreases with increasing altitude. Thus changes in air pressure depend upon both the variations in the density of air and changes in the altitude at which it is measured.

Gas pressure is influenced by four things1.The amount of gas present

2. The temperature of the gas

3. The volume of the gas.

4. The pressure on the gas.

1. The amount of gas present As the number of gas particles increase in

a container increases the volume increases proportionally. (Think of blowing up a balloon)

2. The temperature of the gas

An increase in the temperature of a gas increases the volume of the gas

proportionally. 3. The volume of the gas.

The volume of a gas increases with an increase in temperature or decreases with a increase in pressure. 4. The pressure on the gas.

The pressure of a gas increases with an increase in temperature, decreases with an increase in volume, and increases with an increase in the amount of gas present.