# Forces And Their Effects ! Density !

Density is a way to measure how tightly packed an object is. Denser objects have less empty space in them, they are less holey. The density of an object is often measured in grams per milliliter. So to measure the density of an object you would put it on a balance to measure how many grams it is (the object's mass) then divide that number by the amount of space the object takes up (its volume). The density of something stays the same wherever you take it, on Earth, Mars, or anywhere in the universe.

The density of an object compared to the density of water tells us whether the object will float or not. The density of water is about 1.00 g/mL. If the density of an object is less than the 1.00 g/mL, the object will float in water. If the density of something is greater than 1.00 g/mL it will sink in water.

Have you ever had ice water before? If you have, you'll probably remember that the ice cubes float in water. That's because ice is less dense than water. When you freeze water to make ice, the water expands and takes up more space. So a piece of ice has more volume than the same amount of water. Since ice has more volume for the same amount of stuff it makes sense that it is less dense than water and floats!

Here are some densities of different things. The units are grams per cubic centimeter. A cubic centimeter is about a thimblefull and a gram is a little bit more than the weight of an M & M.
Water = 1.0 Ice = 0.9 People about 1.0 Lead = 11.3 Mercury = 13.6 Air = 0.0013

Remember: things that float have densities less than 1. Things that sink have densities greater than 1.

The density, or more precisely, the volumetric mass density, of a substance is its mass per unit volume. The symbol most often used for density is ρ (the lower case Greek letter rho). Mathematically, density is defined as mass divided by volume:

where ρ is the density, m is the mass, and V is the volume. In some cases (for instance, in the United States oil and gas industry), density is loosely defined as itsweight per unit volume, although this is scientifically inaccurate – this quantity is more specifically called specific weight.

The density of a material varies with temperature and pressure. This variation is typically small for solids and liquids but much greater for gases. Increasing the pressure on an object decreases the volume of the object and thus increases its density. Increasing the temperature of a substance (with a few exceptions) decreases its density by increasing its volume. In most materials, heating the bottom of a fluid results in convection of the heat from the bottom to the top, due to the decrease in the density of the heated fluid. This causes it to rise relative to more dense unheated material.

The reciprocal of the density of a substance is occasionally called its specific volume, a term sometimes used in thermodynamics. Density is an intensive property in that increasing the amount of a substance does not increase its density; rather it increases its mass.

Density Examples:

• Oil and water - In an oil spill in the ocean, the oil rises to the top because it is less dense than water.
• A Styrofoam cup is less dense than a ceramic cup, so the Styrofoam cup will float in water and the ceramic cup will sink.
• A piece of wood will float on water; but, most rocks are more dense than water, so they will sink.
• Helium is less dense than air, so helium balloons rise.
• Oil floats on vinegar because its density is less.
• When adding motor oil to your car, you need to know the recommended density.

# Subtance And Density !

Here are examples of substances and their density expressed as grams per cubic centimeter.

Liquids

• .Mercury=13.6
• Glycerine=1.3
• Seawater=1.03
• Pure water (at 4 C)=1.00
• Olive Oil=0.92
• Turpentine=0.87
• Petrol=0.80
• Alcohol=0.79

Solids

• Aluminum = 2.7
• Copper = 8.9
• Gold = 19.3
• Iron = 7.9
• Platinum = 21.5
• Silver = 10.5
• Glass = 2.5
• Zinc = 6.92
• Ice=0.92
• Pinewood=0.50
• Cork=0.24

Gases

• Air = 0.001 29
• Hydrogen=0.000 09