D ENSITY Density Definitions Density is usually measured in g/cm 3, but in physics is measured in kg/m 3. m Mass is usually expressed in grams, but is measured in kg in physics. V Volume usually expressed in cm 3 or ml. (equivalent terms), but again, in physics, is measured in m 3. Is also measured in liters (l). = m/v (g/cm 3 ) Density = amount of matter per unit volume. Density is the ratio of mass to volume. m V If the volume stays the same and the mass increases... the density will increase. m V If the mass stays the same and the volume increases... the density will decrease. Density Equation The DMV Triangle for Volume, Mass, and Density Space (Volume) What would take up more space ? A kilogram of feathers... or a kilogram of steel? How close the atoms or molecules are to each other. More than heaviness - density includes how much space an object takes up! All substances have density including liquids, solids, and gases. Density is the measure of the compactness of a material Compactness Density is the measure of the compactness of a material How much kinetic energy do the molecules have? The greater the kinetic energy the faster the molecules are moving. The faster the molecules are moving the greater the volume they occupy and hence the less dense that gas is!! Therefore, cold air is more dense than warm air. Density of Gases A low pressure weather system means warmer air which tends to RISE. A high pressure systems indicate a colder more dense air mass that will SINK! Density of Gases The liquid nitrogen quickly cools the air in the balloon, and it shrinks in volume. Balloon in liquid Nitrogen The more dissolved solids in a solution, the more dense it becomes. For example, ocean water has salt dissolved in it, and hence it is more dense than pure water. Cold water in lakes tend to sink (this creates a constant mixing of water, nutrients, and other substances) -- the convection currents set up keep things mixed. Liquids of Pb (lead) is 11.3 g/cm 3 of Hg (mercury) is 13.6 g/cm 3 Hence, the lead ball below floats on liquid mercury! Lead and liquid mercury Most substances are most dense as solids. Water is an important exception. Ice is less dense than water (which is why lakes and ponds have a thin layer of ice covering in winter, with water underneath): Solids This brings us to an important story involving Archimedes about 250 BC. (See the PowerPoint online.) Archimedes and the kings crown Temperature Pressure Factors affecting Density The density of a material or substance, relative to another substance, typically water. Expressed in a ratio: water = 1g/cc This ratio is also known as the specific gravity. Since steel is about 8 times more dense than water, its specific gravity is 8 and its density is 8 g/cm 3. Relative Density How are Submarines like fish? The swim bladder in bony fish control their relative density in order to rise or dive in the water. When air is added to the swim bladder, by diffusion through the blood vessels in the bladder walls, the fish becomes less dense overall. When air is removed fish become more dense. Hence by changing the volume of air in the bladder, the fishs density can be made equal to that of the surrounding water at a given depth. Fish and Submarines The density of a material in its closest packed form. For water: Absolute Density = 1000kg/m 3 or 1 g/cm 3 at 4 o C. In other words, the greatest density of water is at 4 o C. Absolute Density Which of these substances will float in water? To float or not to float, that is the question.