8.8.a Density and Buoyancy Students know density is mass per unit volume. P71 Wood Steel The density of a substance is the same for all samples of that substance. 1. The two blocks shown have the same volume. Which one has the greater density? A. The one with more mass B. The one with more texture C. The one with the darker color D. The one with the lower melting point 2. Why do a small block of steel and a large block of steel have the same density, even though one block is bigger than the other? 1. A 2. The bigger block has a larger volume, but it also has more mass, so the densities are the same.
8.8.a Density and Buoyancy Students know density is mass per unit volume. P72 The density of an object is its mass per unit volume. 1. A sample of an unknown liquid has a mass of 31 grams and a volume of 62 ml. What is the liquid s density? A. 0.5 g/ml B. 2 g/ml C. 62 g/ml D. 93 g/ml 2. Would 120 ml of the same liquid have a higher density? Explain. 1. A 2. No, because a greater volume of the liquid would also have more mass
8.8.b Density and Buoyancy Students know how to calculate the density of substances (regular and irregular solids and liquids) from measurements of mass and volume. P73 You can calculate density by dividing the mass of some quantity of material by its volume. 1. A small apple has a mass of 100 grams and has a volume of 330 cubic centimeters. What is the density of the apple? 2. How can you find the volume of a small apple? A. Weigh it on a scale. B. Measure its length with a ruler. C. Measure how much water it displaces when submerged. D. Its shape makes this impossible. 1. 0.3 g/cm 3 2. C
8.8.b Density and Buoyancy Students know how to calculate the density of substances (regular and irregular solids and liquids) from measurements of mass and volume. P74 You can calculate the volume of a rectangular object by using this formula: Volume = Length Width Height Density = Mass Volume 1. Before it breaks into eight pieces, the large cubical rock has a density of 2.5 g/cm 3. What is the density of each of the eight smaller pieces? A. 0.3125 g/cm 3 B. 2.5 g/cm 3 C. 20 g/cm 3 D. 25 g/cm 3 2. If a cube of unknown material has a mass of 40 g and a length of 2 cm along each edge, what is its density in grams per cubic centimeter? 1. B 2. Its density is 5 g/cm 3.
8.8.c Density and Buoyancy Students know the buoyant force on an object in a fluid is an upward force equal to the weight of the fluid the object has displaced. P75 Buoyant force Fluids exert an upward force called the buoyant force that acts on a submerged object. The buoyant force acts in the direction opposite to the force of gravity, so it makes an object feel lighter. 1. If the fish shown above has a volume of 1,000 cubic centimeters, what is the volume of the water it displaces when it is submerged? A. 10 cm 3 B. 50 cm 3 C. 500 cm 3 D. 1,000 cm 3 2. The weight of the submerged fish is a downward force. What force acts in a direction opposite to the weight of the fish? 1. D 2. The buoyant force
8.8.c Density and Buoyancy Students know the buoyant force on an object in a fluid is an upward force equal to the weight of the fluid the object has displaced. P76 The buoyant force on an object is equal to the weight of the fluid displaced. 1. When the weight of a submerged object is greater than the buoyant force on that object, A. the object sinks to the bottom. B. the object floats on the surface of the water. C. the object sinks into the water but then floats halfway down. D. the object is pushed out of the water. 2. The three objects in the diagram have the same weight. Why does one of them sink? 1. A 2. It weighs more than the water it displaces.
8.8.d Density and Buoyancy Students know how to predict whether an object will float or sink. P77 Substance Wood Corn oil Plastic Water Tar ball Glycerin Rubber washer Corn syrup Copper wire Mercury Density (g/cm 3 ) 0.7 0.925 0.93 1.00 1.02 1.26 1.34 1.38 8.8 13.6 An object more dense than a fluid will sink in that fluid. 1. An object has a density of 0.95 grams per cubic centimeter. Based on the table at the left, in which liquid will the object sink? A. corn syrup B. glycerin C. water D. corn oil 2. Why will a rubber washer sink in a bucket of water? 1. D 2. It will sink because it is denser than the water.
8.8.d Density and Buoyancy Students know how to predict whether an object will float or sink. P78 Displaced fluid Displaced fluid An object less dense than the fluid will float. 1. The liquid in the beaker is water with green food coloring. The density of water is 1 g/cm 3. What is a possible density for the film can on the right? A. 0.5 g/cm 3 B. 1 g/cm 3 C. 1.5 g/cm 3 D. 2 g/cm 3 2. How can you predict if an object will float in a fluid? 1. A 2. If the object is less dense than the fluid, it will float. If an object s weight is less than the buoyant force, it will float.