Buoyancy and Density

Norms for Today’s In-service Welcome! Please be respectful when others are talking. We all want to learn from each other today. Be a participant and get involved in the activities. Please put cell phones on silent so as not to interrupt others. Please excuse yourself when necessary. Bathrooms are down the hall to the left. Remember, you must sign in on the sign-in sheet. In order to pay your stipend, I must have a sign-in signature to match each stipend form. Thank you.

Goals for Today’s In-service To gain a conceptual understanding of buoyancy and density Explain what students should know about buoyancy and density from Grade 3 through grade 6 Model science activities to develop a conceptual understanding of buoyancy and density with your students Evaluate performance assessments and how valuable they are to your students’ understanding of the concepts taught

Grade level Expectations for Grades 3-6 Grade 3-Compare and classify objects on properties determined through experimentation ( e.g. ability to conduct electricity, tendency to float or sink in water) Grade 4-Determine linear, volume, and weight/mass measurements by using both metric system and u.s. system units to compare the results. Grade 5-Compare the physical properties of large and small quantities of the same type of matter. Identify the physical and chemical properties of various substances and group substances according to their observable and measurable properties (e.g. conduction, magnetism, density, light transmission)

Grade Level Expectations Continued By the end of Grade 5, students should also understand that objects in contact exert forces on one another but that some forces do not need to be in contact to act on an object (e.g., gravity and magnetism). They should also learn that each force that acts on an object has a strength (magnitude) and a direction, and the motion of an object depends on the strength and direction of the forces acting on it. Grade 6-Calculate the density of large and small quantities of a variety of substances (e.g. aluminum, foil, water, copper, clay, rock)

Buoyancy (FYI) Buoyancy is the measure of the upward force that a fluid exerts on an object that is submerged. Weight and mass are not the same. Mass is a fundamental property of matter. Mass does not change, no matter where you are. Weight on the other hand is a force caused by the earth’s gravity. Your weight does change from planet to planet due to the difference in gravity. When an object is submerged in water, both gravity and buoyancy are exerting a force on that object.

Newton’s Third Law For every action, there is An equal and opposite Reaction.

Buoyancy explains why some objects sink and others float Buoyancy explains why some objects sink and others float. A submerged object floats to the surface if the buoyant force is greater than its weight. If the buoyant force is less than its weight, then the object sinks. Suppose you place a block of foam in a tub of water. The block sinks partially below the surface. Then it floats without sinking any farther. The upward buoyant force perfectly balanced the downward force of gravity (the block’s weight). If you have a foam block and a wood block of the same size floating, the wood block will sink farther into the water. Wood has a greater density, so the wood block weighs more. A greater buoyant force is needed to balance the wood block’s weight, so the wood block displaces more water. The foam block has to sink only slightly to displace water with a weight equal to the block’s weight. A floating object displaces just enough water to make the buoyant force equal to the object’s weight.

Density (FYI) Whether an object floats or sinks is related to the object’s density. Density is a property of all matter. D=M/V Density describes how much mass is in a given volume of a material Steel has a high density and contains 7.8 grams of mass per cubic centimeter. Aluminum has a lower density and contains 2.7 grams per cubic centimeter or per ML (1 mL=1 cm cubed) Water has a density of 1.0 G/ml

Density (Continued) You can measure the density of regular shaped OBJECTS OR IRREGULARLY SHAPED OBJECTS. For regular shaped objects, you first calculate the volume. If you have a wooden block that measures 4 cm x 3 cm x 2 cm. You multiple to get the volume and measure its mass. If the mass is 20 grams. You calculate D=20g/24 cm cubed; therefore the density of the wooden block would be .83g/CM cubed For irregularly shaped objects, you use displacement to find the volume. An object will displace the amount of water equal to its mass. To find the volume of a key, fill a 100 ML graduated cylinder with 50 ML of water. Gently slide the key into the water and you will see the water level rise. If the level now reads 53.0 ML, then you know the volume of the key is 3.0 ML or 3.0 cubic centimeters. Next, you determine the mass of the key and calculate the density.

Liquids Tend to be Less Dense than Solids The density of a liquid is usually a little less than the density of the same material in solid form. The density of the liquid is lower because the atoms are not packed as uniformly as they are in a solid. Picture a brand new box of toy blocks. When you open the box, the blocks are tightly packed in a repeating pattern like the atoms in a solid. Now imagine dumping the box of blocks out and then trying to pour them back into the box. You would have them all jumbled up and that would take up more space, like the atoms in a liquid. Water is an exception to this rule.

In liquid water each molecule is hydrogen bonded to approximately 3 In liquid water each molecule is hydrogen bonded to approximately 3.4 other water molecules. In ice each each molecule is hydrogen bonded to 4 other molecules.

The ice structure takes up more volume than the liquid water molecules, hence ice is less dense than liquid water. This is why large icebergs can float. It is also very important to understand the wide variety of applications in science and technology. The relative densities of solids can be very important considerations in engineering design. For example, the choice of aluminum over steel for the construction of an airplane becomes an obvious consideration. Without an understanding of how density affects the behavior of gases and liquids, our understanding of weather conditions and ocean currents would be minimal at best.

Performance Assessments Raisins in a cup of sprite (explain) Actually calculate density of a rock Table of densities of objects and have them place them in order from least to highest density A solid brass block measures 2 cm x 2 cm x 3 cm and has a mass of 48 GRAMS. What is its density? PALS performance task on density

Challenge

Legend has it that Archimedes added to his fame by using the concepts of volume and density to figure out whether a goldsmith had cheated Hiero II, the King of Syracuse. The goldsmith had been given a piece of gold of a known weight to make a crown. Hiero suspected the goldsmith had kept some of the gold for himself and replaced it with an equal weight of another metal. Explain the steps you could follow to determine whether or not the crown was pure gold.