Presentation on theme: "OR, WHY DO SOME THINGS FLOAT WHILE OTHERS SINK."— Presentation transcript:
1OR, WHY DO SOME THINGS FLOAT WHILE OTHERS SINK. DENSITYOR, WHY DO SOME THINGS FLOAT WHILE OTHERS SINK.
2D=m/v Where D equals density, m equals mass and v equals volume What Is Density?Density is how heavy something is for its size.Density is mass divided by volume orD=m/v Where D equals density, m equals mass and v equals volume
3Another Way of Looking at It. You will often hear density referred to as “mass per unit volume.”Let’s break this down:Mass is how much matter the object has.Per simply means divided byUnit volume is a measure of how much space it takes up
4So… When you hear mass per unit volume, you should think… Take the mass and divide by its volume!
5What Are the Units of Density? Mass units would includeGrams for very small objects orKilograms for larger objectsVolume units would includeCubic centimeters for small objects orCubic meters for large objectsMilliliters for small amounts of liquidsLiters for larger amounts of liquidLets stick to grams and cubic centimeters for now.
6How to State Density Units You have grams for massYou have cubic centimeters for volumeThese are not the same units, they don’t cancel so you have to include both units in your answer
7How to State Density Units The units of density are:Grams/Cubic Centimeter org/cm3
8length multiplied by width multiplied by height or Why Cubic CentimetersRemember that volume islength multiplied by width multiplied by height orl X w X hIn each case, the distance is in cm and since you are multiplying it three times, you get cubic centimeters.Easy huh!
9Now, How About an Example? l=10 cmh=5 cmw=2 cmMass= 100 gramsUse the information provided to calculate the density of the object
10What Did You Get? If you got 1g/cm3, you are correct! Lets see how this works:V=l x w x hV=10 cm X 5 cm X 2 cmV=100cm3Mass = 100 g (given)D=m/vD=100 g/100cm3=1g/cm3
11A Few Words About the Mass/volume Relationship of Liquids and Solids. When working with a sample of any liquid or solid - wood, steel, or peanut butter,If you decrease the mass, you also decrease the volumeIf you decrease the volume, you also decrease the mass
12A Few Words About the Mass/volume Relationship of Liquids and Solids. When working with a sample of any liquid or solid - wood, steel, or peanut butter,If you increase the volume, you also increase the massIf you increase the mass, you also increase the volume
13Let’s Think About It for a Second. If you add more peanut butter to the pile (more mass) it is going to take up more space (more volume)If you remove peanut butter from the pile (less mass) it is going to take up less space (less volume)
14What About Volume? The same holds true for volume. If you start with the volume of peanut butter in the jar (lets say 16 oz) and you have to move it into another container (say 2 oz). You definitely have less peanut butter in the 2 oz jar than in the 16 oz jar therefore the mass of the 16 oz of peanut butter would be more than the mass of the 2 oz of peanut butter. Remember, we’re not massing the jars but the contents of the jars.
15Does the Density of a Liquid or Solid Change When Mass and Volume Change? What do you think?
16Does the Density Change When Mass and Volume Change? The answer is NO and here is why:Density is a proportional relationship meaning as long as mass and volume are in proportion to each other, the density of a liquid or solid remains the same. This is how we know that certain objects have a predictable density regardless of the sample size.
17Let Me Prove It to You.w=2 cml=10 cmh=5 cmMass= 100 gramsLet’s start with the object from a previous slideRecall that the density was 1.0 g/cm3l=5 cmh=5 cm50 gramsw=2 cmWhat would happen to its density if we cut it in half?
18Let Me Prove It to You.l=5 cmh=5 cm50 gramsw=2 cmThe new volume of either half is: 5 cm x 5 cm x 2 cm = 50 cm3The new mass of either half is: 100g ¸ 2 = 50gSo, the new density of either is: 50g/50cm3 or still 1.0 g/cm3
19What About Gases?By definition, gases have no specific volume and no specific shapeThey expand or contract to entirely fill the vessel containing themSo, gases can have different densities, depending on their conditionsIn fact, that is what distinguishes them from liquidsFor this class, you will be given the density of a gas, as opposed to having to calculate it
20A Word About Irregularly Shaped Objects. If you are given an irregularly shaped object and told to find the density, you might have a problem.If the length, width, and height of the object are not uniform (the same), you cannot use l X w X h to determine volume.
21A Word About Irregularly Shaped Objects. To determine the volume of an irregularly shaped object, start with a graduated cylinder of water filled about half fullRecord the volume of water in the cylinderDrop in the irregularly shaped objectRecord the volume of water in the cylinder with the object in itSubtract the original volume from the second volume to determine the volume of the object
22A Word About Irregularly Shaped Objects. You are measuring the amount of water displaced by the object which is equal to the volume of that objectThis is called:volume by displacementCool!!
23LiquidsSo far we have been talking mostly about solids. Solids have densities that can be easily expressed in g/cm3Liquids tend to be measured in milliliters or liters. This is not a problem because you can convert milliliters or liters into grams per cubic centimeter using a metric conversion factor
24LiquidsYou can also weigh a liquid to determine its mass. Now we are going to use the word massing for weighing to avoid confusionTo do this, measure your cylinder in grams and record its empty massAdd your liquidNow mass the cylinder againSubtract the mass of just the cylinder and you have the mass of your liquid
25LiquidsTo determine the volume, simply look at where the liquid measures in your cylinder and record the amountIn order to convert your volume reading (milliliters or liters) you would use a conversion factor to change milliliters or liters to cubic centimeters.
26LiquidsFor this class, you will be given liquid densities already converted to g/cm3Whew!
27Lets Look at Some Specific Densities. SubstanceDensity (g/cm3)Air0.0013Gasoline0.7Wood (oak)0.85Water (ice)0.92Water (liquid)1.0Aluminum2.7Lead11.3Mercury13.5Gold19.3
28Ever Notice…When you fill a glass with water, there isn’t a large bubble of air at the bottom of your cup.When you put ice in your water it floats to the top.
29That’s BecauseAir is less dense than water and will naturally be displaced by the water and the air will be at the top.Ice is also less dense than water by just a little bit. That is why your ice floats to the top of your water.Now you know!!!
30Some General Comments About Density In order to compare the density of two objects, they must first be in the same units (g/cm3)The material with the lower density will always float to the top of the material with the higher density (think ice and water), as long as the materials don’t react with each other and mix
31Can you guess what kind of solid would sink in water? Sink or Float?Solids can float or sink in a liquid.Can you guess what kind of solid would sink in water?
32Answer:Any solid will sink in water if it has a density greater than the density of water.Water has a density of 1g/cm3. Any solid with a density greater than 1g/cm3 will sink in water!
33Sink or Float?Comparing densities to determine if something will float, only makes sense if at least one of the materials is a liquidA piece of steel (high density) can sit on top of a piece of wood (low density) because the wood cannot float through the steel!