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Density & Buoyancy

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Density Density = Mass/Volume What is the SI Unit for Mass? Volume? SI Unit for Density = kg/m 3 Also…g/cm 3 g/ml and kg/L The Density of Water = 1 g/cm 3 Important to Remember…1 mL = 1 cm 3

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What is density? Density = mass OR mass ÷ volume. Density = mass OR mass ÷ volume. volume volume You can also rearrange the formula and solve for mass or volume. Volume = mass/density Mass = volume x density ALWAYS REMEMBER UNITS!

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Let’s try a density problem together Frank has a paper clip. It has a mass of 9g and a volume of 3cm 3. What is its density? Frank has a paper clip. It has a mass of 9g and a volume of 3cm 3. What is its density? Frank also has an eraser. It has a mass of 3g, and a volume of 1cm 3. What is its density? Frank also has an eraser. It has a mass of 3g, and a volume of 1cm 3. What is its density?

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Work on these problems with your neighbor. Jack has a rock. The rock has a mass of 6g and a volume of 3cm 3. What is the density of the rock? Jack has a rock. The rock has a mass of 6g and a volume of 3cm 3. What is the density of the rock? Jill has a gel pen. The gel pen has a mass of 8g and a volume of 2cm 3. What is the density of the rock? Jill has a gel pen. The gel pen has a mass of 8g and a volume of 2cm 3. What is the density of the rock?

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Now, try these on your own. Al’Licia has a watch. It has a mass of 4g and a volume of 2cm 3. What is the density of the watch? Al’Licia has a watch. It has a mass of 4g and a volume of 2cm 3. What is the density of the watch? Mia has a wallet. It has a mass of 15g and a volume of 5cm 3. What is the density of the wallet? Mia has a wallet. It has a mass of 15g and a volume of 5cm 3. What is the density of the wallet?

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Which one is more dense? Demonstration: People in a square Demonstration: People in a square How about this: Which square is more dense? How about this: Which square is more dense?

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Which one is more dense? Now which one is more dense? Now which one is more dense?

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By Comparing Densities, you can predict whether an object will float or sink in a fluid.

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Liquid Layers If you pour together liquids that don’t mix and have different densities, they will form liquid layers. If you pour together liquids that don’t mix and have different densities, they will form liquid layers. The liquid with the highest density will be on the bottom. The liquid with the highest density will be on the bottom. The liquid with the lowest density will be on the top. The liquid with the lowest density will be on the top.

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Liquid Layers Check out this picture from your book. Which layer has the highest density? Check out this picture from your book. Which layer has the highest density? Which layer has the lowest density? Which layer has the lowest density? Imagine that the liquids have the following densities: Imagine that the liquids have the following densities: 10g/cm 3.3g/cm 3. 10g/cm 3.3g/cm 3. 6g/cm 3.5g/cm 3. 6g/cm 3.5g/cm 3. Which number would go with which layer? Which number would go with which layer?

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Liquid Layers – Try with your neighbor Which liquid has the highest density? Which liquid has the lowest density? Which liquid has the middle density?

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Liquid Layers – Try on your own! Imagine that the liquids on the right have the following densities: Imagine that the liquids on the right have the following densities: 15g/cm 3 10g/cm 3 15g/cm 3 10g/cm 3 3g/cm 3 9g/cm 3 3g/cm 3 9g/cm 3 7g/cm 3 12g/cm 3 7g/cm 3 12g/cm 3 Match the colors to the correct densities. Match the colors to the correct densities. 3g/cm 3 7g/cm 3 9g/cm 3 10g/cm 3 12g/cm 3 15g/cm 3

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How do Submarines Float and then Sink?

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Buoyancy & Archimedes Principle An upward force acting on a submerged object… opposite to the direction of gravity. An upward force acting on a submerged object… opposite to the direction of gravity. If weight of object is > than the buoyant force, the object will sink. If weight of object is > than the buoyant force, the object will sink. If weight of object is < than the buoyant force, the object will float. If weight of object is < than the buoyant force, the object will float. Archimedes Principle = The Buoyant force acting on a submerged object is equal to the weight of the fluid displaced by the object. Archimedes Principle = The Buoyant force acting on a submerged object is equal to the weight of the fluid displaced by the object.

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The pressure on the bottom of a submerged object is greater than the pressure on the top. The result is a net force in the upward direction.

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The block of wood weighs less that the weight of the water displaced. So if floats

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The Oak block sinks…we know that the weight of the wood is greater than the weight of the water displaced

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A balloon filled with air is more dense than surrounding air b/c under pressure. In hot air balloon, air is heated…making it less dense and therefore it floats.

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Review What is the formula for density? What is the formula for density? What happens if you pour together liquids that have different densities? What happens if you pour together liquids that have different densities? Will the liquid on the top have the highest or lowest density? Will the liquid on the top have the highest or lowest density? Will the liquid on the bottom have the highest or lowest density? Will the liquid on the bottom have the highest or lowest density?

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Super Scientist Question of the Day Jake has a book, a ruler, and a balance. Jake has a book, a ruler, and a balance. How can Jake find the density of the book with the tools he has? How can Jake find the density of the book with the tools he has?

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