Name



Name Date Class

Lab 20: Density and Buoyancy

Problem

To learn how to predict whether an object will float or sink

Background

Eureka! According to popular legend, in the 3rd century B.C. the Greek mathematician Archimedes discovered that there was a relationship between the amount of water he displaced when getting in a bathtub and the buoyant force that made him feel lighter in the water. This discovery lead to the

principle named after him, which states that “an immersed object is buoyed up by a force equal to the weight of the fluid it displaces.”

Why do some objects float and others sink? The answer depends partly on the densities of the object itself and the liquid it is placed in. Density is the mass of

a material in one unit of volume. The mathematical formula for density is given below.

Density = or d =

Mass

Volume

m

V

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The answer also depends on the volume of fluid that is displaced by the object.

The shape of an object affects how much fluid is displaced–which explains why

a solid block of iron will sink, while the same mass of iron when shaped in the form of a boat would float.

Skills Focus

Problem solving, calculating, applying concepts, predicting, drawing conclusions

Procedure

Density of a Solid

1. Start Virtual Physics and select Density and Buoyancy from the list of assignments. The lab will open in the Density laboratory.

2. You will be measuring the density of solid objects and of various liquids to predict whether the solids will float or sink. You will also calculate the buoyant force on the solids in one of the fluids. Find the ice ball on the lab wall. Pick up the ball and drag it to the spotlight on the balance. Record its mass in Data Table 1.

[pic]

3. Use the Up and Down arrows on the control panel to toggle through the options of fluids to use in the lab. Select Virtual Fluid B. This is a unique

Virtual Fluid that is used only in this virtual laboratory. Click the Full button underneath that display to select the amount of fluid to be added to the cylinder. Click the Fill button to release the chosen amount of fluid into the

250 mL graduated cylinder on the laboratory bench. Click on the top of the cylinder to see a zoomed in view of the level of the fluid. Record the volume

of the Virtual Fluid in Data Table 1.

64 Density and Buoyancy

Virtual Physics Lab Workbook, by Brian F. Woodfield, Steven Haderlie, Heather J. McKnight, and Bradley D. Moser. Published by Prentice Hall.

Copyright © 2008 by Pearson Education, Inc.

Name Date Class

4. Drag the ice ball to the top of the cylinder and drop it in the cylinder of

Virtual Fluid. Click the green Drop button to let the ball fall into the fluid. Look at the close-up view window to note the new volume of the cylinder with both the Virtual Fluid and the ball. Record the volume in the table.

5. Problem Solving How can you determine the volume of the ice ball from your measurements?

Record the volume of the ice sample in Data Table 1. Click the handle at the

bottom of the cylinder to empty the contents of the cylinder.

6. Repeat Steps 2–5 for two more samples: aluminum and pine wood. Record your measurements in Data Table 1.

7. Calculate the weight of each of the objects. Remember Weight mass

force of gravity (g). Use the mass of the objects in kg and use g 9.8 m/s2.

Data Table 1

| | |Volume of |Volume of Virtual | | | | |

| |Mass of Sample|Virtual Fluid |Fluid and Sample |Volume of Sample| | |Buoyant force in|

| |(g) |(mL) |(mL) |(mL) | | |olive oil |

| | | | | |Weight of solid| |(N) |

| | | | | |(N) |Density | |

|Sample | | | | | |(g/mL) | |

|Ice | | | | | | | |

|Aluminum | | | | | | | |

|Pine Wood | | | | | | | |

Density of a Liquid

8. Use the Up and Down arrows on the control panel to toggle through the options of fluids to use in the lab. Select Ethanol. Click the Full button underneath that display to select the amount of fluid to be added to the cylinder. Click the Fill button to release the chosen amount of fluid into the cylinder. Click on the cylinder to see a zoomed in view of the level of the fluid. Record the volume in Data Table 2.

9. Drag the empty beaker on the counter to the balance and record its mass in

Data Table 2.

[pic]

10. Pick up the cylinder filled with ethanol and pour it into the empty beaker. Record the mass of the ethanol and beaker in Data Table 2.

Density and Buoyancy 65

Virtual Physics Lab Workbook, by Brian F. Woodfield, Steven Haderlie, Heather J. McKnight, and Bradley D. Moser. Published by Prentice Hall.

Copyright © 2008 by Pearson Education, Inc.

Name Date Class

11. Problem Solving How can you determine the mass of the ethanol in the

beaker?

Record the mass of the ethanol in Data Table 2. Click the handle at the

bottom of the cylinder to empty the contents of the cylinder.

12. Repeat Steps 8–11 to obtain the densities of water and olive oil. Record your measurements in Data Table 2.

Data Table 2

| | |Mass of Empty Beaker |Mass of Beaker and | | |

| |Volume of |(g) |Sample (g) |Mass of |Density |

| |Sample (mL) | | |Sample (g) |(g/mL) |

|Sample | | | | | |

|Ethanol | | | | | |

|Water | | | | | |

|Olive oil | | | | | |

Analyze and Conclude

1. Calculating Use the formula for density to calculate the density of each of the solid samples. Record your answers in Data Table 1.

2. Calculating Use the same formula as above to calculate the density of each

of the liquid samples. Record the answers in Data Table 2.

3. Applying Concepts Does the weight of an object or its density determine whether or not it will float in a fluid? Explain.

[pic]

66 Density and Buoyancy

Virtual Physics Lab Workbook, by Brian F. Woodfield, Steven Haderlie, Heather J. McKnight, and Bradley D. Moser. Published by Prentice Hall.

Copyright © 2008 by Pearson Education, Inc.

Name Date Class

4. Predicting Which of the solids will float in the olive oil? Explain.

Calculate the buoyant force on each of the objects in the olive oil. You have

calculated the volume of fluid displaced by each of the objects and the density of the olive oil. Use the density equation to calculate the mass of

olive oil displaced, from the density and volume that you have. To calculate the buoyant force, you need to calculate the weight of the displaced olive oil

in each case.

Buoyant force on object = Weight of displaced fluid =

Mass of displaced fluid X g

Record your results in Data Table 1. You can tell if each object will sink or float by comparing the weight of the object and the buoyant force. If the

buoyant force is larger, the object has more force pushing up than the weight

of the object pulling it down, so it will float. Compare the two forces to predict which objects will float.

5. Test your prediction by filling the cylinder with olive oil. Then move the

dispenser head over the next cylinder by clicking it and dragging it until it clicks in place above the cylinder. Fill three cylinders with olive oil. Place

each of the objects in one of the cylinders and release them all to see whether or not they will float.

6. Predicting What would you observe if olive oil and water were poured together?

Test your prediction by half filling one of the cylinders with olive oil by

1

clicking the -- button on the dispenser control before filling. That will allow

you to add half a cylinder of oil, then toggle through the fluids and select

1

water and dispense -- of a fill of water to the same cylinder.

[pic]

7. Drawing Conclusions If all three solids and all three liquids were mixed

in the same cylinder at the same time, what would you observe? List the solids and liquids that you would see from the top of the cylinder to the bottom. Explain how you determined your placement.

Density and Buoyancy 67

Virtual Physics Lab Workbook, by Brian F. Woodfield, Steven Haderlie, Heather J. McKnight, and Bradley D. Moser. Published by Prentice Hall.

Copyright © 2008 by Pearson Education, Inc.

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ISBN 0-558-83901-0

ISBN 0-558-83901-0

ISBN 0-558-83901-0

ISBN 0-558-83901-0

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