Buoyancy (Titus Kim)

Title: Sink or Swim!

Principle(s) Investigated: Buoyancy, Buoyant Force; Archimedes Principle; Density.

Standards : Eight Grade Physical Science Principle 8: Density and Buoyancy

8. All objects experience a buoyant force when immersed in a fluid.

• C. 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.
• D. Students know how to predict whether an object will float or sink.

Materials:

1. Plastic bin or Large beaker (1000 ml or 500 ml)
2. Water
3. Aluminum foil ( 12 cm x 12 cm)
4. Pennies ~ 40

Procedure:

1. Cut foil into 12 x 12 cm squares. Repeat three times.
2. Fold each square into a boat.
   1. Fold the first one into canoe shape. (BOAT 1)
   2. Fold the second square into a wide u-shaped boat. (BOAT 2)
   3. Choose your own shape for the third one. (BOAT 3)
3. Fill bin or beaker with water.
4. Place BOAT 1 into water.
5. Place pennies one at a time until boat sinks.
6. Record number of pennies onto data table.
7. Dry pennies
8. Repeat 4-7 for boats 2 and 3.

Student prior knowledge:

1. Density of a substance is the mass per unit volume.

2. Density = Mass/Volume

3. Density of water is 1.0 g/mL

4. An object that is more dense than the fluid in which it is submersed sinks.

5. An object that is less dense than the fluid in which it is immersed floats to the surface.

6. The density of an object can be changed by changing the mass or the volume.

7. Buoyant force acts in an upward direction against the force of gravity.

8. Archimedes’ Principle states that the buoyant force on an object is equal to the weight of the fluid displaced by the object.

9. More fluid displaced means a greater buoyant force. Less fluid displaced means a smaller buoyant force.

Explanation: The purpose of this lab experiment is to help students witness buoyant force in action. This is the force that acts in the opposite direction of gravity. When the buoyant force is greater than the force of gravity acted upon an object, that object will float. Adding the pennies into the foil boat increases the force of gravity enacted upon the foil boat. It also increases the amount of water displaced. According to Archimedes' Principle, the buoyant force is equal to the weight of the water displaced. Thus, the more water a boat displaces, the greater the buoyant force counteracting the force of gravity. Each of the foil boats will continue to float in the water until the force of gravity becomes greater than the buoyant force.

The design of boat 2 has a greater surface area than boat 1. The greater surface area allows boat 2 to occupy/displace a greater volume within/of the water. This is important because by increasing the volume of water occupied/displaced, it has a lower density than boat 1 because density is mass/volume. Both boats 1 and 2 will have the same mass because it is constructed with the same amount of foil.

Questions & Answers:

1. Which boat was able to support the most pennies? What allowed this boat to carry more pennies?
   1. The boat that has a greater surface area in contact with the water allows for a greater displacement of water. This is what allows that boat to support more pennies
2. Which boat would require greater energy to forward in the water?
   1. The boat with the greater surface area would require greater energy because it has to displace more water when moving forward.

Applications to Everyday Life:

1. Ships stay afloat in the sea by using the same principle examined in this activity.
2. Submarines are able to dive and resurface by manipulating its' volume through the usage of a ballast tank.

Photographs:

https://docs.google.com/spreadsheet/ccc?key=0At-chnAhFxY4dGttRWhpbEVwZGVhLWc4V0l6RUlCLUE#gid=0

Videos: http://www.youtube.com/watch?v=YiSl1wvP4Lc

Sources:

Kevin Takeuchi: Master Teacher at Lawrence Middle School