Montana State University

Canoes and Buoyant Force

Subject: Physical Science, Forces, Buoyancy
Classtime: 3, 50 minutes class periods
Grade Level: 4-5

Integration & Introduction:

This lesson introduces the concept of buoyant force and how the shape and weight of objects can influence their ability to float. Students will use a spring scale to measure the buoyant force exerted on their handmade canoes, thereby conducting scientific inquiry. They will work in teams to compare the buoyancy of different-sized canoes, recording results in their journals and discussing possible explanations for different results. Readings about the use of canoes for travel and fishing by the Makah tribe of Washington State illustrate the real-world significance of buoyancy.

Learner Outcomes:

Students should be able to:
  • Recognize that weight, design, and size influence how well an object (in this case, student canoes) is able to float.
  • Feel the upward, buoyant force of water on their canoes.
  • Measure the buoyant force exerted on different canoes (different sizes), after predicting how the buoyant force for each canoe will compare to others.

Content Standards:

  • Science as Inquiry Content Standard A: Abilities to do scientific inquiry
  • Physical Science Content Standard B: Position and Motion of Objects

Problem/Purpose:

The purpose of this activity is to give students a basic understanding of natural forces, in this case, the buoyant force. Students will observe some of the variables affecting the respective buoyancy of different objects. Students will record and discuss their team experiment results, practicing both written and verbal accounts of scientific data. Cultural and economic Native American tribal traditions that center around canoes encourage students to think about practical applications of buoyancy.

Background:

The Makah people have lived on the Olympic Peninsula in the northwestern corner of present-day Washington State since ancient times. The ocean has always been an important part of their culture and their livelihood. Traditionally, there were various types of wooden Makah canoes designed for different purposes. Young children were taught how to handle smaller canoes made especially for them. Many Makah still make a living by fishing in the Pacific Ocean and Puget Sound. They are highly skilled mariners, taking long ocean voyages in their canoes over hundreds of miles along the coast of Washington and British Columbia.

When an object like a canoe is placed in water, the water exerts what is called a buoyant force upward on the canoe, allowing it to float. A buoyant force is exerted on all objects placed in liquid, whether or not those objects float. If an object floats, the buoyant force exerted on it is greater than the force of gravity. If the object sinks instead, the force of gravity is greater than the force of buoyancy. Buoyancy, then, is the tendency of a body to float or to rise when submerged in a liquid.

Materials/Technology:

  • Bars or partial bars of soap (one for each student)
  • Two-gallon tubs of water (one for each group of three students)
  • Spring scale (one for each group)
  • Small suction cups with a loop on top (one for each group of three students)
  • String
  • Carving tools (spoons, clay working tools)
  • Research journals

Vocabulary:

  1. Buoyancy - upward pressure on a floating object
  2. Spring Scale - a weighing scale used to measure force, such as the force of gravity, exerted on a mass
  3. Mass - the quantity of matter in a body as measured by its inertia
  4. Volume - the amount of space occupied in three dimensions

Procedure:

  1. Read to the students the literature on the Makah people and their canoes (see information at www.makah.com). Ask students whether they have ever been in a canoe or other types of boats.
  2. Distribute soap and carving tools to each student and allow time for each to carve a small canoe. An alternative approach would be forming the canoes from modeling clay that is then fired in the oven.
  3. Divide the students into groups of three, making certain that the three canoes in each study group are different sizes. To each group, distribute a tub of water, a suction cup, a piece of string, and a spring scale.
  4. Instruct the students to record in their journals the weight of each of the three canoes (out of water).
  5. Ask students to write their predictions as to which of the three canoes will have the most buoyant force exerted on it (i.e., on which will the water have to push up hardest to make the canoe float) and which will need the least force to float.
  6. Allow the students to feel the buoyant force by pushing their own canoe under the water. (Afterwards, dab the canoe dry with paper towels so it doesn’t get too slippery.)
  7. Give students the following instructions:
    1. Tie one end of your string to the hook on the spring scale.
    2. Thread string through the loop on the top of the suction cup.
    3. Tie the other end of the string around your canoe.
    4. With your group partners, hold the canoe and the scale out of the water while pushing the suction cup against the bottom of the tub so it sticks.
    5. Canoe Setup
    6. Place the canoe in the water so it floats. Pull on the spring scale until there is tension on the string but the boat is not being pulled out of position. Record the amount on the spring scale.
    7. Now slowly pull on the spring scale until the canoe is submerged just under the surface of the water. Record in the journal what the scale reads.
    8. Repeat this for each of the three canoes.

Conclusions:

Example:
The weights and shapes of different objects affect how those objects float in liquids such as water.

Assessments:

Student participation in team activity

Student journals

Extensions:

Make boats out of other materials and conduct the same experiment. Does the material used affect how something floats?

What other variables could be changed? (e.g., the liquid, whether the floating objects are hollow)

Think of examples in your life when buoyancy is important, such as swimming in the pool or fishing from a boat. Have you noticed that you can float on water when you spread your arms and legs flat, but if you curl into a ball, you sink?

References/Resources:

Author: Shane Doyle
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