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Finding the Center of Gravity

This lesson on finding an object's center of gravity teaches students about gravitational force and the influence of object mass. Students observe, test, and record how objects behave in the pull of gravity. Team activity and class discussion encourage cooperation and verbal skills. Experimentation and hypothesis development introduce students to scientific inquiry. A Crow story about tipi poles illustrates that the position and shape of objects have significance.

Key Concepts

Physical Science, Forces, Gravity

Program/Collection

Multicultural Classroom Activities  View All »

Duration

1 - 50 minute period

Audience

3-5

Partners

National Teachers Enhancement Network

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http://btc.montana.edu/courses/aspx/lessons.aspx?TheID=14

tipi

Topics

Physical Science

Resource Type

Extended Lesson Plan

Format

Website

Updated

1/23/2017

You'll find additional information specific to this extended lesson plan below.

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More Info for this Extended Lesson Plan

Background

The Crow tribe has lived for many years on the northern Plains, where they once hunted wild game, lived in tipis, and moved often to follow herds of buffalo. Today the Crow reservation is located in southeastern Montana. In the Crow language, the tribal name is Apsáalooke. Tipis are still used for special occasions and gatherings to celebrate tribal culture and community. A force is a push or a pull on an object, and gravitational force is a force of attraction between two bodies such as the earth and a dropped object. The gravitational force, or pull, on a body is directly proportional to the mass of that body; the greater the mass, the greater the gravitational force. All objects exert a gravitational force on all other objects, but the size of that force depends on the mass of the objects and the distance between them. The movement of the moon around the earth is due to the gravitational pull of the earth, keeping the moon in orbit around the earth. The attraction between the earth and the moon also is responsible for the ocean tides on the earth.

Problem/Purpose

The purpose of this activity is to give students a basic understanding of the force of gravity and to illustrate that an object's mass affects how it reacts to gravity. Teams of students will conduct experiments that show the center of gravity of a tapered object is not at the physical center of that object. Students will use both class discussions and journal entries to develop hypotheses explaining why this is true. A Native American story about the tapered poles used to build tipis encourages students to think about the significance of object shapes.

Author

Shane Doyle
Cultural Consultant -CrowCultural Consultant

My name is Shane Doyle, and I live and teach on the Crow reservation. As an enrolled member of the Crow tribe, I grew up in the town of Crow Agency. I attended Montana State University in Bozeman, and returned home after college to teach 5th grade the Lodge Grass Public School. I'm just finishing my fourth, and most successful, year of teaching. I enjoy teaching, I enjoy science, and I'm looking forward to working with other teachers at the NTEN course!

Vocabulary

  • Mass - the quantity of matter in a body as measured by its inertia
  • Gravity - force that tends to draw all bodies in the earth's sphere toward the center of the earth
  • Center of Gravity - the average location of the weight of an object
  • Apsáalooke - The Crow tribal name in the Crow language
  • Tipi -is a conical tent originally made of animal skins or birch bark
  • Force - A push or pull on an object
  • ashé - home in the Crow language
  • ashtáale - real home in the Crow language
  • baalaawaasúua - a wooden house in the Crow language
  • awappóoshawaasua - brick house in the Crow language
  • póopahtataale - Great horned owl in the Crow language
  • bilíkaashee - Screech owl in the Crow language

Learner Outcomes

  • Recognize that gravitational force moves objects.
  • Develop hypotheses to explain why an object’s center of gravity may not be the physical center of that object.
  • Record and analyze data from in-class experiments.
  • Realize that the shape and mass of an object influence how it reacts to gravity

Content Standards

  • Science as Inquiry Content Standards – Center of Gravity Science as Inquiry Content Standard A: Abilities to do scientific inquiry
  • Physical Science Content Standard B: Properties of objects and materials
  • Science and Technology Content Standard E: Understanding about science and technology
  • Science in Personal and Social Perspectives Content Standard F: Types of Resources

Materials

  • Rulers and yardsticks
  • Branches or poles (thicker at one end and preferably straight), three for each group of two
  • Markers
  • Two long poles for teacher demonstration, one uniform thickness (e.g., dowel from hardware store) and one tapered

Lesson Procedures

  • Draw or show students an image of a tipi and explain that the framework of tipis is constructed from lodge poles, which are lodgepole pine trees about 20 to 30 feet long. See - Setting up a Tipi. Using a pole of this length for teacher demonstrations makes a dramatic illustration. Explain that Native American families have to cut enough lodgepole pine trees, take off the branches, and peel off the bark from each pole. The number of poles used may vary.
  • Read the Crow story about building tipis and the meaning of the different poles. It is helpful to use model-sized sticks, adding each to a tipi framework at appropriate points in the story (have the four base poles already in place, attached together with a rubber band or string).
  • Preview the following story:
    The Apsáalooke call the tipi ashé, home, or ashtáale, real home. The term ashtáale began to be used after the Apsáalooke started living in homes that the government had built. These homes were either made of logs or bricks. The log homes were called baalaawaasúua, wood house, and the brick houses were called awappóoshawaasua (awappóoshe is the term for bricks).

    The different parts of the tipi have significance to Apsáalooke people. There are usually twenty-one poles for a tipi, and each one of them represents something. Apsáalooke legend says that a warrior named Yellow Leggings brought the first tipis to the tribe. He was sent by the spirit man White Owl to kill an elk that had captured all of the winds. The elk had two helpers, the owl and the coyote. When Yellow Leggings killed the elk, he brought back the owl and the coyote. On the tipi there are two outer poles which are used to move the smoke flaps; the one on the south side is the coyote and the one on the north side is the owl. According to the religious history of the Apsáalooke, these two smoke-flap poles are sentries that watch the home and warn of evil or bad things. This idea is contrary to other native peoples' beliefs who say the owl is a symbol of bad fortune. To the Apsáalooke, however, póopahtataale, the great horned owl, is thought of as one who protects the home. The Apsáalooke do believe, as others, that bilíkaashee, the screech owl, warns families of impending death. The two outer poles then are the coyote on the right side and the owl on the left side.

    The two door poles on each side of the tipi door represent the mountain lion and the grizzly bear, also protectors of the home. The north pole of the door is the mountain lion, which was a gift to Yellow Leggings from his brother-in-law Juniper On The Bridge Of The Nose. The south pole is the grizzly bear, which was a gift from White Owl.

    The four base poles that the Apsáalooke use represent the four seasons. Since the Apsáalooke think of Spring as the beginning of the year, they consider the first base pole, the one on the southeast, to represent Spring. The southwest base pole is Summer, the northwest base pole is Fall, and the northeast base pole is Winter.

    There are five additional poles on the north and on the south, between the four base poles. The first two on the north, from the rear toward the front, represent well-being and health. The first two on the south, also from the rear toward the front, represent good fortune and wealth. There are then three poles remaining on each side. Whatever the owner of the lodge feels is sacred, these things are assigned to the six remaining poles. In addition to these meanings, the two pair of five also represent the ten lunar months of pregnancy.

    At the rear of the lodge are three more poles. The pole that the covering is tied to is called the chief pole, which represents the owner of the lodge. The two poles that are to each side of the chief pole are referred to as helpers. One helper pole represents a helper from the natural world and the other is a helper from the spiritual world.

    The tipi covering itself is white because White Owl instructed Yellow Leggings that the home is not to be touched by anything that is bad or evil, therefore the cover should be white to represent purity. Therefore, the Apsáalooke did not paint their tipis except in very few cases when they were instructed to do so by visions or dreams. Even today when the Apsáalooke set up their tipis at Crow Fair in Montana, the Tipi Capital of the World, most are pure white, with only a few painted.

    When the Apsáalooke first received the tipi they did not use stakes or pins, instead they used stones to set on the bottom portion of the lodge. The Apsáalooke refer to that time period as Biiakaashiisshipee, When They Placed Stones On Their Homes. The tipi rings that can be found on the Northern Plains are the stones that were used on the bottoms of the lodges. Many years after Yellow Leggings had brought the first tipis, a man by the name of Big Metal was visited by the badger. The badger instructed Big Metal to fashion stakes and pins for the tipi. He told Big Metal that there was no force on earth that could move Badger from his home, because he could dig his claws into the earth. Therefore by making stakes the tipi could be similarly held tight to the ground so that even strong winds could not knock it down. To represent the badger's gift to the Apsáalooke, some people will make stripes out of bark on their pins and stakes, to symbolize the stripes on the badger's back.

    The rear of the lodge is the place of honor. People who are respected, either spiritual leaders or outstanding warriors, are allowed to sit at the rear of the lodge. The area at the door is for the bravest men, because if the camp were attacked by enemies or some other emergency arose they could respond quickly and meet the crisis.

    Certain protocol and behaviors were expected inside the Apsáalooke home. People were instructed not to cry or say words that were intended to ruin another's good mood. People were expected to joke, laugh, and sing inside the home. The Apsáalooke would say that a good home could be recognized by the singing and cheery voices that could be heard from inside the lodge, even when there was stormy weather outdoors.

    The home is said to be the property of the woman, and in fact is believed to represent a woman, a second mother. The Apsáalooke say that a person's biological mother is on loan from the Creator, but as long as an individual has a home that offers security and happiness, then he or she has a second mother. As property of the woman, it is said that the others in the home live there by her grace and invitation. Even when the most outstanding chief would invite people over to eat, he would say, in reference to his wife, "We will eat some of her food, and she might provide us with soup." The women had pride in being a good hostess, especially in providing good food for her visitors.

    The Apsáalooke say that a good home has a road to it. This refers to a home where people like to come and visit. A good home is one where people can come and be fed, take home gifts and good thoughts. The sharing ethic of the Apsáalooke is well-expressed when people come to the home. They say, "If you have nothing at all, at least send your guest home with a kind word. Or, if you are down to your last morsel of food, cut it in half. Share, even if it is just a cup of water."

    The Apsáalooke also believe it is impolite to knock or ask to be admitted to a home. At a good home, visitors are always welcome. Even today, in Apsáalooke homes a visitor is at least given a meal, if not extra food or gifts to take home. If the visitor should be from a great distance or end up staying late, a bed is often offered. This practice is based on the Apsáalooke belief that, "When people grace your home, they bring with them good feelings, good thoughts, and good fortune. And since they have shared that with you then you should try to treat them well and make them feel good so when they leave they are happy."

    Source: Dale D. Old Horn and Timothy P. McCleary, Apsáalooke Social and Family Structure (1995), Crow Agency, Montana: Little Big Horn College, pp. 51-65
  • Discuss the Crow or Apsáalooke story with students. Ask students how difficult would it be to build a real tipi? Use a lodge pole, or a smaller version also with a tapered end, to demonstrate that in order for one person to carry a long pole, he or she must hold the pole at its center of gravity so the pole stays balanced. Show that any deviation from the center of gravity will make it more difficult. Comparing a pole of uniform thickness to one that is tapered, demonstrate to students that the center of gravity of the uniform pole is at its center, whereas that is not true for the tapered one. First let students speculate where on the poles the centers of gravity will be.
  • Separate the class into teams of two, each with three tapered sticks of different sizes. Have students divide in half a page in their journals. On one side, draw each stick as a line and label each with the length of the stick; then mark on each line what they predict will be the center of gravity. Balancing the sticks on their hands or fingers, students will find the actual center of gravity for each stick and mark the spot with a marker. For each stick, students will draw another line on the other side of the journal page, then measure and add to the drawing the stick's width on either end, as well as the measured location of the actual center of gravity. Have the students create a hypothesis that explains why the center of gravity is not always in the middle of the stick.
  • Have the teams write their hypotheses on the board. As a class, the students will test each hypothesis using the teacher's longer poles. Discuss the hypotheses and the student observations and conclusions from the class exercise. Guide the class to the correct explanation for the variation in the center of gravity, that the center of gravity is not actually the center of distance or length, but at the center of mass. A tapered stick has more mass at one end than the other, thus moving the center of gravity toward the larger end of the stick.

Conclusions

  • The center of gravity of a long object is not necessarily half of the length of that object.
  • The force of gravity pulls objects toward the earth’s surface.

Assessments

  • Student participation in team/class activity and discussion
  • Journal entries

Extensions

  • Take a set of dowels of different lengths to a second or third grade classroom and have the younger students find the center of gravity by balancing the dowels on their fingers.
  • Repeat the balancing exercise using a uniform stick like a dowel and hanging various weights on one or both ends of the stick.
  • Discuss things outside the classroom that might illustrate similar principles, such as playground teeter-totters, mobiles hanging from the ceiling, and high-wire walkers who carry long poles for balance.

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