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6-8 > Physical Science
Grade level: 6-8 Subject: Physical Science Duration: Two class periods
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Students will understand the following:
1. Without air resistance, all objects would fall with the same acceleration, regardless of mass.
2. Gravity is the force that causes objects to fall.
3. Air resistance, a type of friction, works against gravity to decrease the acceleration of a falling object.

An encyclopedia or a computer with Internet access should be available to students. The following materials should be provided for each group:
A variety of object pairs, such as balls of different sizes and weights, or a book and a sheet of cardboard the same length and width as the book
Objects, such as a feather or a sheet of paper, that encounter more air resistanc when dropped than the other objects

1. Ask your students if they predict that a heavier or larger object, if dropped from a height, will fall to Earth faster than a lighter or smaller object. Tell them that Galileo Galilei (1564-1642) performed a famous experiment that they are going to replicate in order to confirm or refute their predictions.
2. Have students use the Internet or an encyclopedia to find out about Galileo’s experiment in which he dropped objects from the Leaning Tower of Pisa in Italy.
3. Divide the class into groups, giving each group a variety of objects to experiment with (see Materials).
4. Instruct groups to meet in order to design their own experiments. Remind them that a good experiment should have a control and introduce only one variable at a time. Each group’s experimental design should include a chart on which to record the results of each test performed.
5. Have students experiment with the object pairs, dropping them, one at a time, while standing on a chair or desk. Other students in the group should observe closely to see whether one object reached the floor before another or both objects reached the floor at the same time. Students should carefully record their results on their charts. (Students should find that balls of different sizes and weights fall at the same rate of speed, as do a book and a sheet of cardboard the same length and width as the book.)
6. When students try dropping a feather or a sheet of paper from the same height from which they dropped the other objects, they will discover that the feather and the paper fall more slowly. Suggest that they bunch the sheet of paper up into a ball and drop it from the same height. They will find that the ball of paper reaches the floor in less time than the sheet of paper.
7. Have students meet in their groups to discuss possible reasons for these results. They should conclude that air resistance, a type of friction, is slowing down the feather and the sheet of paper.
8. Ask students what they think would happen if they performed the same experiment in a vacuum tube, which has no air in it. (The feather would fall at the same rate of speed as a ball or a brick.)
9. Each student should write a report explaining the results of the experiments and drawing conclusions regarding the effects of both gravity and air resistance on the acceleration of falling objects. Encourage students to accompany their paragraphs with labeled drawings and diagrams.
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Adaptations for Older Students:
Have students research and explain Newton’s three laws of motion. They should identify the law that builds on Galileo’s experiments with gravity.
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Discussion Questions

1. Describe how the human body has adapted to the force of gravity on Earth over time. How might it have evolved if gravity had not been present?
2. Compare and contrast the force of gravity on Earth with the force of gravity on one of the other planets in our solar system. Which planet has a stronger gravitational force? What would be the effects on astronauts’ bodies when visiting this planet for an extended period of time?
3. Describe the physical features on the Earth’s surface that were influenced by gravity during their formation or that are influenced by it now. How would these features look if Earth’s gravitational force were significantly weaker or stronger?
4. Discuss Newton’s Laws of Motion. Which law or laws involve the force of gravity? Give examples of each law of motion that occurs in your everyday life.
5. A roller coaster is usually designed to give its riders the sense of defying the laws of gravity. Sometimes the back car is moving slightly faster than the others, thanks to the acceleration due to gravity, and at other times the first car is going slightly faster. With that in mind, which seat in a roller coaster is the scariest? How might a roller coaster designer make a roller coaster that gives the greatest sensation to the passengers?
6. Describe the gravity experiments conducted by Galileo, Newton, and Cavendish. How were these experiments similar? How did these scientists build on each other’s research and observations to make their discoveries?
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You can evaluate your students on their reports using the following three-point rubric:

  • Three points:results accurately reported; illustrations or diagrams clearly labeled; conclusions explained logically in well-written, well-organized paragraphs

  • Two points:results adequately reported; illustrations or diagrams included; paragraphs lacking in organization

  • One point:reporting of results sketchy or inaccurate; no illustrations or diagrams; conclusions lacking in logic; paragraphs poorly organized

You can ask your students to contribute to the assessment rubric by determining which results should be reported and which conclusions explained.
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Swinging Pendulums
One of Galileo’s key experiments involved observing pendulums. Using a variety of different lengths and weights, he carefully noted each pendulum’s period (the amount of time it takes for a pendulum to make one complete swing). Galileo’s observations allowed him to determine that the period of a pendulum’s swing is affected by its length but not its weight—an observation that may run counter to what students intuitively expect. To begin this activity, lead a class discussion about the fact that scientists cannot rely on intuition alone, but must perform experiments to test their hypotheses. Ask your students to replicate Galileo’s pendulum experiments using varying lengths of string with different numbers of washers attached to the ends. Students should measure each pendulum’s length, weight, and period, making sure that they keep the amplitude, or angle to which the pendulum is raised, consistent. When their observations are complete, gather their data into a chart for easy reference.

Defying Gravity in Your Own Home
Considering recent developments in space programs around the world, it may not be long before humans are able to live for a long time in outer space—not just astronauts, but families. Have your students imagine that their own homes are going to be transported to an orbit around Earth; then ask them to design a room that could exist in a microgravity environment in outer space. The students’ descriptions should be detailed—how, for example, will they prevent food from floating up off kitchen plates? How will they stay in bed while sleeping? You can start by brainstorming a list of common activities that are performed in each room of a standard house; then make sure that students take all of those needs into consideration. If time permits, students can build a model of their newly designed room. You can even divide students into groups and have each group work together to design a model outer-space house.

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Suggested Readings

Six Easy Pieces: Essentials of Physics Explained by Its Most Brilliant Teacher
Richard Feynman. Addison-Wesley Publishing Company, 1995.
This witty discussion of gravity, written without equations and technical jargon, is an ideal introduction to the concept written by one of the most admired and accessible scientists of our time.

The Character of Physical Law
Richard Feynman. MIT Press, 1994.
Fascinating writing by a truly great scientist—gravitation is Richard Feynman’s principal law in this outstanding book. His approach and enthusiasm for the subject makes this title classic reading in the field.

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Universal Gravitation
Teacher Tom Henderson provides us with an interactive physics textbook that helps students to understand gravity with text and animations.

Exploring Gravity
An introductory, intermediate and advanced online tour of our understanding of gravity from ancient times to theoretical constructs like black holes. Check your knowledge of this phenomenon with the online gravity test.

Virtual Lab on Projectile Motion
Guided by a student lab manual, students will observe and analyze how an object projected in a gravitational field is affected by changes in the angle of projection, velocity, and the resistance of the air through which the projectile moves.

Orbit Simulator
Here's your chance to reconstruct the Solar System. Add a new star and a couple of planets; change masses and positions; press the button and watch how the gravitational interactions cause each object to orbit around one another.

Amusement Park Physics - Roller Coaster
Put your knowledge of gravitational potential energy to work and design and test drive your very own roller coaster at this hands on and "physics is phun" web site.

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Click on any of the vocabulary words below to hear them pronounced and used in a sentence.

speaker    atrophy
Definition:Decrease in size or wasting away of a body part or tissue.
Context:Muscles atrophy when a body spends time in space.

speaker    gravity
Definition:A fundamental physical force that is responsible for interactions that occur because of mass between particles, between aggregations of matter (as stars and planets), and between particles (as photons) and aggregations of matter, that is 1039 times weaker than the strong force, and that extends over infinite distances but is dominant over macroscopic distances especially between aggregations of matter.
Context:Gravity is the force of attraction all celestial bodies have upon objects at their surface.

speaker    ligament
Definition:A tough band of tissue connecting the articular extremities of bones or supporting an organ in place.
Context:Ligaments become loose after time in weightlessness.

speaker    supernova
Definition:The explosion of a very large star in which the star may reach a maximum intrinsic luminosity one billion times that of the sun.
Context:The rare phenomenon of the supernova is caused by the force of gravity on a star.

speaker    weightless
Definition:Lacking apparent gravitational pull.
Context:When an object or body is released into outer space, it seems to be weightless.

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This lesson plan may be used to address the academic standards listed below. These standards are drawn from Content Knowledge: A Compendium of Standards and Benchmarks for K-12 Education: 2nd Edition and have been provided courtesy of theMid-continent Research for Education and Learningin Aurora, Colorado.
Grade level:6-8, 9-12
Subject area:science
Understands essential ideas about the composition and structure of the universe and the Earth’s place in it.
Benchmark 6-8:
Knows characteristics and movement patterns of the nine planets in our solar system (e.g., planets differ in size, composition, and surface features; planets move around the sun in elliptical orbits; some planets have moons, rings of particles, and other satellites orbiting them).

Benchmark 6-8:
Knows that gravitational force keeps planets in orbit around the sun and moons in orbit around the planets.

Benchmark 9-12:
Knows that although the origin of the universe remains one of the greatest questions in science, current scientific evidence supports the “big bang” theory, which states that between 10 and 20 billion years ago, the entire contents of the universe expanded explosively into existence from a single, hot, dense chaotic mass; our solar system formed from a nebular cloud of dust and gas about 4.6 billion years ago.

Benchmark 9-12:
Knows ways in which technology has increased our understanding of the universe (e.g., visual, radio, and x-ray telescopes collect information about the universe from electromagnetic waves; computers interpret vast amounts of data from space; space probes gather information from distant parts of the solar system; accelerators allow us to simulate conditions in the stars and in the early history of the universe).

Grade level:6-8, 9-12
Subject area:science
Knows the kinds of forces that exist between objects and within atoms.
Benchmark 6-8:
Understands general concepts related to gravitational force (e.g., every object exerts gravitational force on every other object; this force depends on the mass of the objects and their distance from one another; gravitational force is hard to detect unless at least one of the objects, such as the Earth, has a lot of mass).

Benchmark 9-12:
Knows that the strength of the gravitational force between two masses is proportional to the masses and inversely proportional to the square of the distance between them.

Grade level:6-8, 9-12
Subject area:science
Understands the scientific enterprise.
Benchmark 6-8:
Knows ways in which science and society influence one another (e.g., scientific knowledge and the procedures used by scientists influence the way many individuals in society think about themselves, others, and the environment; societal challenges often inspire questions for scientific research; social priorities often influence research priorities through the availability of funding for research).

Benchmark 9-12:
Knows that creativity, imagination, and a good knowledge base are all required in the work of science and engineering.

Grade level:9-12
Subject area:science
Understands basic features of the Earth.
Knows how life is adapted to conditions on the Earth (e.g., force of gravity that enables the planet to retain an adequate atmosphere, intensity of radiation from the sun that allows water to cycle between liquid and vapor).

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Mary C. Cahill, middle school science coordinator, Potomac School, McLean, Virginia.
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