Lesson 7
Gravity
Mercury: Exploration #1
Gemini: Add Exploration #2
Apollo: Add the Extensions
Materials: Two balls, one
large, one small
Spring scales (5) with planet
overlays, objects to weigh.
Overview: All humans have an instinctual familiarity with
gravity-the point of this lesson is to focus and develop that
knowledge. Gravity is an essential aspect of our “systems”
approach to the Solar System-it is what connects the various
bodies of the Solar System.
Objectives: (1) Earth's gravity pulls any object on or near it
toward the Earth, but without touching it. It pulls everything
with the same strength. The student can:
a. state that gravity “holds” objects to Earth
b. chart the strength of gravity on different objects
c. contrast the force of gravity, which works at a distance,
with more familiar forces, which only work through
direct contact.
Marbles (at least two sizes),
flexible track, meter sticks,
tape, stop watch (optional), clay
Books/Videos: Magic School
Bus Lost in The Solar System
(ISBN #0590414283)
Bill Nye’s Gravity (*****)
(Available from DIRC)
WWW:
http://www.apolloarchive.com
A great source for video clips
(on computer) of astronaut
activities in zero and low
gravity. See Appendix.
http://www.athena.ivv.nasa.gov
/curric/space/planets/agewt.htm
l provides one’s age and weight
on other planets.
(2) All objects in the Solar System also have gravity. The
more weight the body has, the stronger it’s gravity is. The
student can:
a. state that more massive planets have stronger gravity
than less massive planets
b. determine the strength of gravity on different planets
c. contrast the force of gravity on Earth with the force of
gravity on other bodies.
(3) The Sun and planets are round because gravity pulls
everything as close as possible. The student can:
a. state that planets are spherical due to gravity.
b. model this through inquiry.
c. compare the concept to actual shape of Solar System
objects.
Background: Gravity is so obvious we tend to ignore it, except during and after a fall or other mishap! But
gravity is an all-important force, one of only four forces in the Universe.
Gravity is a property of all massive objects-everything made of matter also has gravity. (Matter, by the
way, is defined as something that takes up space, and has inertia.) Although gravity has an infinite range
(that is, your gravity stretches to the stars), it is very, very, very weak (your gravity is less than a billionth
of the force of a handshake!)
Gravity produces weight; in a place with no gravity, things are weightless. Earth's gravity produces our
weight, and causes objects to accelerate toward Earth's center. While different objects might have different
weights, they will fall toward Earth at the same rate, as Galileo was supposed to have done on the Leaning
Tower of Pisa. (But probably didn't!) Weight = mass and gravity acting together.
Vocabulary: Gravity, spherical, force, mass, matter, slope
Preparation: Assemble materials at your teaching site. For the inclined plane experiment, you will need a
table for each group. Scales for weights on other planets can be set up as a center.
Exploration #1: Being Galileo Galilei
Engage: Display the two balls, and hold them high above the floor. Ask the students what they think will
happen if you let go (Don't say "drop"!) the balls. As usual, record answers. Focus the student's attention to
how fast the balls will fall. Why do they fall at all? Will they fall at the same or different speeds? Do
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heavier things fall faster than lighter things? The answer to all of these questions lies in the nature of
gravity. Objects made of matter, like the Earth, people, and balls, have gravity, and so pull each other
towards themselves.
Explore: Let go of the balls, observing whether one ball falls faster than another. Again develop, through
discussion, factors that might make balls fall faster or slower. Does weight matter? Does size matter?
(Other variables, such as color of the object, are possible. Pursue them as you see fit.) How might the
students test these hypotheses? Some students may point out that it is hard to see which falls fastest,
because the balls go too fast. Then tell them we can slow things down by using an inclined plane.
History of Science Note: This experiment is exactly the same one Galileo did in 1589. See
http://www.rit.edu/~flwstv/galileo.html for a wonderful discussion of Galileo and his contributions to
science. To put his triumphs into perspective, Galileo was doing his experiments while the Pilgrims who
would found the Massachusetts Bay Colony were still infants! Galileo was 25 years old at the time he did
these experiments!
The students will test their hypotheses about how gravity works by rolling marbles down an inclined plane.
As in all our inquiry-based experiments, students should follow the standard process:
1.
Ask what affects the speed at which objects fall down the ramp (weight, size, density, color, angle
of slope...).
Figure 22: Galileo’s Experiment
Different slopes?
Different weights?
Different colors?
Different weights
Different colors?
Experiment
Different slopes
Results
2.
Plan a series of experiments that test their questions (use different weight, size, density, color of
balls, change the slope of the ramp)
3.
Conduct the experiments by collecting data. Use two ramps at a time, and vary one of the
variables between them, while keeping the others fixed. For example, here the ramps’ slope are
different, with the result that the ball on the steeper ramp hits the table first. Regardless of the
experiment, record as data which ball hits the table first. Always do at least three trials of the same
experimental set-up!
4.
Decide what variables affect speed by looking at the data. Students should see that only the slope
of the ramp matters, with steeper ramps allowing faster falls. The lesson is important: All objects
fall at the same rate, regardless of size, weight, color etc.
5.
Communicate your results by bringing the class back together and discussing the results. What
variable affect the speed of fall? What other experiments could the students do? Could you use a
feather in this experiment? Why/Why not? What about a helium balloon?
Explain: As usual, bring the class together to review the results. One way is to make a column graph of the
results, which might look like the one shown in Figure 23. Each column represents the results of all the
experiments in all the class. The column labeled “Hvy.” above the “Weight” variable indicates that of the
36 weight-comparison experiments done in the class, in 18 of them the heavy ball was first to the end of the
track, and in 18 of them the light ball was first. This is the result you, but perhaps not your students, would
expect. The second variable shown on Figure 23, Color, shows slightly different results. Here the Red balls
were faster 19 times, and the blue balls only 17. Does this mean that the red balls are being pulled more
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strongly that the blue ones? NO! It means that in the real world, one rarely gets perfect results. Even the
results for the size variable, 20 to 16, isn’t very good evidence for a difference; after all, only two trials had
to be mis-judged to give these results. The results for slope are pretty clear though-the steeper track is
definitely and consistently faster than the shallower track. We interpret this to mean that gravity attracts
things at the same strength, regardless of weight, size, or even color! This is a good time to watch Gravity
by Bill Nye
Figure 23: Results of Galileo’s Experiment
Number faster
40
30
20
10
0
Hvy. Light
Red Blue
Big Small
Steep Shallow
Weight
Color
Size
Slope
Variable
Exploration #2: Which way is up?
Engage: Which direction do things fall on Earth? Do they fall up? Down? Left, or right? Conduct a (rather
simple) experiment with the students in their groups. Using a globe, ask students to come up and
demonstrate which way things would fall on Earth, choosing different places on the globe as examples. At
the top of the globe, for example, students should correctly identify that an object would fall down, toward
Earth’s center. Now choose a point on the side of the globe, and again ask which way an object would fall.
Figure 24: Down is toward the center!
down!
Again the answer should be "down toward the center," which will confuse students, because he center of
the globe is, from their point of view, to the side. Now choose a place on the bottom of the globe, and
repeat the exercise. Again the answer should be "down toward the center," which is now “up” from your
student’s point of view. This is a tough result for third graders to understand-gravity everywhere pulls
things into the center, and this defines down!
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Explore: What does this mean for the shape of planets? Send the students back to their groups with a bag
of clay, where they will discover for themselves how gravity makes planets spherical (or round).
Have the students begin by making small (marble-sized) pieces of clay. After they have a significant pile,
ask the students to put their ideas of gravity to work. Show then two pieces of clay, and have them imagine
that each is a asteroid out in space. Bring them close to each other, and tell the students that in space gravity
would bring these two asteroids together. Another asteroid (aka a piece of clay) might happen by and
suddenly our little asteroids have combined to form a single, bigger one.
Now, ask the students some questions. Imagine that the large asteroid starts attracting other small ones; As
more and more little ones add on, gravity will force them as close to the center of the asteroid as possible.
What shape will the asteroid become? Have the class experiment, first by bringing two pieces of clay
together, and then by adding, piece by piece, additional pieces to their pile. Their goal is to always add the
newest piece as close to the center of their growing planet (the pile of clay) as possible.
What shape did the students get? While I might be a bit lopsided, they probably have a sphere-the same
shape as the planets and most other large bodies of the Solar System!
Explain: Bring the students back together. Have each group send up a representative to explain both the
shape they got, and why they got that particular shape. A final group activity is to combine all of the groups
results in to one large planet. (This process, of asteroids making little planets, and then little planets making
a big planet is exactly the way the planets are thought to have formed!)
Exploration #3: Weighty Matters
Engage: Remind the students that we've seen that gravity pulls us towards the center of the Earth. What
about other planets? The gravity of any body tends to pull things towards its center, so on the Moon things
fall toward the Moon's center, and so on. But the amount of gravity is different on each planet.
Watch, for example, a film clip from the www.apolloarchive.com site of people walking on the moon (try
"Aldrin demonstrates kangaroo hop" under Apollo 11), or a clip from Apollo 16
www.hq.nasa.gov/office/pao/History/40thann/mpeg/ap16_salute. Note how the astronauts bounce up and
down very easily. Is gravity stronger or weaker on the Moon as compared to the Earth? (Weaker-about 1/6
of Earth). The force with which gravity (on any planet) pulls something towards its center is called weight.
Let's explore the strength of gravity on other planets by seeing how much things weigh on other planets.
(This is the perfect time to read the Magic School Bus Lost in the Solar System.)
Explore: This is a perfect center activity. Students weigh common objects on scales with the scales
readings adjusted accurately for each planet. This activity also forms the basis for the assessment.
Distribute the four scales (labeled Earth, Moon, Jupiter and Sun) at the center, along with a number of
common objects and the worksheets (in the notebooks). Student weigh a few objects (at least three) on each
scale, recording the weight of each object on the worksheet. After completing the weighing, students must
decide what feature of the planet controls the weight they measured. Included on the worksheet are a
number of features, including color, temperature, atmosphere and mass. Students can determine the
relationship by graphing, or ranking, the answers. Students should discover that only mass of the planet has
any relationship to weight-and the heavier the body, the heavier the weight!
Table 11: Weights on Other Planets
Body
Gravity (relative to Earth)
Body
Gravity (relative to Earth)
Sun
28
Mercury
4/10
Venus
9/10
Earth
1
Moon
2/10
Mars
4/10
Asteroids
3/100
Jupiter
3
Saturn
1 2/10
Uranus
9/10
Neptune
1 2/10
Pluto
1/10
How to use this table: This table provides an easy way of calculating weights on any planet.
For example, a 60 pound third grader would way 60 x 28 =1680 pounds on the Sun, and
60 x 3/100 = 1.8 pounds on Ceres, an asteroid!
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Evaluate: The first exploration provides an excellent way of assessing the students' practice and content
knowledge. Students’ notebooks can be examined for use of the inquiry rubric, graphing fluency, use of
data and factual content. Alternatively, use the Gauge Page for Lesson 7 and ask the following questions:
1.
On the left hand side of the page, please draw a picture of the Earth, with two people on it, one on
the top, one on the bottom. On the right hand side, please tell me, in words, why the person on the
bottom doesn’t fall off.
2.
Now add a picture of the Moon to the left hand side of the page. Remember that the Moon is
smaller, and has less mass, than Earth. On the right hand side of the page, tell me if you would
weigh more on the Moon or the Earth. Also tell me how you know this.
3.
Look at the page after your Gauge Page. Pretend that you are an astronaut, traveling in an
unknown Solar System. You send a robot to explore the three planets, and the robot reports that it
weighed 3000 grams on the first planet, 500 grams on the middle planet, and 1000 grams on the
third. On Earth, the robot weighs 1000 grams. Can you tell which planet has the most mass?
Which planet has the least mass? How do you know this?
Extend: This is a great Lesson for incorporating mathematics, specifically graphing and even a little solid
geometry. Try graphing the results of Inquiries #1 and #3. For #3, an excellent line graph can be made
between the weight of objects on Earth, Moon, Jupiter and Mars, and their masses. See Figure 26.
Figure 26. Graphing Results of Exploration #3
Measuring Gravity
12
Jupiter
10
8
Mass of Planet
6
4
2
00
Earth
Mars
Moon
200
400
600
800
1000 1200 1400 1600
Weight of Object (grams)
Appendix: To find Dave Scotts’ experiment of dropping a hammer and feather on the Moon, use the
following steps. First, go to http://www.apolloarchive.com. Click “multimedia” on left window, scroll
down to “Apollo 15” and Choose “Dave Scott performs…” It’s best to download the movie for later
viewing.
Bill Nye’s Gravity is a very good to excellent survey of gravity, with good connections to the planets.
Themes are:
Down is towards the center of the Earth.
Shows Dave Scott’s gravity experiment on the Moon!
Planets are round because of gravity.
Some really spectacular falls, that is of gravity in action, are included
No errors detected.
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Help us improve this curriculum! Tell us what you liked and disliked, what you would change, omit, or
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• What aspects of Lesson 7 worked poorly or not at all?
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Please return to Science Instructional Resource Specialist, DIRC, 110 McDonald Drive
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