Barycenter Lab
Background Information
Binary bodies are two celestial bodies held together by mutual gravitational attraction. Gravity is a force of attraction
between all objects in the universe. Examples of binary bodies are two stars, a planet and its sun, or a planet and its moon.
Binary bodies behave somewhat as if they were connected by a dowel. Their center of gravity or center of mass is called the
barycenter (the point between two binary bodies where their mass seems to be concentrated and the point about which they
rotate). If the masses of the binary bodies are equal, the barycenter lies at an equal distance from each body. If the masses of
the binary bodies are not equal, the barycenter is located closer to the larger mass. Binary bodies revolve (move in a circular
path about a point) about their barycenter.
The average barycenter of our solar system lies just outside the surface of the Sun. It changes depending on the location of the
planets. Jupiter, the most massive planet, has the greatest effect. Find out more about the barycenter of binary bodies in the
solar system. Where does the barycenter lie for most planet-satellite (moon) systems? Which planet has such a massive moon
that the barycenter lies in the space between them? For information, see Thomas R. Watters, Planets: A Smithsonian Guide
(New York: Macmillan, 1995).
Materials
 Hole paper punch
 1/2-by-3-inch (1.25-by-7.5-cm) piece of thick paper, such as a file folder
 Cord
 Play-doh
 Scale
 Wooden dowel
 Ruler
Procedure
1. Use the paper punch to make a hole in each end of the piece of paper.
2. Bend the paper to bring the holes together. Thread one end of the cord through the holes. Tie a knot to hold the holes
together. You have made a paper sling for the dowel.
3. Tape the string to a supporting object (like a desk or table edge). If you do not have a desk or table edge, you can
simply hold the string.
4. Using the food scale, measure two 50 gram pieces of play-doh. Shape each piece into a ball.
5. Stick one end of the dowel into one of the clay balls to a depth equal to the radius of the ball, celestial body #1.
6. Slide the free end of the dowel through the paper sling.
7. Repeat step 5 using the remaining clay ball on the other end of the dowel.
8. Determine the balancing point by moving the dowel back and forth in the sling until it balances (see Figure 11.1).
9. Measure the distance between the center of celestial body #1 and the center of the paper sling. The center of the
paper sling is the barycenter between celestial body #1 and celestial body #2.
10. For trials 2 -5 you will mold the play-doh into a larger ball (M1) and a smaller ball (M2). Record the mass and
distance for each play-doh ball.
Pre-Lab Discussion:
1.
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4.
A celestial body can be a _____________________, ___________________, or ___________________.
In this lab, the celestial body is represented by _____________________________________________.
The distance between two celestial bodies is represented by _________________________________.
The point of balance between the two balls of play-doh is their center of gravity. This represents the
center of _____________________, or the ___________________________ of two celestial bodies.
Hypothesis:
1. The independent variable(s) for this lab include: _____________________________________________
2. The dependent variable(s) for this lab include: ______________________________________________
3. My hypothesis for this lab is:____________________________________________________________
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Data:
Table: _______________________________________________________________________
Trial
Mass of Celestial Body
Mass
A of Celestial Body
Distance
B
to CelestialDistance
Body A to Celestial Body B
Trail 1
Trail 2
Trail 3
Trail 4
Trail 5
Data Analysis:
Graph: __________________________________________________________________
Results:
What did your graph show you? Was your hypothesis supported or not?
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