Our solar system consists of the Sun and the many millions of celestial bodies, including large planets and microscopic dust particles, which orbit around it. As a unit, the solar system has a center of mass, its balancing point. At this point, the system would balance like a spinning plate atop a circus performer's balancing stick. This point, called the barycenter, is the exact point about which all the bodies in the solar system orbit. Since the Sun is vastly larger and heavier than all the other bodies combined, the solar system's barycenter is very close to the Sun—but not at the Sun's center. Thus, while all the other solar system bodies seem to orbit the Sun, they, including the Sun, are actually orbiting a point in space just beyond the Sun's outer layer.
Binary bodies are two celestial bodies held together by mutual gravitational attraction. In this project, you will learn how mass affects the location of their barycenter. You will discover the mathematical relationship between the masses of binary bodies and their distances from their barycenter. You will prepare a model of the Earth-Moon system and determine the orbit that each body follows as it orbits the barycenter of the system. You will also discover the location of the barycenter for most planet-moon systems.
Purpose: To model the barycenter of binary bodies.
Pre-Lab Questions:
1.
What is barycenter? Is the sun our solar system’s barycenter? If not, what is?
2.
What are binary bodies? List an example from our solar system?
3.
Which binary systems will this lab model?
4.
What force holds binary bodies together? Which astronomer hypothesized about this force?
1.
Cut a length of string about 12-15 inches long
2.
Secure the string around the PVC pipe so that it is stable but still able to be moved along the length of the pipe.
3.
Using the scale, measure two 50g pieces of Play Doh. Shape each piece into a ball.
4.
Stick one end of the dowel into one of the clay balls to a depth equal to the radius of the ball.
5.
Repeat step 5 using the remaining clay ball on the other end of the dowel.
6.
Determine the balancing point by moving the dowel back and forth in the sling until it balances
(see Figure 2.1).

7.
Measure and compare the distance between the center of each clay ball and the string. Record this on your paper in the data table. Include both distances.
8.
Gently push one of the balls so that the dowel turns. Observe the motion of the clay balls.
Record your observations on your paper.
9.
Adjust the size of one of the clay balls so it is noticeably smaller than the other. Adjust the string until there is a balance and then re-measure the distance between each clay ball and the rope.
Record your observations.
10.
Gently push one of the balls so that the dowel turns. Observe the motion of the clay balls.
Record your observations on your paper.
11.
What happens if you adjust the barycenter so that it isn’t balanced? Describe the revolution of the pipe and Play Doh.
The balancing point is in the center of the dowel, an equal distance from each of the clay balls. The balls move in a circular path around the sling.
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 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. Binary bodies revolve (move in a circular path about a point) about their barycenter.

Q: What is barycenter? Is the sun our solar system’s barycenter? If not, what is?
A:
1.
2.
Q: What are binary bodies? List an example from our solar system?
A:
3.
Q: Which binary systems will this lab model?
A:
4.
Q: What force holds binary bodies together? Which astronomer hypothesized about this force?
A:
Trial 1: Clay spheres are the same size
Distance between clay ball 1 and string:
Distance between clay ball 2 and string:
What did you observe about the motion of the spinning?

Trial 2: Clay spheres are different sizes
Distance between clay ball 1 and string:
Distance between clay ball 2 and string:
What did you observe about the motion of the spinning?
Post-Lab Questions
1.
In trial 1, where was the center of mass (indicated by the location of the string) located?
2.
In trial 2, where was the center of mass (indicated by the location of the string) located?
3.
If the center of mass is located exactly halfway between two objects, how does that affect the revolution?
4.
If the center of mass is not located exactly halfway between two objects, how does that affect the revolution?
5.
What can you conclude of the barycenter of the sun and Earth system? (Hint: Does the earth spin around the sun’s center?)