Download Planets or other objects orbiting a star are accelerating

Gravity, Orbits and Kepler’s Laws
LEARNING OBJECTIVES
 To learn how the orbit of an object is affected by mass, speed, and radius.
 To observe Kepler’s and Newton's laws
INTRODUCTION
Planets or other objects orbiting a star are accelerating -- they are continually changing
direction. The force that produces this acceleration is the gravitational attraction to the
star.
PROCEDURE
Go to http://phet.colorado.edu, click on “play with sims,” then click “physics” and open
the “Gravity and Orbits” simulation.
Keep it on the “cartoon” tab with just the Earth and Our Sun. Begin the activity with
original values. Hit Reset if you need to return to the original values. Click on: Gravity
Force, Velocity, Path, and Grid.
1. Hit the start arrow and observe the path of the Earth around the sun.
a. Look carefully at the orbit and the gridlines. What is the shape of the orbit?
b. Which of Kepler’s laws is illustrated? (State the law – don’t simply give the
number of the law.)
c. RESET and measure the time for one period of revolution. (You may have to
practice stopping at “one period” and you may want to slow the Sim speed.)
RESET
2. Slide the “Planet” selector to the left. You will be maintaining the distance to the
Sun but replacing Earth with a planet with a smaller mass.
a. Before hitting start, what changes do you notice in the vectors?
b. Which law does this illustrate?
c. Hit start and carefully observe the orbit and determine and record the period.
RESET
3. Slide the “Planet” selector to the right – maintaining the distance to the Sun but
replacing Earth with a planet with a larger mass.
a. Before hitting start, what changes do you notice?
b. What law is being illustrated?
c. Hit start and carefully observe the orbit and determine and record the period.
d. What do you learn from your measurements in parts 1, 2, and 3?
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RESET
4. You can adjust the speed of the orbit by moving the “V” at the end of the velocity
arrow up or down. Adjust the speed of the planet to make a more stretched-out
elliptical orbit.
a. Did you lengthen or shorten the velocity?
b. Describe what you observe about the speed (the length of the velocity arrow)
and the force (the length of the force arrow) as the planet travels the orbit.
c. Which of Kepler’s laws did this illustrate? (State the law – don’t simply give
the number of the law.)
RESET
5. Move the planet farther from the sun (so that it is still able to make a complete
orbit).
a. What changes in vectors did you notice?
b. Which law is represented?
c. RESET and hit start and carefully observe the orbit and determine and record
the period.
d. Compare this period with the period from #1. Which of Kepler's laws is
illustrated?
RESET
6. Switch modes, so that you are now looking at just the earth and the moon.
a. How long does it take for the moon to orbit the earth? (What is it's orbital
period?)
b. Describe what the Earth is doing at the moon orbits. (Describe the motion of
the objects AND the size and direction of the vectors.)
RESET
7. Switch modes, so that you are now looking at just the earth and a satellite (leave the
mass set to the Space Station)
a. How long does it take for the Space Station to orbit the earth? (What is it's
orbital period?)
RESET
8. Switch modes to view the Sun, Earth, and Moon.
a. Make a sketch showing all three objects and their paths for one full year.
b. Turn gravity off. How do the Earth and Moon move after gravity is
removed?
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Name:_______________________________________________________ Block___________
Gravity, Orbits, and Kepler’s Laws
1. Original Earth & Sun
a. ________________________________________
b. __________________________________________________________________
__________________________________________________________________
c. ______________________________
RESET
2. Smaller mass planet & the Sun
a. ___________________________________________________
b. ___________________________________________________
c. ______________________________
RESET
3. Larger mass planet & the Sun
a. ___________________________________________________
b. ___________________________________________________
c. ______________________________
d. __________________________________________________________________
__________________________________________________________________
RESET
4. Earth with a change in velocity & the Sun
a. ________________________________________
b. __________________________________________________________________
__________________________________________________________________
c. _______________________________________________
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RESET
5. Earth with a change in distance & the Sun
a. _____________________________________________
b. ______________________________________________
c. _________________________________
d. _________________________________________________________________
RESET
6. Earth and the moon.
a. __________________________________
b. __________________________________________________________________
__________________________________________________________________
RESET
7. Earth and the Space Station)
a. _____________________________________
RESET
8. Sun, Earth, and Moon.
a. Sketch
b. __________________________________________________________________
__________________________________________________________________
__________________________________________________________________
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