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PLANETARY MOTION SIMULATIONS KEY

Simulation 1: Universal Gravitation|
2.
d.

3. Change the planet’s mass. RESET. What happened to the orbit?

Planet keeps a constant path, but the force vector increases with an increase in mass

4. Change the star’s mass. RESET. What happened to the orbit?

Increase mass and the planet crashes into sun, decrease and the earth leaves the orbit

5. What do you have to change to make the planet leave the orbit of the star only changing masses?
Have the sun mass set to 0.5

6. What do you have to change to make the planet crash into the star only changing the masses?
Increase the suns mass
.

7. FULL RESET, then recheck all the boxes. Select Sun, Earth, & Moon:
a.
b. What happened to the planet’s orbit?
Planets orbit remains the same
c. What does the moon’s orbit look like?
Looks scalloped like flower petals or a sea shell

8. Change both the star and planet’s masses. RESET. What happened to the orbits of the moon
and planet?
Increasing Earth’s mass causes the Moon to crash into it. Decreasing the Earth’s mass causes the
moon to orbit the Sun instead. Increasing the Sun’s mass causes both the planet and the moon to
crash into the Sun. Decreasing the Sun’s mass causes the Earth to leave orbit, but the Moon’s orbit
seems to stay the same relative to the Earth.

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9. Copy this table into your journal and fill it in by describing what you observe:
Explain what you What other changes
How can you…. Draw the orbital path
changed did you observe?
Decrease Earth’s The force of gravity
mass between the Earth
and Sun decreases
…make the Moon
orbit Earth in an
orbit with a larger
radius?

Decrease Sun’s mass The force of gravity

between the Earth
and Sun decreases
…make the Earth
take more time to The Moon’s orbit is
orbit the Sun? also shorter

Increase Sun’s mass The force of gravity

between the Earth
and Sun increases
…make the Earth
take less time to The Moon’s orbit is
orbit the Sun? also longer

10. Along the bottom bar, choose “To Scale”

d. Describe what you see.
The Sun and planet are much smaller and further apart. The orbit is stable and nearly
circular.
11. Choose one: Change the star’s mass, change Earth’s mass, change both masses. What happened
to the orbit?
Increase Sun’s Mass: more eliptical path
Decrease Sun’s Mass: Earth escapes orbit
Decrease Earth’s Mass: more eliptical path
Increase Earth’s Mass: Sun moves around during orbit more
Increase both mases: more eliptical path
Decrease both masses: Earth escapes orbit
12. FULL RESET, then recheck all the boxes. Select Sun, Earth, & Moon:
a. Set the Star Mass to “1.5” and the Planet Mass to “Earth”
b. Start the simulation with and allow 365 days to pass
c. What happened to the planet’s orbit?
Earth’s orbit become more eliptical and shorter (nearly 2 years pass)
d. What happened to the moon’s orbit?
The moon now orbits the Sun at about the same path and pace as Earth
e. What happens when you turn off gravity?
The moon and Earth both continue in a straight line instead of orbitting the Sun

13. FULL RESET, then recheck all the boxes. Select Earth & Satellite:
a. Start the simulation with
b. What happens to the orbit when you change the satellite’s mass?
The orbit is the same, but the gravitational force increases
c. What happens to the orbit when you change the planet’s mass?
Increasing the mass causes it to crash; decreasing the mass causes it to escape orbit

Simulation 2: Orbital Motion Vectors|

16. Describe the object's velocity during the course of the elliptical orbit. Comment on both the
magnitude and the direction.
During the elliptical orbit, the magnitude of object’s velocity changes. It is largest close to the Sun.
Its direction is always tangential (pointing in a straight line in its direction of motion)

17. Describe the net force acting upon the object during the course of the elliptical orbit. Comment
on both the magnitude and the direction.
During the elliptical orbit, the magnitude of object’s force changes. It is largest close to the Sun. Its
direction is always pointing towards the center of the Sun.

18. How does changing the eccentricity of the orbit effects the shape of the orbit? What eccentricity
results in a perfectly circular orbit?
Increasing the eccentricity creates a more elliptical orbit. Decreasing it creates a more circular
orbit. Zero eccentricity results in a perfect circle.
19. Sketch the diagrams below in your journals. Then draw and label the net force (FNET) and
velocity (v) vectors for each of the six marked positions for a clockwise orbit. Draw the vectors
in the appropriate direction and of the proper relative magnitude.

Elliptical Orbit Circular Orbit

20. Fill in the blanks in the following statements for a planet orbiting the Sun:
a. The direction of the velocity of the planet is always (for circular orbit) perpendicular to
the net force acting upon the planet.
b. The planet will move fastest when it’s closest to the Sun.
c. While moving towards the Sun (along the path from D to A) there is a component of force
in the same direction as the motion; this causes the planet to speed up.
d. While moving away from the Sun (along the path from A to D) there is a component of
force in the opposite direction as the motion; this causes the planet to slow down.

21. Summarize what you’ve learned from this simulation. Be sure to describe the features of a
planet’s elliptical orbit including velocity, acceleration and force.
A planet’s velocity and force of gravity are always largest in magnitude closest to the Sun and
smallest far away. The velocity is always tangential and the force always points towards the center
of the Sun. Acceleration should be in the same direction as the force.