Orbits and gravity
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School
Arizona Western College *
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Course
130
Subject
Astronomy
Date
Apr 3, 2024
Type
Pages
5
Uploaded by ChefFangRabbit32
Lab #2:
Understanding Gravity and Orbits
Objectives:
-
Observe the direction and amount of gravitational force in different
scenarios
-
Use mathematics to compare the results of different scenarios
–another highly appreciated skill in the workplace
-
See the same sorts of observations that Kepler saw, and connect
them with Kepler’s laws
-
Apply your understanding to make predictions –You can use this
skill to avoid ending up in a failarmy/instant regret tiktok
Name:
Lab partner(s):
Date:
Part 1: Getting Started
For this lab we are going to use the following simulation:
https://phet.colorado.edu/sims/html/gravity-and-orbits/latest/gravity- and-orbits_en.html
Select the “
To Scale”
option of the simulation. This will start you off with just the Sun and
Earth. Next turn on the
Grid, Velocity, Path, and Gravity
Force
options. Then set the speed to Fast and hit play.
Once the Earth has completed one orbit of the sun, push pause and
sketch, as accurately
as possible, what you see on the next page before proceeding to answer these
questions.
1. What color line is used to represent:
a. the velocity? Green
b. the orbit or path? Gray
c. gravitational force? Blue
2. What direction is the
gravitational force
of the Earth?
The direction of the gravitational force of the earth is towards the center of the earth.
3. What direction is the
gravitational force
of the Sun?
The gravational force of the Sun
is directed towards the center of the Sun
.
4. What direction is the
velocity
of the Earth?
The direction of the earth velocity is
tangential to its orbit around the Sun.
5. Push play, and while the Earth is orbiting, click on the toggle to turn
Gravity OFF. (note that this is not the checkbox). What happens?
When the gravity turns off while the earth is orbiting, the earth will move in straight
line path at a constant speed.
6. Using vocabulary words from our lecture on Gravity and Kepler’s laws (Lecture 5 , for all
2023) explain why this happens:
Happens because when the gravity turns off during the earth is orbiting , the earth will
continue to move straight at a constant speed, that happen because is newtons first law of
motion.
Part 1: Sketch of the system after one orbit. Be as
accurate as possible. Also label the following: force,
velocity, orbit (path), Earth, Sun
Part 2: What Happens to Gravity?
Now you are going to experiment with the force of gravity. Hit the “reset” button, and then
turn on the
grid, velocity, path, gravity force, and Fas
t again. Also turn on measuring
tape. Let the Earth orbit once and hit pause again.
7. Use the tape measure to measure the following things
(note: you only should need
about 3 significant figures or digits for accuracy; so if you measure 344,010.0 km, you
only need to write down 3.44 x 105 km)
a. Gravitational force experienced by the Sun ______________
b. Gravitational force experienced by the Earth ______________
(note: this is going to give a general idea of how strong the force is—but not an actual
realistic number, since the tape measure is in units of kilometers and force is not)
8. Now reset the simulation as above (turning on everything you need). Click on Earth and
drag it to move it. Double its distance from the Sun (it is about two squares of the grid away
from the Sun, so move it to about 4 squares. You may need to zoom out). Now measure
(with three significant figures for accuracy):
a. Gravitational force experienced by the Sun __________
b. Gravitational force experienced by the Earth ____________
9. Divide your number for 8a (above) by your number for 7a (remember, 3 significant
figures): ____________________
10. You are comparing the force the experienced by the Sun normally (in the original
scenario, 7) to what force it experiences when the distance between the two masses is
doubled (2x as far, in the second scenario or 8). Is the force doubled, or 2x what it was? Is it
halved, or 1/2x what it was? Using your understanding of gravitational force, explain why or
why not:
11. Now hit play, and wait about 350 Earth Days (about a year). Describe what happens, and
why it has happened (in terms of force, velocity, acceleration etc):
Part 3: What else happens to Gravity?
Hit the “reset” button again, and then turn on
grid, velocity, path, gravity force, tape
measure and Fast
again. Let the Earth orbit once and hit pause again.
12. Use the tape measure to measure the following sizes (3 significant figures)
a. Gravitational force experienced by the Earth __________
b. The
velocity
of the Earth ____________
(note: this is going to give a general idea of the speed of the Earth—but not an actual
realistic number, since the tape measure is in units of kilometers)
13. Now
reset
the simulation as above (turning on everything you need). Then set the mass
of the sun to 2.0 (or, 2 times the mass of our sun). Now measure the gravitational force
experienced by the Earth: _______________________
14. Divide your answer for Q13 by your answer for Q12a ((remember, round to 3 significant
figures)). By dividing your number for 13 by your number for 12a, answer the following
question: Using the above method, answer this question: The gravitational force experienced
by the Earth is HOW much more, or less, in this high solar mass scenario (13) than the first
scenario (12)? _________________
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Now hit play, and wait about 350 Earth Days (about a year) before hitting pause. Sketch the
system on the next page, and
label perihelion and aphelion before proceeding
.
15. Does the force of gravity stay the same throughout the planet’s orbit? Why or why not?
16. Run the simulation until the planet is at perihelion (closest approach) and pause it.
Measure the velocity of the planet at this point: _______________ in the (approximate)
direction of: _______
17. Run the simulation forward until the planet is at aphelion (furthest away) and pause it to
measure the velocity of the planet at this point: _______________ in the (approximate)
direction of: _______
18. Compare your answers for Q16 and Q17. Which of Kepler’s laws does
Part 3: Accurately draw the system after about a year. Be sure to label the following: force,
velocity, orbit (path), Earth, Sun, perihelion, aphelion; and make sure their relative sizes are
appropriate. BONUS +1 EC EACH: draw and label the major and minor axes (also
acceptable: semimajor and semiminor axes)
Part 4: Orbits
Hit the “reset” button again, and then turn on
grid, velocity,
path, gravity force, tape measure and Fast
again. Let the
Earth orbit once and hit pause again.
19. Turn on the tape measure and use it to measure the
distance from the center of the sun to the orbit of Earth
directly to the
right
. What distance do you measure? (3
significant figures). Note that this is what the measuring
tape was originally designed for, so you need units with
this as well: __________________________________
20. Now use the tape measure to measure the distance from
the center of the sun to the orbit of Earth directly to the
left.
What distance do you measure? _______________________________
21. Compare these two distances. What do you notice about
them? And what does this mean about the
shap
of Earth´s orbit?
Part 5: Think about it
In 1-2 sentences or bullet points?
22. Why did I stop asking you to report the gravitational force
for BOTH the Sun and the Earth about halfway through the lab?
23. What would happen if you were to repeat part 2 but
halve the distance instead of doubling it? Explain why this happens:
24. What would happen if you repeat Part 3 but double the
mass of the Earth instead of doubling the mass of the sun ? Explain why this happen