PHYSICS WEB QUEST: GRAVITY
Name____________________________________ Date ________________________
Go to www.phet.colorado.edu
Open Gravity Force Lab
(http://phet.colorado.edu/simulations/sims.php?sim=Gravity_Force_Lab)
Part I: Force of Gravity
1. What is true about the force of m2 on m1, and the force of m1 on m2 (two
things)?
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2. Show the calculation for the force of gravity for the current set up:
3. Set both masses equal to 25 kg, and set them both 4 m apart. You can move the
masses and rulers.
What is the force of gravity? ___________________
Compare the following answers to this answer.
4. Keep the masses 4 m apart, but change one mass to 50 kg.
What is the new force of gravity? _____________________
How did it change (compared to #4)? ___________________________________
5. Change the 50 kg mass down to 12.5 kg.
What is the new force of gravity? _____________________
How did it change (compared to #4)? ___________________________________
6. Set both masses back to 25 kg, but move them so they are 8 m apart.
What is the new force of gravity? _____________________
How did it change (compared to #4)? ___________________________________
7. Given the two masses of 25 kg 8 m apart, how can you change one mass to get
back to the answer in #4? _____________________________________________
Now open My Solar System
(http://phet.colorado.edu/simulations/sims.php?sim=My_Solar_System)
Part II: Planetary Motion
1. Select the preset “Sun and Planet”
2. Set the speed in between fast and accurate
3. Adjust the settings for body 2. Leave the mass = 10, set x position to 150, y = 0,
and x velocity = 0, and y velocity = 75.
4. Describe the shape of the orbit. ________________________________________
5. Describe how the planet moves when it is close to the sun. __________________
6. Describe how it moves farthest from the sun. _____________________________
7. Stop. Reset
8. Adjust the settings for body 2. Leave the mass = 10, set x position to 150, y = 0,
and x velocity = 0, and y velocity = 200.
9. What happened to the planet? Why? ____________________________________
10. Calculate the escape speed of this planet (assuming distances are in meters and
mass in kilograms). This won’t match the simulation as we aren’t given units.
11. Stop. Reset
12. Adjust the settings for body 2. Leave the mass = 10, set x position to 150, y = 0,
and x velocity = 0, and y velocity = 80.
13. Calculate the initial moment of inertia for the planet considering it to be a point
mass where I = mr2.
14. Calculate the initial angular speed of the system from v = ωr
15. Calculate the initial angular momentum L = Iω
16. Hit stop when the planet is on the left hand side of the sun.
17. Mouse over the planet and record the x and y components of the planet’s velocity.
Use the components to calculate the tangential speed (pythagorean theorem).
vx =
vy =
v=
18. Mouse over the planet and record the x and y components of the planet’s position.
Use the components to calculate the actual radius (pythagorean theorem).
rx =
ry =
r=
19. Calculate the final moment of inertia for the planet considering it to be a point
mass where I = mr2.
20. Calculate the final angular speed of the system from v = ωr
21. Calculate the final angular momentum L = Iω
22. Compare 15 and 21. _________________________________________________
23. Stop. Reset. Click 3 bodies at the bottom of the screen – leave the presets. Click
Start.
24. Describe the motion of the blue object relative to the purple planet. ___________
__________________________________________________________________
25. What could this object represent?
26. Stop. Reset. Click 4 bodies at the bottom of the screen – leave the presets. Click
Start.
27. Use the tape measure to determine the entire horizontal axis of the purple planet.
Divide this value by 2. This is the semi-major axis of the ellipse which represents
the radius of the orbit. Purple Radius: ___________
28. Use the tape measure to determine the entire horizontal axis of the green planet.
Divide this value by 2. This is the semi-major axis of the ellipse which represents
the radius of the orbit. Green Radius: ___________
29. Time one revolution of the purple planet (you may have to reset). This is the
period of that planet. Period of purple planet (T): _________________________
30. Use the following formula to approximate the period of the green planet. (Let a =
green planet info, and b = purple planet info).
Ta2 ra3

Tb2 rb3
31. Reset and actually time the green planet. What is the measured period? T = ____
32. Is this value close to # 30?
33. Click a few of the other presets to observe some other astronomy phenomena.
Now open Lunar Lander
(http://phet.colorado.edu/simulations/sims.php?sim=Lunar_Lander)
Part III: Lunar Lander
1. Play the game. Your goal is to have the maximum number of landings before you
run out of fuel. Try to keep the landings soft and avoid the boulders.
2. How many landings did you achieve? ___________________________________
3. What was difficult about this game? ____________________________________
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Last Open Torque
( http://phet.colorado.edu/simulations/sims.php?sim=Torque )
Part IV: Torque
1. Click the tab at the top that says torque
2. Set the force equal to 1 N.
3. Click Go let this run for at least 10 seconds
4. What is the torque on the wheel (include direction).
5. What eventually happens to the lady bug? _______________________________
6. From Newton’s second law, a force will cause an __________________________
7. When considering angular motion, a torque will cause an ___________________
____________________ (consider both torque equations)
8. What must be the centripetal force that keeps the lady bug moving in a circle?
__________________________________________________________________
9. Why does this force eventually fail? ____________________________________
__________________________________________________________________
10. Reset all, and set the force back to 1 N.
11. Observe the acceleration vector as you start. Describe how it changes. _________
__________________________________________________________________
12. Will the acceleration vector ever point directly to the center? ________ Why /
Why not? (the next steps might help you answer this question) _______________
__________________________________________________________________
13. Reset all. Set the force back to 1 N.
14. Hit start, wait about 2 seconds, and set the brake force to 1 N. Hit enter and
observe.
15. Describe the motion of the wheel: ______________________________________
16. What happened to the acceleration vector? __________________________ Why?
__________________________________________________________________
__________________________________________________________________
17. What is the net torque? _________
18. Reset all. Set the Force back to 1 N. Hit Start.
19. After a few seconds, set the brake force equal to 3N and hit enter.
20. Right after you set the break force, calculate the net torque (check with the
graph):
21. Eventually the disc stops and the net torque is zero. This is because the breaking
torque changed as you can see in the graph. Why did it change?
Part V: Moment of Inertia
1. Click the Moment of Inertia Tab at the top.
2. Disregard any millimeter units. They should all be meters.
3. To best see the graphs, set the scale of the torque graph to show a range of 20 to
-20.
4. Set the Moment of Inertia Graph to show a range of 2 kg m2 to – 2 kg m2
5. Set the angular acceleration graph to show 1,000 degrees / s2 to –1000 degrees / s2
6. Calculate the moment of Inertia for the disk with the given information.
7. Hold the mouse over the disk so the mouse finger is pointing anywhere between
the green and pink circles.
8. Hold down the left mouse button. Move your mouse to apply a force.
9. Look at the graph and try to apply a force that creates a torque of 10.
10. Use the ruler to determine the radius at any point between the green and pink
circles. r = ___________m
11. Calculate what the applied force must have been.
12. Calculate the angular acceleration of the disk. Work in SI units, and then convert
to degrees / s2. Compare to the graph to check your answer.
13. Predict what will happen to the moment of inertia if you keep the mass of the
platform the same, but you create a hole in the middle (increase inner radius).
__________________________________________________________________
14. Set the inner radius equal to 2. Calculate the moment of inertia for this shape.
Set the disk in motion and check your answer by looking at the moment of inertia
graph.
15. Even when the force on the platform changes, the moment of inertia graph
remains constant. Why? _____________________________________________
__________________________________________________________________
16. Fill in the blanks: When the mass of an object increases, the moment of inertia
________________. When the distance of the mass from the axis of rotation
increases, the moment of inertia ___________________.
Part VI—Angular Momentum
1. Click the Angular Momentum tab at the top.
2. Set the scale of the moment of inertia and angular momentum graphs to show a
range of 2 to -2.
3. Set the angular speed to be 45 degrees / s.
4. What is the SI unit for angular momentum? ______________________________
5. Calculate the angular momentum in SI units (you should have already calculated
the moment of inertia in part II).
6. While the disk is moving, change the inner radius to 2.
7. Observe the graphs.
8. Changing the inner radius automatically changes the angular velocity to 36
degrees / s. Why? (mention moment of inertia and angular momentum in your
answer).