1. Gravity is a System

2. Gravitation explains things
Gravitation explains why objects fall toward the center of Earth.
Gravitation explains why the moon, the ISS, and satellites orbit around Earth.

3. Gravity is a system
There have to be 2 objects in order for gravitation to happen.

4. Gravity is a system
Every particle in the universe attracts every other particle.

5. Gravity is a system
Gravitation depends on the mass of the objects and the distance between them.

6. A little virtual experiment…
We are going to see how the mass of a planet and a star can change the force of gravitation between them.
It’s not really possible to change the mass of Earth or another planet, so we’ll have to do this in the virtual world.
Click on the link below to go to the pHET site: ulation/gravity-and-orbits You might want to open it in a separate browser window.

7. Click on the “Download” button

8. Switch to “To Scale”
(Double Click)

9. Select the
Path,
Grid,
and Measuring Tape Options

10. Click on the Play-Circle button to start the simulation and observe.

11. Be ready to click on the Pause-Circle button as soon as the planet makes it back to EXACTLY where it started, at the grid intersection lines.

12. # of Earth Days to complete 1 orbit box
On your worksheet, in the first column,
complete the
Shape of Orbit box
and the
# of Earth Days to complete 1 orbit box
(it should be 365)

13. Next, move the blue and yellow end of the measuring tape until the little red x is centered on the star in the middle of the orbit.

14. Drag the other end to the shortest side of the orbit (on the right) and note the distance on your worksheet.
Make sure to add 3 zeroes at the end of the number

15. Now repeat for the other side of the orbit (to the left.)
Record your measurement in the “longest” box.
Make sure to add 3 zeroes at the end of the number

16. Now, click on the Refresh-Circle button in the lower right.
Re-check the boxes for Path, Grid, and Measuring Tape
Slide the bar on Planet Mass from “Earth” to “1.5” times the mass
Repeat the simulation.
Do this for each of the 4 scenarios.

17. Pretty much the same every time, right?
Now, see what happens if you change the mass of the STAR…
Start with the Star Mass set to “Our Sun” then repeat the simulation for 1.5 times the Star Mass, 2 times the Star Mass, and 0.5 times the Star Mass.
Record your measurements and observations each time.

18. Draw some conclusions based on your data…
What happened when the mass of the planet was increased or decreased?
What happened when the mass of the star (which has MUCH more mass than the planet) was increased or decreased?

19. Gravity is a system
Gravitation depends on mass: What do you think would happen if the mass of one of the objects changed?
If one of the objects gets bigger, the gravitation between them gets bigger.
If one of the objects gets smaller, then gravitation between them gets smaller.

20. Gravity is a system
Gravitation also depends on distance: What do you think would happen if the distance between the objects changed?
If one of the objects moves farther away, the gravitation between them gets smaller.
If one of the objects moves closer, then gravitation between them gets bigger.