1. Bell Work What are Kepler’s three laws about planetary motion?

2. 1. The orbits of the planets are ellipses, with the Sun at one focus of the ellipse. 2. The line joining the planet to the Sun sweeps out equal areas in equal times as the planet travels around the ellipse. 3. The ratio of the squares of the revolutionary periods for two planets is equal to the ratio of the cubes of their semimajor axes

3. Physical Science – Lecture 27 Weight on Other Planets

4. What does Gravity Depend on? It depends on your mass and the mass of the planet you are standing on.

5. Example If you double your mass, gravity pulls on you twice as hard. If the planet you are standing on is twice as massive, gravity also pulls on you twice as hard.

6. On the other hand, the farther you are from the center of the planet, the weaker the pull between the planet and your body. The force gets weaker quite rapidly. The force drops off with the square of the distance.

7. Newton’s Equation

8. The Variables M = mass of planet (kg) m = mass of object near planet (kg) r = distance between two masses from center of planet (kg)

9. Steps for Calculator Step 1: Enter an Open Parenthesis Step 2: Enter the first mass Step 3: Hit Multiply Step 4: Enter the second mass Step 5: Close Parenthesis Step 6: Hit Divide Step 7: Enter an Open Parenthesis Step 8: Enter an Open Parenthesis Step 9: Enter the value for r. Step 10: Enter closed Parenthesis Step 11: Enter x 2 or ˆ2. Step 12: Enter closed Parenthesis

10. Only really big things like planets, moons, and stars, have enough gravity to be measured.

11. Example If a 150 kg person is standing on earth, what is the force of gravity between them and the earth? Radius = 6,353,000 m, Mass = 5,973,600,000,000,000,000,000,000 kg

12. The masses are in the numerator because the force gets bigger if they get bigger. The distance is in the denominator because the force gets smaller when the distance gets bigger. Note that the force never becomes zero no matter how far you travel.

13. Extreme Example Standing on a neutron star makes you unimaginably weighty. Not only is the star very massive to start with (about the same as the Sun), but it is also incredibly small (about the size of San Francisco), so you are very close to the center and r is a very small number. Small numbers in the denominator of a fraction lead to very large results!

14. Weight and Mass on Earth For everyday matters, here on earth, your weight and your mass are the same, but that is not true everywhere!

15. Traveling to Another Planet If you ever become an interplanetary traveler, you will see that your weight is different on each planet. This is because a planet's gravity is determined by its own size, or mass. Larger planets have more gravity; smaller planets have less gravity. This is true for suns and moons too.

16. Weight on Other Planets If each planet has it’s own gravity, do we all weigh the same on other planets as we do on Earth?

17. To calculate your weight: mass x gravity = weight

18. Weight on Other Planets If you weight 150 pounds on Earth, how much will you weigh on other plants?

19. Location MassGravityWeight Earth 1 Outer space 0 Earth's moon 0.17 Venus 0.90 Mars 0.38 Mercury 0.38 Jupiter 2.36 Saturn 0.92 Uranus 0.89 Neptune 1.13 Pluto.07

20. Why are all planets round? Planets and stars are round because of gravity. Gravity pulls equally in all directions.

21. Of course a planet is not perfectly round — look at the mountains and valleys on the Earth and on Mars!

22. Suppose you had a great big, tall mountain. As time goes by, rocks and dirt loosen up and fall down the mountain side. Eventually the mountain is worn down. Similarly a deep, deep valley will fill up.

23. Also the bigger the planet, the stronger the gravity. So bigger planets will be rounder. Tiny planets may not be very round. For instance, some of the moons around Jupiter are not very big and are not round — sort of oblong and irregular. Asteroids, which may be only a few miles long, are also irregular.

24. Why does every planet have gravity? Every planet has gravity, because EVERYTHING has gravity! All matter in the universe has gravity. The bigger something is, the more gravity it has.

25. The earth has strong gravity, but the sun is much bigger and has much stronger gravity. You also have gravity much, much, much smaller than the earth, so your gravity is very small.

26. How do planets stay in orbit? The sun's gravity holds all the planets in orbit. Their orbits are a balance between gravity and the motion of the planet If the planet wasn't moving, it would fall into the sun.

27. Why doesn't gravity pull the planets into the sun? One approach is to go back to Sir Isaac Newton's reasoning — the famous apple falling from a tree event. Newton asked himself — why doesn't the moon fall to Earth, the way the apple falls from the tree?

28. Newton’s Reasoning He reasoned that the motion of the moon around Earth had something to do with it.

29. Suppose you threw the apple very hard. It would fall to earth, but not at the same spot. Suppose you could throw the apple so hard that it never fell to earth. It would keep circling Earth, just as the moon does.

30. Earth's motion in its orbit keeps it from falling into the sun. On the other hand, the moon can't escape from Earth (or Earth from the sun) because of gravity.

31. Holding it together The solar system is held together by gravitational forces, balanced by the forward motion of all its components.

32. How do gas planets stay together? How can gas have gravity? All matter, whether it is solid, liquid, or gas, has gravity. So the gas planets stay together because of the planets' gravity.

33. Stars A star is a massive, luminous ball of plasma held together by gravity. The combination of the radius and the mass of a star determines the surface gravity.