1. Newton’s laws & gravity & tides
Chapter 4
Newton’s laws & gravity & tides

2. Summary of gravity & tides
What does force of gravity depend on?
What objects A) give gravity? B) Experience gravity?
If you change mass(es), how does gravity change? If change distance or radius?
What mechanism causes tides in general?
What objects cause tides here on Earth?
Name 2 other worlds that experience significant tides, and what are the tide sources?
How have tides affected the Moon’s appearance from Earth?
What is changing in the Earth-Moon system as a result of tides?

3. Universal Law of Gravity:
Newton hypothesized that gravity’s force depends on three things.
The mathematical relationship is called an Inverse Square Law and it looks like this:
FOR ANY 2 OBJECTS (any  Universal)
G is a number you look up in a book. We’ll ignore it. (Usually)

4. Gravity is forcing you towards the person next to you.
True
False 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

5. Is there gravity on the Moon?
Yes No 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

6. Is there gravity on you if you were in the empty space between the planets?
Yes No
Why not or from what? 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

7. Which of the following would make you weigh half as much as you do right now? Select all that apply.
Take away half of the Earth’s atmosphere.
Double the distance between the Sun and the Earth.
Make the Earth spin twice as fast.
Take away half of the Earth’s mass.
There are two things, not on this list, you could do to weigh half as much. What are they? 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

8. It would be four times as strong It would be twice as strong
If you were twice as far from Earth as you are now, how would gravity change?
It would be four times as strong
It would be twice as strong
It would be the same
It would be half as strong
It would be 1/4th as strong 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

9. It would be four times as strong It would be twice as strong
If the Earth shrank to half its current size but didn’t lose mass, what would gravity be on the new surface?
It would be four times as strong
It would be twice as strong
It would be the same
It would be half as strong
It would be 1/4th as strong 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

10. It would be 9 times as strong It would be 6 as strong It would be 3x
If the Earth grew to 3 times its current size but didn’t change mass, how would surface gravity change?
It would be 9 times as strong
It would be 6 as strong
It would be 3x
It would be the same
1/9 as strong
1/6 as strong
1/3 as strong 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

11. Real triple-size planets
We just said planets that are 3x the Earth’s size have _________ gravity.
Do you think real planets that are 3x bigger than Earth have weaker gravity?
Why not?

12. Masses Earth’s mass is 6 x 1024 kg. 6 trillion trillion kg.
6,000,000,000,000,000,000,000,000 kg (24 zeroes)
When a pen falls down, why does the pen move & the Earth doesn’t?
Earth DOES accelerate up, VERY slowly!
Similar to planets tugging on stars.  Found 405* planets around other stars (since 1995**)!
*as of 11/23/2009
**According to exoplanet.eu (NOT edu) 1 system was discovered in 1989, around pulsars! The first “regular” planet detection was in 1995.

13. Tides
End of chapter 4.
Clicker questions coming soon.

14. Big questions What causes tides? What worlds do they affect?
How do they affect these worlds?
How are the tide-creators affected?
Give examples.

15. Objects A, B, C are above the Earth, A at top, B middle, C bottom
Objects A, B, C are above the Earth, A at top, B middle, C bottom. They all start falling at the same time. Which one experiences the largest acceleration due to Earth’s gravity at the moment they start falling? A B C
All 3 are the same B C A
Original location
Earth 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

16. Which picture shows the spacing after a little while, when all objects are still falling?
Original location 1 2 3 4 5
1 = Original spacing 2 3 4 5 A A A A A B B B B B C C C C C 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

17. Tidal forces Gravity stretches an object out as it falls.
This is called tides, they
are caused by
Differences in gravity’s acceleration between
the near side,
center,
and far side.
Technically there is no such thing as a “tidal force.”
A tidal force is:
a difference
in gravity’s acceleration that
stretches things out.
Tidal forces
Neat info: The “tidal forces” are what kill you as you fall into a black hole.
Good rubber band analogy on p. 140.

18. More on tides Gravity and (therefore) tides work on:
Liquid (water) and
Land (rocks)
Common misconception: many people think you need liquid on the surface to experience tides. But the land experiences tidal bulges (high tides) too!
Water bulges are more noticeable because
land is harder to lift up.

19. Tides on worlds
Which objects do you think can cause noticeable tides on Earth?
On the Moon?
Mercury?
Venus?
Io? Famous for…
Europa? Famous for …
Titan? Famous for …
Pluto? Pluto’s moon Charon?

20. Tidal Forces – see also figure 4.23, page 140

21. Tidal Forces – again, see also figure 4. 23
Tidal Forces – again, see also figure And see also “common misconception” on page 141
With your neighbor, decide where water would be deep & shallow if we added a LOT of water to Earth.

22. Tidal Forces – still figure 4.23
How many high tides are there on Earth right now?
How often are there high tides are there at the beach each day?
The arrow in this picture shows the Moon to the right. Where else could the Moon be to have identical Earth tides?

23. Tide table, part 1 – monthly cycle
Where are the highest high tides?
On the left orange box, what phase is the Moon?
Note: Would you expect the 2 high tides each day to be the same strength?
Are they? Why not? (Not answered in class.) Think three dimensionally!
2nd box?
3rd box?

24. Tide table, part 2 (one day)

25. Which object causes stronger tides on Earth?
Sun
Moon
Both are equal 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

26. Tidal Effects from Moon – see fig 4.26
Tides cause the oceans to move, causing friction inside Earth and with ocean floor. (clicker)
[to be continued]

27. Because of tidal friction, days are getting ____.
Longer
Shorter 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

28. Tidal Effects from Moon – see fig 4.26
Tides cause the oceans to move, causing friction inside Earth and with ocean floor. (clicker)
Earth slows down (by ~ 2ms/century).
Friction also causes internal heat
Volcanoes on Io
liquid water on Europa & now probably Enceladus
See fig 4.26 for more detailed explanation.

29. Tides on the Moon
Just like Moon causes tides on the Earth, Earth causes tides on the Moon.

30. Tidal Effects on the Moon
Friction causes Moon to
Slow its rotation until…
The bulges lined up exactly with Earth.
Synchronous rotation (synchronized)
Keeps the same face towards Earth.
Fancy words: The rotation time
of the Moon is the same as its revolution time.
See pages 44-45

31. A bit about Jupiter’s Galilean moonsIo
Volcanoes (mechanism shown soon, similar to Europa)
Europa
Salt water under the ice surface
Ganymede & Callisto maybe also?
All 4 are approximately the size of Earth’s Moon
All 4 are about the same distance from Jupiter
as Earth’s Moon is from Earth

32. Do you think Jupiter’s moons are synchronized
Do you think Jupiter’s moons are synchronized? (Note: The moons don’t affect each other.)
Yes No 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

33. Which planet had its moon(s) get synchronized quicker?
Earth
Jupiter
Same 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

34. Another result of tidal friction
As the Earth slows due to tidal friction reaction causes …
Moon to move away from the Earth, 4cm per year.
As Moon moves away, what would we observe?
Weaker tides
Moon appears smaller
no more total eclipses
As Moon moves away, will its orbit time increase, decrease or stay the same? Tides were weaker or stronger? (no clicker)
Important for forming life! (water less stagnant)

35. Tides affecting other moons’ orbits
Phobos is one of two tiny Mars moons
Orbits very close to Mars.
tides make Phobos get closer because it orbits more than once per day.
Phobos will be split into ring particles in 50 million years (Roche limit) or maybe crash
Triton is Neptune’s largest moon
Orbits clockwise.
Triton will also crash/split into rings, in 100 million years.

36. Summary of gravity & tides
What does force of gravity depend on?
What objects A) give gravity? B) Experience gravity?
If you change mass(es), how does gravity change? If change distance or radius?
What mechanism causes tides in general?
What objects cause tides here on Earth?
Name 2 other worlds that experience significant tides, and what are the tide sources?
How have tides affected the Moon’s appearance from Earth?
What is changing in the Earth-Moon system as a result of tides?