Chapter 4
Newton’s laws & gravity & tides
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?
Universal Law of Gravity:
• Newton hypothesized that _____________________
_______________________________________.
• The mathematic relationship is called
an Inverse Square Law and it looks like this:
Gravity is forcing you towards the
person next to you.
1. True
2. False
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Is there gravity on the Moon?
1. Yes
2. No
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If you were twice as far from Earth as you
are now, how would gravity change?
1.
2.
3.
4.
5.
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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
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If the Earth grew to 3 times its current size but
didn’t change mass, how would surface gravity
change?
1.
2.
3.
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7.
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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
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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?
Masses
• Earth’s mass is ________________________.
– _____________________________kg (__ zeroes).
• When a pen falls down, why does the pen move
& the Earth doesn’t?
• Earth _____________________.
• Similar to _____________________.  Found
____+ planets around other stars (since 1995)!
Tides
• End of chapter 4.
• Clicker questions coming soon.
Big questions
•
•
•
•
•
What causes tides?
What worlds do they affect?
How do they affect these worlds?
How are the tide-creators affected?
Give examples.
Objects A, B, C are above the Earth, A at top, B middle, C
bottom. They all start falling simultaneously. Which one
experiences the largest acceleration due to Earth’s gravity?
0
0
0
0
1.
2.
3.
4.
A
B
C
All 3 are the same
Original
location
A
B
C
Earth
Which picture shows the spacing after a little while,
when all objects are still falling?
1.
2.
3.
4.
5.
1=
Original
spacing
Original
location
A
1
2
3
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5
B
C
2
3
A
B
A
B
C
A
B
C
C
5
A
A
B
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C
B
C
This slide left blank.
__________ forces
Gravity _____________
___________ as it falls.
This is called ____, they
are _______________
__________________
Between the ________,
_______________,
and _______________.
Technically there is no
such thing as a “tidal
force.” _____________ Neat info: The “tidal forces” are what kill
___________________ you as you fall into a black hole.
___________________. Good __________________ on p. 140.
More on tides
______________ (therefore) ____ work on:
• _______________________ and
• ________________________
Common misconception: many people think
you need liquid on the surface to
experience tides. But the _____________
_____________________________ too!
• __________________________ because
– land is ______________________.
Tides on worlds
• Which objects do you think can cause
noticeable tides on Earth?
• On the Moon?
• Mercury?
• Venus?
• Io?
• Europa?
• Titan?
• Pluto? Pluto’s moon Charon?
Tidal Forces – ___ also _________
Tidal Forces – again, see also figure 4.23. 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.
Another blank slide
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?
Tide table, part 1 – monthly cycle
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!
Tide table, part 2 (one day)
Tidal Effects from Moon – see fig ____
• Tides cause the ____________, causing _____
__________________. (clicker)
– Earth _____________________
(by ~ 2ms/century).
• _____________ also causes ______________
– ____________________________________
– __________________ & now probably ___________
•
See fig 4.26 for more detailed explanation.
Tides on the Moon
• Just like Moon causes tides on the Earth,
_______ causes tides on ___________.
Tidal Effects on the Moon
• ________ causes ____ to _________
• _________________________ until…
• The __________________________.
__________________ (synchronized)
_______________________________.
Fancy words: The _________________
of the Moon is ______________________
________________.
See pages ___________.
A bit about _____________ moons
• ____
– __________________
• _____________
– _______________________________
– _________& _______________________________?
• All 4 are approximately the size of ___________
• All 4 are about the _______________________
– as _______________________________________
Do you think Jupiter’s moons are
synchronized? (Note: The moons
don’t affect each other.)
0
0
1. Yes
2. No
Another result of tidal friction
As the _______________ due to ____________________
__________________________________________.
 ______________________________________________.
As Moon ______________________, what would we observe?
1. ___________________________________
2. Moon ____________________________
 ______________________________________
As Moon ______________________, will its orbit time increase,
decrease or stay the same? Tides were weaker or stronger?
(no clicker)
Important for forming life! (water less stagnant)
Tides affecting other moons’ orbits
• _____________ is ____________________________
– tides __________________ because it orbits more than once per
day
– Phobos will ________________________ in 50 million
years (_________________) or maybe _______
• ________________ is __________________ moon
– Orbits _____.
– Will also ________________________, in 100 million years.
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?