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 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)
Fgravitybetween 2 objects 
Gmobject1mobject2
d
2
G is a number you
look up in a book.
We’ll ignore it.
(Usually)
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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Is there gravity on you if you were in
the empty space between the planets?
1. Yes
2. No
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Why not or from what?
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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.
1.
2.
3.
4.
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There are two things, not on this list, you could do to weigh half as
much. What are they?
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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 shrank to half its current size but didn’t lose
mass, what would gravity be on the new surface?
1.
2.
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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.
4.
5.
6.
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 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.
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 at the same time. Which one
experiences the largest acceleration due to Earth’s gravity
at the moment they start falling?
1.
2.
3.
4.
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A
B
C
All 3 are the same
Original
location
A
B
C
Earth
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Which picture shows the spacing after a little while,
when all objects are still falling?
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2.
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4.
5.
Original
location
A
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C
1=
Original
spacing
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A
A
B
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C
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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.
Tidal forces
Technically there is no
such thing as a “tidal
force.”
A tidal force is:
Neat info: The “tidal forces” are what kill
a difference
you as you fall into a black hole.
in gravity’s acceleration
Good rubber band analogy on p. 140.
that
stretches things out.
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.
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?
Tidal Forces – see also figure 4.23,
page 140
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.
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
Where are the highest high tides?
On the left orange box, what phase is the Moon? 2nd box?
3rd box?
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)
Which object causes stronger tides
on Earth?
1. Sun
2. Moon
3. Both are equal
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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]
Because of tidal friction, days are
getting ____.
1. Longer
2. Shorter
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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.
Tides on the Moon
• Just like Moon causes tides on the Earth,
Earth causes tides on the Moon.
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
A bit about Jupiter’s Galilean moons
• Io
– 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
Do you think Jupiter’s moons are
synchronized? (Note: The moons
don’t affect each other.)
1. Yes
2. No
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Which planet had its moon(s) get
synchronized quicker?
1. Earth
2. Jupiter
3. Same
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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?
1. Weaker tides
2. 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)
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.
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?