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?

221

2 dmGm

F objectobjectobjectsbetweengravity

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)

Gravity is forcing you towards the person next to you.

0

0 1. True2. False

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Is there gravity on the Moon?

0

0 1. Yes2. No

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Is there gravity on you if you were in the empty space between the planets?

0

0 1. Yes2. No

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.

There are two things, not on this list, you could do to weigh half as much. What are they?

0000 1. Take away half of the Earth’s atmosphere.

2. Double the distance between the Sun and the Earth.3. Make the Earth spin twice as fast.4. Take away half of the Earth’s mass.

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If you were twice as far from Earth as you are now, how would gravity change?

0

0

0

0

0 1. It would be four times as strong2. It would be twice as strong3. It would be the same4. It would be half as strong5. 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?

0

0

0

0

0 1. It would be four times as strong2. It would be twice as strong3. It would be the same4. It would be half as strong5. 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?

0

0

0

0

0

0

0 1. It would be 9 times as strong2. It would be 6 as strong3. It would be 3x4. It would be the same5. 1/9 as strong6. 1/6 as strong7. 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?

B

C

A

Original location

Earth

0

0

0

0 1. A2. B3. C4. All 3 are the same

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Which picture shows the spacing after a little while, when all objects are still falling?

B

C

A

1 = Original spacing

BC

A

3

B

C

A

2

BC

A

5

B

C

A

4

B

C

A

Original location

00000 1. 1

2. 23. 34. 45. 5

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Tidal forcesGravity stretches an object out as it falls.This is called tides, theyare caused by Differences in gravity’s acceleration betweenthe near side,center,and far side.

Technically there is no such thing as a “tidal force.”A tidal force is:a differencein gravity’s acceleration thatstretches things out.

Neat info: The “tidal forces” are what kill you as you fall into a black hole.Good rubber band analogy on p. 140.

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 4.23How 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?

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?

Tide table, part 2 (one day)

Which object causes stronger tides on Earth?

0

0

0 1. Sun2. Moon3. Both are equal

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• Tides cause the oceans to move, causing friction inside Earth and with ocean floor. (clicker)– [to be continued]

Tidal Effects from Moon – see fig 4.26

Because of tidal friction, days are getting ____.

0

0 1. Longer2. Shorter

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• 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.

Tidal Effects from Moon – see fig 4.26

Tides on the Moon

• Just like Moon causes tides on the Earth, Earth causes tides 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 timeof the Moon is the same as its revolution

time.See pages 44-45

Tidal Effects on the Moon

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.)

0

0 1. Yes2. No

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Which planet had its moon(s) get synchronized quicker?

0

0

0 1. Earth2. Jupiter3. Same

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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 tides2. 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)

Another result of tidal friction

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?