Gravity Revisited

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Gravity Revisited Gravity and the Tides

• Why gravity does so much more than keep you from
  flying off into space!

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Our Planetarium So Far

• Enduring Understanding Forces, waves, and energy
  influence the Universe, past, present, and
  future.
• This Units Big Picture Forces and Energy
  influence the motion of celestial objects.

• Gravity Unit Big Goals!
• Understand how to calculate and apply the FORCE
  of gravity.
• Understand how gravity affects the tides.
• Understand how gravity keeps our planets and
  other celestial bodies in motion.

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Lets Review Gravity

• Remember, the acceleration due to gravity was
  calculated using the formula a F/m
• For an object in free-fall a 9.8 m/s2
• The FORCE of gravity is much different! It is
  calculated using the formula F G m1 m2
  d2
• The force of gravity tells us how strongly two
  objects are pulling on each other.

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First, Take Apart the Formula

• F G m1 m2 d2
• F the force of gravity measured in Newtons
• G Universal Gravitational Constant, which is
  6.6726 x 10-11
• m1 Mass of the first object in kg
• m2 Mass of the second object in kg
• d distance between the objects centers in m

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Great, But What Does it Mean?

• The force of gravity depends on two things
• 1. The MASS of the objects
• The more massive the object, the greater the
  gravitational force it exerts.
• 2. The DISTANCE between the objects centers.
• The closer together the objects, the greater the
  gravitational force between them.

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Here are Some Examples

• The force of gravity between you and the Earth
• F 6.6726 x 10-11 70 kg 5.98 x 1024 kg
  (6.37 x 106 m)2
• This equals 688.35 Newtons
• The force between a suitcase and the Earth
• F 6.6726 x 10-11 2 kg 5.98 x 1024 kg
  (6.37 x 106 m)2
• This equals only 19.66 Newtons!

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This Explains Our Pal the Moon

• We are a stable planet largely because of our
  relatively massive neighbor, the Moon!
• If the moon were less massive, or significantly
  farther away, the gravitational force between
  the Earth and Moon would not be strong enough to
  stabilize our wobble or control our tides as
  effectively!

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It Also Explains Why Our Weight Changes On Other
Planets

• Since different planets have different diameters
  and different masses, the amount of gravity at
  each planet's surface is different.
• Greater mass means greater surface gravity. But,
  greater diameter (distance between centers) means
  lesser surface gravity.
• Homework Your Age Weight on Other Planets
• http//www.exploratorium.edu/ronh/age/index.html
• http//erc.arc.nasa.gov/HowBig/gravity.htm

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Masses of the Planets

• Mercury 0.33 x 1024 kg
• Venus 4.87 x 1024 kg
• Earth 5.98 x 1024 kg
• Mars 0.65 x 1024 kg
• Jupiter 1900 x 1024 kg
• Saturn 570 x 1024 kg
• Uranus 87 x 1024 kg
• Neptune 100 x 1024 kg
• Pluto 0.7 x 1024 kg

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Recap and Look Ahead

• Target Review
• Tomorrow we put on our waders and jump into the
  tides!

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Our Planetarium So Far

• Enduring Understanding Forces, waves, and energy
  influence the Universe, past, present, and
  future.
• This Units Big Picture Forces and Energy
  influence the motion of celestial objects.

• Gravity Unit Big Goals!
• Understand how to calculate and apply the FORCE
  of gravity.
• Understand how gravity affects the tides.
• Understand how gravity keeps our planets and
  other celestial bodies in motion.

12
Gravity Revisited Gravity and the Tides

• Why gravity does so much more than keep you from
  flying off into space!

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Gravity and the Tides

• We are going to study TWO types of tides
• Daily Tides (High and Low tides happen on a daily
  basis around the world)
• Monthly Tides (Spring Tides and Neap Tides which
  coincide with Moon phases)
• The Force of GRAVITY is the common thread that
  ties our study of tides together.

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Key Tide Facts!

• All tides are caused by gravity from the Sun and
  the Moon pulling on and deforming large bodies of
  water.
• The greatest influence on our tides is the
  differences in how much the MOON pulls on
  different parts of the Earth.
• True, the Sun plays a part, but its so very far
  away! (Remember as d2 increases, the force due to
  gravity decreases).

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Daily Tides (High vs. Low)

• High Tides occur on the side of Earth closest to
  the Moon and the side opposite the Moon.
• High tides draw water away from the areas
  between, causing Low Tides to form.

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For most places on Earth, high tides occur every
12 hours (twice a day).High and Low are about 6
hours apart.
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• Some places on Earth have only one high and one
  low tide everyday.
• High and low are about 12 hours apart.

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Monthly Tides (Spring Tides)

• When Earth, Moon and Sun are in line, the forces
  of gravity add, giving the high spring tides.
• This happens right after the new or full moon,
  about every 14 days.

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Monthly Tides (Neap Tides)

• Halfway between spring tides, the Sun and Moon
  are at right angles their forces of gravity
  partially cancel, and we get the low neap tides.
• Water levels vary less during neap tides.

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Spring Tides High Tide at noon and midnight
Neap Tides High Tide at sunrise and sunset
http//earthsci.terc.edu/content/investigations/es
2406/es2406page03.cfm?chapter_noinvestigation
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Recap and Look Ahead

• Target Review
• Homework Finish the Tides Lab Write-Up
• Next Stop We look at how the forces of Gravity
  determine orbits and keep celestial bodies in
  motion. (Hello Kepler!)