LESSON 10: Gravity and Orbits
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Transcript of LESSON 10: Gravity and Orbits
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LESSON 10: Gravity and Orbits
Modified from http://www.psci.unt.edu/~roberts/Pics/Fall/Chapter
%2005.ppt#295,6,Isaac Newton (1643 - 1727)
http://www.csub.edu/Physics/phys110/Ch3.ppt#257,2,Section 3.1 Inertia
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The Universal Law of Gravity
• Any two bodies are attracting each other through gravitation, with a force proportional to the product of their masses and inversely proportional to the square of their distance:
F = - G Mm
r2
(G is the Universal constant of gravity.)
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Understanding Orbital MotionThe universal law of gravity allows us to understand orbital motion of planets and moons:
• Earth and moon attract each other through gravitation.
Example:
Earth
Moon
• Since Earth is much more massive than the moon, the moon’s effect on Earth is small.
• Earth’s gravitational force constantly accelerates the moon towards Earth.
• This acceleration is constantly changing the moon’s direction of motion, holding it on its almost circular orbit.
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Orbital Motion and Gravity
• A force is any kind of push or pull exerted by one object on another.
• Besides contact, friction, electric, magnetic, elastic, pressure, etc. forces, Newton said that objects also exert a gravitational force on each other.
• The force of gravity causes all bodies to attract all other bodies.
• Gravity, coupled with laws of motion, enabled Newton to explain exactly how orbits work.
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The Moon and the Earth• the Earth will exert a
gravitational forces on the Moon pulling the Moon towards the Earth.
• So, what holds the Moon up?
• Why doesn’t it fall down like if you drop a rock?
Earth
Moon
GravitationalForce
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The Moon and the Earth• If the Moon was just sitting up
there, it would fall straight down onto the Earth.
• But the Moon is moving, “sideways” at a pretty high speed.
• The Moon does fall down, but it is moving sideways at the same time.
• Just like if I throw a baseball, it moves across the room while falling downwards.
Earth
Moon
GravitationalForce
Moon’s velocity
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The Moon and the Earth (3)• Without gravity, the Moon would
move in a straight line, flying away from the Earth.
• The orbit is a balance between the natural straight-line motion and the attractive pull towards the Earth.
• The Moon is always falling towards the Earth but it is also always shooting away from the Earth.
Earth
Moon
GravitationalForce
Moon’s velocity
Path followed by the Moon
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The Sun and Planets• Orbits of planets around the Sun work just
like the Moon’s orbit around the Earth.• If the gravitational force and orbital speed are
exactly balanced, a planet will orbit in a perfect circle.
• If the planet’s speed is a little faster or slower, a non-circular orbit results.
• If the planet’s speed is much too fast or slow it may escape the Sun altogether or fall into the Sun.
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Orbital MotionIn order to stay on a closed orbit, an object has to be within a certain range of velocities:
Too slow => Object falls back down to Earth
Too fast => Object escapes Earth’s gravity