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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. The Universal Law of Gravity.

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## 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**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 theirdistance: Mm F = - G r2 (G is the Universal constant of gravity.)**Understanding Orbital Motion**The universal law of gravity allows us to understand orbital motion of planets and moons: Example: • Earth and moon attract each other through gravitation. • 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. Moon • This acceleration is constantly changing the moon’s direction of motion, holding it on its almost circular orbit. Earth**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.**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? Moon Gravitational Force Earth**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. Moon’s velocity Moon Gravitational Force Earth**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. Moon’s velocity Moon Gravitational Force Earth Path followed by the Moon**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 areexactly 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.**Orbital Motion**In 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

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