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# LESSON 10: Gravity and Orbits - PowerPoint PPT Presentation

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

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

• 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.)

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

• 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 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

• 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

• 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

• 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.

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