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**Ch 13 Gravitational Interactions**

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**Newton DID NOT discover gravity.**

He discovered that gravity is universal. - It pulls everything with the same force.

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**He also discovered that all objects in the universe attract each other.**

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**A force field exerts a force on objects in its vicinity.**

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**Gravitational Field (g)**

A force field that exists in the space around every mass or groups of masses.

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**The strength of a gravitational field is the force per unit mass.**

g = F/m

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**Newton’s Law of Universal Gravitation**

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**Using the earth’s radius and mass we get 9.8 m/s2.**

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**Remember at or near the earth’s surface the gravitational field strength is 9.8 m/s2 or 10 m/s2.**

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**The acceleration due to gravity, ‘g’, decreases with distance from the earth’s center as 1/d2.**

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As an object moves away from the center of the Earth, by a distance equivalent to the earth’s radius.

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**Distance RE Fg 2 RE 1/4 Fg 3 RE 1/9 Fg 4 RE 1/16 Fg 5 RE 1/25 Fg**

Force of Gravity RE Fg 2 RE 1/4 Fg 3 RE 1/9 Fg 4 RE 1/16 Fg 5 RE 1/25 Fg

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**Gravitational Field Inside a Planet**

There exist a gravitational field inside the earth as well as outside.

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**As you get closer to the center of the earth the gravitational acceleration decreases.**

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**When you get to the center of the earth your acceleration is zero.**

There is no gravitational field at the center of the earth The mass above and below you cancel each other out.

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**Weight (Fg) The force of gravity.**

The pressing against the earth is the sensation we interpret as weight.

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**Think about being in an elevator.**

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**Stage A Stage B Stage C Stage D Fnet = ma Fnet = 0 N Fnet = 400 N, up**

Stage B Stage C Stage D Fnet = ma Fnet = 0 N Fnet = 400 N, up Fnet = 400 N, down Fnet = 784 N, FN equals Fg FNorm = 784 N FN > Fg by 400 N FNorm = 1184 N FN < Fg by 400 N FNorm = 384 N FN < Fg by 784 N FNorm = 0 N

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This last situation is the reasoning you might feel a little queasy in the stomach when you go over a good size bump in the road at a really fast speed or sudden drops on roller coasters. You catch a little air and the seat drops out from you briefly.

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**Rather than define weight as the force of gravity that acts on you.**

We can say that it is the force you exert on a supporting floor.

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Weightlessness Is not the absence of gravity, rather it is the absence of a support force.

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Newton showed that the ocean tides are caused by differences of the moon’s gravitational pull on opposites sides of the earth.

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**When the sun, Earth, and the moon are all lined up, the tides due to the moon and sun coincide.**

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**These are called spring tides.**

Resulting in higher than average high tide and lower than average low tides. These are called spring tides.

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Lunar Eclipse Is produced when earth is directly between the sun and the moon.

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Solar Eclipse Is produced when the moon is directly in between the earth and sun.

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Neap Tides When there is a half moon, the sun and moon partly cancel each other out. Produces lower than average high tides and lower than average low tides.

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Black Hole A mass that has collapsed to so great a density that its enormous local gravitational field prevents light from escaping.

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Anatomy of a Black Hole Black Holes

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Gravitational Interactions

Gravitational Interactions

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