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**Law of Universal Gravitation**

Chapter 12 November 9/10

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**Objectives Analyze how Earth is falling**

State Newton’s law of universal gravitation Evaluate the significance of inverse-square law

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**Introduction All objects have gravity**

Therefore, all objects attract one another

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**The Falling Apple An apple falls to Earth because of gravity**

Does Earth’s gravity pull the moon? YES Why doesn’t the moon fall into Earth? It does but it doesn’t crash because of tangential speed

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**The Falling Moon Earth’s gravity pulls moon = centripetal force**

Moon falls beneath the straight line it would follow if no force acted on it Below the tangential speed line Figure 9.2 Confirmed with experiment Figure 9.4

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**The Falling Earth Earth (and other planets) falls around the sun**

Why don’t planets crash into sun? Because of tangential velocities What would happen if tangential speeds went away? Crash into sun This has happened to asteroids and other matter without a big enough tangential speed

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**Check for understanding**

In the picture below use vectors and words to describe how the Earth orbits the sun Tangential speed Sun Earth Fc = gravity Earth stays in orbit as the resultant of the sun’s gravity and Earth’s tangential speed

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**Newton’s Law of Universal Gravitation**

Newton discovered gravity is universal Everything pulls on everything else Law of Universal Gravitation “Every object attracts every other object with a force that for any two objects is directly proportional to the mass of each object” Greater masses = greater force of attraction Greater distance (farther away) = less force of attraction

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**Newton’s Law of Universal Gravitation (con’t)**

Law can be expressed as an equation FG = force of gravity (N) G = universal constant = 6.7 x Nm2 kg2 m1 = mass of one object (kg) m2 = mass of other object (kg) r = radius (m)

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Class work- Honors Use the equation that relates to the law of universal gravitation to solve the problem

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Inverse Square Law When a quantity varies as the inverse square of its distance from its source Figure 9.4 and 9.5 Force of gravity depends on distance from Earth.

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Closing An apple falls into Earth. Why doesn’t the moon fall and crash into Earth? It does but it doesn’t crash because of tangential speed AND Force of gravity is weaker since moon is farther from Earth’s surface than apple

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**Homework Next class we will: The following class is the unit exam on:**

Take a quiz on all units and symbols- STUDY! review for the unit exam The following class is the unit exam on: Newton’s 3 laws Centripetal force and tangential speed Law of universal gravitation

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