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**Chapter 7: Circular Motion and Gravitation**

Section 3: Motion in Space

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**Objectives Describe Kepler’s laws of planetary motion.**

Relate Newton’s mathematical analysis of gravitational force to the elliptical planetary orbits proposed by Kepler. Solve problems involving orbital speed and period.

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**Kepler’s Laws Kepler’s laws describe the motion of the planets.**

First Law: Each planet travels in an elliptical orbit around the sun, and the sun is at one of the focal points. Second Law: An imaginary line drawn from the sun to any planet sweeps out equal areas in equal time intervals. Third Law: The square of a planet’s orbital period (T2) is proportional to the cube of the average distance (r3) between the planet and the sun.

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**Kepler’s Laws, continued**

Kepler’s laws were developed a generation before Newton’s law of universal gravitation. Newton demonstrated that Kepler’s laws are consistent with the law of universal gravitation. The fact that Kepler’s laws closely matched observations gave additional support for Newton’s theory of gravitation.

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**Kepler’s Laws, continued**

According to Kepler’s second law, if the time a planet takes to travel the arc on the left (∆t1) is equal to the time the planet takes to cover the arc on the right (∆t2), then the area A1 is equal to the area A2. Thus, the planet travels faster when it is closer to the sun and slower when it is farther away.

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**Kepler’s Laws, continued**

Kepler’s third law states that T2 r3. The constant of proportionality is 4p2/Gm, where m is the mass of the object being orbited. So, Kepler’s third law can also be stated as follows:

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**Kepler’s Laws, continued**

Kepler’s third law leads to an equation for the period of an object in a circular orbit. The speed of an object in a circular orbit depends on the same factors: Note that m is the mass of the central object that is being orbited. The mass of the planet or satellite that is in orbit does not affect its speed or period. The mean radius (r) is the distance between the centers of the two bodies.

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Planetary Data

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**Sample Problem Period and Speed of an Orbiting Object**

Magellan was the first planetary spacecraft to be launched from a space shuttle. During the spacecraft’s fifth orbit around Venus, Magellan traveled at a mean altitude of 361km. If the orbit had been circular, what would Magellan’s period and speed have been?

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**Sample Problem, continued**

1. Define Given: r1 = 361 km = 3.61 105 m Unknown: T = ? vt = ? 2. Plan Choose an equation or situation: Use the equations for the period and speed of an object in a circular orbit.

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**Sample Problem, continued**

Use Table 1 in the textbook to find the values for the radius (r2) and mass (m) of Venus. r2 = 6.05 106 m m = 4.87 1024 kg Find r by adding the distance between the spacecraft and Venus’s surface (r1) to Venus’s radius (r2). r = r1 + r2 r = 3.61 105 m 106 m = 6.41 106 m

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**Sample Problem, continued**

3. Calculate 4. Evaluate Magellan takes (5.66 103 s)(1 min/60 s) 94 min to complete one orbit.

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**Weight and Weightlessness**

To learn about apparent weightlessness, imagine that you are in an elevator: When the elevator is at rest, the magnitude of the normal force acting on you equals your weight. If the elevator were to accelerate downward at 9.81 m/s2, you and the elevator would both be in free fall. You have the same weight, but there is no normal force acting on you. This situation is called apparent weightlessness. Astronauts in orbit experience apparent weightlessness.

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**Weight and Weightlessness**

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