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254 Chapter 6 | Gravitation and Uniform Circular Motion
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• Newton's law of gravitation applies universally.
6.6 Satellites and Kepler's Laws: An Argument for Simplicity
• Kepler's laws are stated for a small mass orbiting a larger mass in near-isolation. Kepler's laws of planetary motion are then as follows:
Kepler's first law
The orbit of each planet about the Sun is an ellipse with the Sun at one focus.
Kepler's second law
Each planet moves so that an imaginary line drawn from the Sun to the planet sweeps out equal areas in equal times.
Kepler's third law
The ratio of the squares of the periods of any two planets about the Sun is equal to the ratio of the cubes of their average distances from the Sun:

where is the period (time for one orbit) and is the average radius of the orbit.
• The period and radius of a satellite's orbit about a larger body are related by

1. There is an analogy between rotational and linear physical quantities. What rotational quantities are analogous to distance and velocity?
6.2 Centripetal Acceleration
2. Can centripetal acceleration change the speed of circular motion? Explain. 6.3 Centripetal Force
3. If you wish to reduce the stress (which is related to centripetal force) on high-speed tires, would you use large- or small- diameter tires? Explain.
4. Define centripetal force. Can any type of force (for example, tension, gravitational force, friction, and so on) be a centripetal force? Can any combination of forces be a centripetal force?
5. If centripetal force is directed toward the center, why do you feel that you are ‘thrown' away from the center as a car goes around a curve? Explain.
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Conceptual Questions
6.1 Rotation Angle and Angular Velocity
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