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Kepler's laws, as presented, describe orbits satisfying two conditions. What are they?
AThe orbit must be a perfect circle, and friction must be present
BThe satellite must be visible from Earth, and its period must be known
CA small mass orbits a much larger mass, and the system is isolated from other masses
DThe orbiting body must have exactly the mass of a planet
Answer & Solution
Correct answer: C. A small mass orbits a much larger mass, and the system is isolated from other masses
1. The first condition is that a small mass m orbits a much larger mass M, which lets the larger mass be treated as stationary.
2. The second condition is that the system is isolated from other masses, so outside gravitational effects can be neglected.
3. Neither a perfectly circular orbit nor the presence of friction is required by these two conditions, and Kepler's laws were derived without reference to visibility from Earth.
4. The orbiting body does not need to have planetary mass at all, since moons, artificial satellites, and asteroids can all satisfy the same two conditions.
_Source: OpenStax College Physics (CC BY 4.0), Ch 6 "Uniform Circular Motion and Gravitation", section 6.6 Satellites and Kepler's Laws: An Argument for Simplicity_
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