Lesson 21
Central Force Motion; Equations of Motion, intro to Planetary Motion

 Name:faculty Section:M2 Start Time:12:20:32 Instructor:pate Course:355

1) There are at least three ways to describe or characterize the motion of a system governed by a central force. Each is useful in certain cases. For example, we might want to know the path or orbit of the particle(s).

What are three different ways to characterize the system's motion? For each, refer to an equation number from the textbook that would help us if we actually had a system we wanted to study. Also, when would each be useful? (Do we really need all three?)

2) Suppose you are, once again, an astronomer. This time you are observing a binary star system (two stars in orbit about one another). By carefully observing the motion of the pair of objects, you can infer the ratio of the masses. Give this a try by looking at the simulation on the physlet page. By observing the motion of the red mass-blue mass system (feel free to "Pause" and "Step" the simulation!), estimate the ratio of the masses. Please be sure to state any assumptions you make and briefly explain the thought process you use to arrive at your estimate.

Note: To obtain your answer, you'll need to give another Physlet page a try. Important: This page will open in a NEW window, so you'll need to come back to THIS window (using the taskbar) to submit your preflight. Once again, if the physlet page does NOT load properly, please tell me what happens, including any error messages it displays, in the comments box below. Thanks. Click this link to get to the physlet page.

3) Consider a planet orbiting our Sun. In the expression

for the square of the period of an orbit (Equation 8.48 in Marion and Thornton, 4th edition), the reduced mass m
1. can be approximated by the mass of the planet
2. can be approximated by the mass of the Sun
3. indicates that the orbit is elliptical
4. indicates the expression includes the translational motion of the entire system

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