Calculating Orbital Period from Semi-Major Axis Alone

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Homework Help Overview

The discussion revolves around determining the orbital period of an object based solely on its orbital radius, with a specific focus on the context of orbits around the Earth. Participants explore the implications of missing information, particularly the mass of the central body.

Discussion Character

  • Exploratory, Assumption checking

Approaches and Questions Raised

  • Participants question whether the mass of the central body is necessary for calculating the orbital period and discuss the implications of assuming the mass is unknown. There is also a consideration of the definitions of orbital radius versus semi-major axis.

Discussion Status

Some participants suggest that the mass of the Earth should be accessible, implying that it is a standard piece of information expected in the context of the problem. Others clarify that introductory physics often simplifies the situation by focusing on circular orbits, which may affect how the problem is interpreted.

Contextual Notes

There is a mention of the problem being part of a test review, which raises concerns about the availability of information during the actual test. Participants also note that the terminology used, such as "orbital radius," may lead to confusion regarding the nature of the orbit being considered.

colbenstein
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Homework Statement



Is it possible to figure out the orbital period of an object with only the orbital radius given? --- It seems to be with the equations given too little information

Homework Equations





The Attempt at a Solution

 
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Do you know the central mass about which the object is orbiting?
 
No... I am just assuming that the teacher forgot to give us the mass. I just wasn't sure if i was missing some kind of equation which would make that unnecessary. I understand that orbit is an elipse, and that what it orbits is one of the foci. is there any way to use that information without having the mass of what is being orbitted? The problem relates the central mass which it orbits (the earth), but gives us nothing about it. It's a test review question, so that's something that I wouldn't be able to look up during the actual test.
 
Last edited:
You are told that it is orbiting the Earth, so you are expected to look up the mass of the Earth; perhaps it is given in your textbook. I doubt the teacher forgot.

On a test, that information would most likely be provided for you; I've never heard of a teacher who expects you to memorize the mass of the Earth for a physics test.
 
Also, usually in introductory physics classes, only circular orbits are considered. The term "orbital radius" implies circles, as ellipses don't have a radius. You don't have to worry about elliptical orbits, foci, or anything like that.
 
Or more generally, interpret "orbital radius" to mean semi-major axis (a bit of a misnomer, but it is common). Once again you do not have to worry about eccentricity.
 

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