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# help T is proportional to r^2/3 watch

1. hi is there anyone out there who could help me understand why they are proportional and how to solve this question?

Before I lose my sanity lol
2. (Original post by physicsstudent27)
hi is there anyone out there who could help me understand why they are proportional and how to solve this question?

Before I lose my sanity lol
What does Kepler's third law state?
Think of the summary equation.
3. (Original post by physicsstudent27)
hi is there anyone out there who could help me understand why they are proportional and how to solve this question?

Before I lose my sanity lol
Falls out of Newton's gravitational law and formula for centripetal acceleration.
And you have to relate T to the velocity.
4. (Original post by physicsstudent27)
hi is there anyone out there who could help me understand why they are proportional and how to solve this question?

Before I lose my sanity lol
Use circular motion formulae with gravitational attraction to derive Kepler's equation that'll show that .

Remember what

Edit: lol **** this nvm got ninjaed twice over
5. (Original post by physicsstudent27)
hi is there anyone out there who could help me understand why they are proportional and how to solve this question?

Before I lose my sanity lol
Equate the gravitational force to the centripetal
6. (Original post by EricPiphany)
Falls out of Newton's gravitational law and formula for centripetal acceleration.
And you have to relate T to the velocity.
Just use the form of centripetal force with omega
7. (Original post by samb1234)
Equate the gravitational force to the centripetal
The way you phrased this is a bit misleading, isn't it? The gravitational force provides the centripetal force, you don't equate the two.
8. (Original post by samb1234)
Just use the form of centripetal force with omega
velocity could mean 'angular velocity' too
9. (Original post by Zacken)
The way you phrased this is a bit misleading, isn't it? The gravitational force provides the centripetal force, you don't equate the two.
Yeah I guess the phrasing isn't the best tbh, but as you say since the gravitational force is the centripetal force that is essentially the method

10. hope this helps.
its all I could remember from A level physics since starting my degree.
11. Thanks a lot guys I am pretty comfortable with the idea that when a object in circular orbit that the centripetal force is the resultant force. I am just not comfortable with the way the questions are worded or the idea of proportionality. Especially when they give me a quest like t1/r1 = t2/r2 work out blah blah something like that
12. Probably just need to practice... they do seem to love ratio/proportionality questions in astronomy papers.

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