• Support PF! Buy your school textbooks, materials and every day products via PF Here!

Orbital velocity help

  • Thread starter tinksy
  • Start date
5
0
hi, could someone please help me with this problem??

If a small planet were discovered with a distance from the sun eight times that of the Earth, what would you predict for its period in (Earth) years. (i.e. how many times longer would it take to go round the sun than the Earth does.)
 
Last edited:
18
0
i will not give you the answer straight away, but here's the hint---
get orbital velocity v as a function of radius r

dynamics of the system(*) :

from newton's second law and law of gravitation,
(m*v^2)/R = (G*M*m)/R^2


get v(r) and substitute in the kinemetical relation which you got correct,that is, T=2(pi)r/v

this will give you T(r) which is usually called "kepler's third law".


*assumption: all orbits are circular

justification : though the orbits that actually elliptical ,the eccentricity is very small.(dont worry if you don't understand this,take this as a side remark).

cheers :smile:
 
Last edited:
5
0
thanks teddy...

could u tell me if i'm right?

i've used kepler's law T^2 (directly proportional to) R^3
so if R = 8R (as for Earth, R = 1AU so for the planet, R = 8AU)
kepler's law: R^3/T^2 = 1
therefore, T^2 = 8^3/1
so T = (root)512 = 16(root)2 .....???? is that correct???
 
18
0
yup,its right.
bye :smile:
 
5
0
yaaaaaay!!! thanks
 

Related Threads for: Orbital velocity help

  • Posted
Replies
5
Views
2K
  • Posted
Replies
1
Views
2K
Replies
2
Views
13K
Replies
11
Views
2K
  • Posted
Replies
1
Views
6K
  • Posted
Replies
1
Views
9K
  • Posted
Replies
11
Views
2K
  • Posted
Replies
1
Views
1K

Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving
Top