- #1
RoboNerd
- 410
- 11
Homework Statement
The picture has the problem question.
OK.
When the moon is at point B, the distance from the moon to the center of the planet is most nearly:
A) (1/25) * rA
B) (1/5) * rA
C) (1/ sqrt(5) ) * rA
D) rA
E) sqrt(5) * rA.
Homework Equations
Newton's law of gravitation. kepler's third law
The Attempt at a Solution
[/B]
Ok. I am told that the correct answer is B but I do not know why.
I got A instead. Here's what I did.
I solved from my rA for the planet at point A.
I apologize for my bad math formatting, but I am in a rush and can not use latex. Hope you understand.
Gravitational force causes centripetal acceleration:
G * mPlanet * mMoon mMoon * (v0)^2
--------------------------- = -----------------------
(rA)^2 rA.
I cancel mMoon and one of my rA terms and I solve for v0^2. I got v0^2 to be equal to
G * mPlanet/r.
I then did the same thing for my planet in position B.
G * mPlanet * mMoon mMoon * (5v0)^2 mMoon * 25 * v0^2
--------------------------- = ----------------------- = ----------------------
(rB)^2 rB. rB
I plugged in my calculated result for v0^2 for position A into the expression and solved for rB.
My G and my mPlanet terms cancel. I got rB = rA/25.
Where did I mess up?
Thanks in advance.