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student34
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Homework Statement
Consider a ring-shaped body in a fixed position with mass M. A particle with mass m is placed at a distance x from the center of the ring and perpendicular to its plane. Calculate the gravitational potential energy U of the system (the picture has a small sphere traveling towards the center of a ring perpendicular to the ring's plane).
Homework Equations
Fg = (G*m*M)/r^2
Fg*r = U = (G*m*M)/r, where r is the radius, and G = 6.67*10^(-11) (gravitational constant)
The Attempt at a Solution
Let r be the hypotenuse of a right triangular distance to any part of the ring and a be the distance from the center of the ring to join the hypotenuse. x will make the 90° angle with a.
So, I thought that I would multiply the perpendicular component of force to the perpendicular distance x to get a function for gravitational potential energy,
F(perpendicular component) = (G*m*M)/(x^2) = (G*m*M)/(r^2 - a^2), where x^2 = r^2 - a^2.
Then, U = (G*m*M)/(r^2 - a^2)*(r^2 - a^2)^(1/2) = (G*m*M)/(r^2 - a^2)^(1/2).
But apparently this is wrong.
The right answer is U = (G*m*M)/(r^2 + a^2)^(1/2), the same as my answer except for the +.
I just don't understand why my answer is not right.
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