A at disk of material has the same mass as the Earth, 5.98E+24 kg, and has a radius of 6.25 E+07 m. Point A is

Question

located a distance of 5.8E+06 m above the center of the disk. Point B is located right at the center of the disk. Treat all of the mass as if it were located in the x-y plane.
An object of mass 250 kg is located near the disk.
a. Find the gravitational potential energy (J) when the object is at point A.
b. Find the gravitational potential energy (J) when the object is at point B.

Answers:
a)-2.88E+09 (I keep getting 1.59 E+09)
b)-3.19E+09 (I can kinda get this)

I used the Following
a) (-GMm)/x, where I used pythagorean and them to get x based on radius and height

b) (-Gmm)/r and I used (the r given/2)
that seems to work

I am really confused on what I am doing wrong on part A and if any suggetions on how to correct part B so i don't have to fudge it would be wonderful

Answer 1

The formula you are using for potential energy applies to a spherical object, not a disc.

See http://www.physicsforums.com/showthread.php?t=449947

for the appropriate formula

Answer 2

So I get most of what the website is saying however, it says M squared, but I have two masses
and only one radius, so I am still a little confused. Please help

Answer 3

As I recall from that link, the gravitational PE for a point above a disc involves both the distance to the center of the disc and the distance to the edge of the disc.

In both cases, the point must be at or above the center of the disc. That is true in your case.

Answer 4

Right and I have two different masses that need to be multiplied. The my r0 is 6.25 E 7, and R is 5.8 E 6, but using these number i am still getting 1.48 E 9 and not 2.88 E 9, so I am not sure, I have tried it different ways, am I missing something obvious here?