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Posted by on Saturday, December 31, 2011 at 10:40am.

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
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

  • Physics - , Saturday, December 31, 2011 at 11:52am

    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

  • Physics - , Saturday, December 31, 2011 at 12:09pm

    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

  • Physics - , Saturday, December 31, 2011 at 1:16pm

    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.

  • Physics - , Sunday, January 1, 2012 at 10:27am

    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?

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