TBR, Physics, Forces, Passage 2, #11

[Sammy1024]

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11. If the space station is located a distance of twice the radius of the Earth away from the surface of the Earth, the acceleration due to gravitational attraction from the Earth on the space station is:

A. one-ninth of that on Earth.
B. one-quarter of that on Earth.
C. one-half of that on Earth.
D. equal to that on Earth.

So I used the correct equation. F = Gm/r^2.

In the answer they said that r becomes 2r because the distance is 2x the radius of the Earth, but you also have to add the radius of the Earth so r becomes 3r.

I don't understand when you would know to do that??

 

[BerkReviewTeach]

How far are you from the center of the Earth? That is true distance you need to consider. On the surface, you are a distance of "r" from the center. If you then move to a position that is 2r above the Earth, you are actually 3r from the center of the Earth. The distance has tripled, so the force decreases by a factor of 9.

 

[Sammy1024]

I understand that part, but I was wondering why I would consider from the center of the Earth, when the question doesn't say center.

 

[ipmed]

Grav. Force is an attractive force that gives rise to grav. acceleration and is taken from the center of mass of two objects. The force r value in the eqn is between the centers of mass