Gravitational Acceleration-Orbital Motion

[physgrl]

Homework Statement

The weight of a 0.60 kg object at the surface of Planet V is 20 N. The radius of the planet is 4 x 10^6 m. Find the gravitational acceleration at a distance of 2 x 10^6 m from the surface of this planet.

a. 8.9 m/s2
b. 11 m/s2
c. 13 m/s2
d. 18 m/s2

Homework Equations

F=ma
a=MG/r^2

The Attempt at a Solution

I did
acceleration at surface=20N/.6kg=33.33m/s^2
acceleration at surface=MG/r^2
MG=33.33m/s^2*(4x10^6)^2

acceleration at a distance=MG/(r+d)^2
a at a distance=(33.33m/s^2*(4x10^6)^2)/(2x10^6+4x10^6)^2
a=14.8m/s^2 which is not in the options

 

[technician]

I can't see anything wrong with your method. I did exactly the same and got the same answer as you !
It will be interesting to see if there are any other responses to your posting.

 

[phyzguy]

I think you did it right and the question is wrong. Anyone else see an error?

 

[sandy.bridge]

I get 8.89...

 

[technician]

The weight will be 8.89N

 

[sandy.bridge]

Settled! Answers wrong

 

[phyzguy]

It doesn't ask for the weight, it asks for the acceleration, which should be 14.8 like the OP calculated.

 

[physgrl]

thanks :D