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Your starship, the Aimless Wanderer, lands on the mysterious planet Mongo. As chief scientist-engineer, you make the following measurements: a 2.50-kg stone thrown upward from the ground at 15.0 m/s returns to the ground in 7.00 s; the circumference of Mongo at the equator is 1×10^5 km; and there is no appreciable atmosphere on Mongo. What is the mass of Mongo?

well i started out by finding the acceleration of the rock. I used V=V_o + at and from the problem, v_o = 15, and t=7.

Then I used F=ma to get the gravity force.

Then I used the general formula for force due to gravity. from the problem I used: R=1*10^5 / 2*pi (because they gave us the circumference) everything else is pretty self-explanitory. Since I have F, G, the mass of the rock, and R, I solved for M and got the wrong answer. Could anyone tell me where I went wrong?