What went wrong in calculating the mass of planet Mongo?

[ninjagowoowoo]

Q:
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?

 

[Berislav]

I used V=V_o + at and from the problem, v_o = 15, and t=7.

How do you know v?

 

[NateTG]

It would help a little if you put some numbers in. A shot in the dark:
If you got an acceleration of gravity (a) that is less than $4\frac{m}{s^2}$ then you calculated that incorrectly.

 

[ninjagowoowoo]

thrown upward from the ground at 15.0 m/s returns to the ground in 7.00 s

Is this not an acceptable value for v_o? and, if it returned to the ground, wouldn't v be 0?

 

[Berislav]

Is this not an acceptable value for v_o? and, if it returned to the ground, wouldn't v be 0?

No, v would take that value for maximum height.

 

[ninjagowoowoo]

...yeah I got a value less than 4...

 

[ninjagowoowoo]

So to get the equation to work out, I would just halve the time right? That would give me a V=0 at the max height. and an A of about 4.3m/s

 

[ninjagowoowoo]

Thanks a lot for the help guys, I got it now.

 

[Päällikkö]

How do you know v?

V = -V_o (?)

 

[Berislav]

V = -V_o (?)

The OP didn't state that, which lead me to think that he overlooked it.