# Calculus 1 help?

Okay I had a calculus test and we got it back and I got this question wrong. I tried it again but I can't figure it out.

The weight, w, in kg of an object is inversely proportional to the square of the distance, r, in km, from the center of the earth. Assume a stone of the ground has weight 100 kg and that the diameter of the earth is approximately 12,756 km. What is the formula of w in terms of r?

I said w=k/r^2 but the question was multiple choice and I wasn't entirely sure so i chose the answer:
w=(100*12756^2)/r^2
w= (25*12756^2)/r^2

SteamKing
Staff Emeritus
Homework Helper
Why did you use the diameter for r? Where is the center of the earth in relation to its diameter?

That's what I'm confused about. I don't know what 12,756 stands for in the equation.

SteamKing
Staff Emeritus