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## Homework Statement

Calculate thow long it would take a ball to orbit the earth once , just above the surface , at a speed of 7920 ms^-1 . The radius of earth is 6400km.

## Homework Equations

Well Im not sure to be honest.

## The Attempt at a Solution

I tried using S=ut+1/2(a)(t)^2

Then used (-b +/- √b^2-4ac)/2a

But got stuck in (-b +/- √b^2-4ac)/2a because got a negative sign answer in the √

Cant find a formula in the book for this - probably have to manipulate some other formulas, but as I said im not sure