This vCalc equation computes the velocity of an Earth satellite in a circular orbit at a specified distance from the center of the Earth, `r`.

This derives from Newton's second law: `sum vecF = m * veca` as:

`(G*m_E*m)/r^2 = (m* v^2)/r`

And solving for v we get:  `v = sqrt((G*m_E)/r)`

References

Young, Hugh and Freeman, Roger.  University Physics With Modern Physics.  Addison-Wesley, 2008. 12th Edition, (ISBN-13: 978-0321500625 ISBN-10: 0321500628 ) Pg 394 eq 12.10