Please Help! Question on Universal Gravitation

[JReiss]

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I'm sure this question is easily solved by most but if someone could take the time to explain it, I would be very grateful. EK 1001, question 207 says: "The earth has a radius of approximately 6400km. If an object could orbit the earth just at its surface, how fast would it have to travel?"

Apparently the answer is choice A - 8km/s. But I'm really not sure how they came up with this.

Thanks!

 

[MT Headed]

in motion at the surface of the earth, a = g
in circular motion, a = v^2/r

combining them, v = sqrt(gr) = sqrt(10 x 6400x10^3) m/s
= sqrt(64 x 10^6) m/s
= 8 x 10^3 m/s

or 8km/s