# Homework Help: Acceleration Due to Gravity on earth

1. Apr 13, 2012

### eagles12

1. The problem statement, all variables and given/known data
At what altitude above the earth's surface is the acceleration due to gravity equal to g/3?

2. Relevant equations

ag=G Me/(Re+H)^2

3. The attempt at a solution

I solved for H and got H=√3 Re-Re
using this i got 17*10^6
but it is saying this is not correct

2. Apr 13, 2012

### Staff: Mentor

Can you show the details of your calculation? What value did you use for Re?

3. Apr 13, 2012

### D H

Staff Emeritus
17*10^6 what? Distance isn't just a number. It has units. What are the units of this number 17*10^6, what units is the answer supposed to be in, and what value did you use for the radius of the Earth?

4. Apr 13, 2012

### eagles12

for re i used 6.37*10^6m
the answer is also sopossed to be in meters

i started with ag=g Me/(Re+H)^2
then plugged in 1/3*9.8=G Me/(Re+H)^2
I reduced that to ag=1/(Re+H)^2=1/3(1/Re^2)
from there i got
(Re+H)^2=3Re^2
Re+H=sqrt(3)*Re
H=sqrt(3)Re-Re

5. Apr 13, 2012

### Staff: Mentor

Your derivation looks okay, so you must be having finger problems with the calculator