Posted by **jawn** on Wednesday, October 7, 2009 at 8:52pm.

At what altitude above the Earth's surface is the acceleration due to gravity equal to g/128?

i did

g/128=(G*Earth's mass)/r^2

then solved for r and got 72080960. its within 10% of the correct answer but not correct. what am i doing wrong? (ive already rounding differently,etc)

- physics -
**bobpursley**, Wednesday, October 7, 2009 at 8:58pm
you need to subtract earth radius from r, the question asked for altitude.

There was a easier way to do this.

g/128=g*(re/(re+alt))^2 notice the g's divide out, or

(re+alt)^2/re^2=128

or (1+alt/re )^2=128

take the sqrt of each side...

alt= re * (sqrt128 -1)

Now you see why your answer was within ten percent, as the sqrt 128 is about 11.

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