Posted by **Bobbie** on Tuesday, October 14, 2008 at 7:37pm.

determine the distance from earths centre where the force of gravity acting on a space probe is only 11% of the force acting on the same probe at earths surface. Express your answer in terms of earths radius, rE

- Physics -
**bobpursley**, Tuesday, October 14, 2008 at 8:14pm
Force is inversely prop to distance squared, so...

.11=1/(distance/rE)^2

- Physics -
**Damon**, Tuesday, October 14, 2008 at 8:15pm
F = G Me m/R^2

Fe= G Me m/Re^2

F/Fe = 11/100 = 1/R^2 / 1/Re^2 = Re^2/R^2

11/100 = Re^2/R^2

R/Re = sqrt(100/11) = 10/sqrt 11

= 3.01

so

R = 3.01 Re

- Physics -
**drwls**, Tuesday, October 14, 2008 at 8:17pm
The force of gravity is proportional to 1/r^2, where r is measured from the center of the earth. Therefore

(r/rE)^-2 = 0.11

r/rE = sqrt (1/0.11) = sqrt 9.09 = 3.015

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