# Gravity

• Jan 22nd 2007, 03:58 PM
symmetry
Gravity
When a satellite is a distance R from the center of Earth, the force due to gravity on the satellite is F. What is the force due to gravity on the satellite when its distance from the center of Earth is 3R?

(1) F/9 (3) F
(2) F/3 (4) 9F
• Jan 22nd 2007, 09:07 PM
CaptainBlack
Quote:

Originally Posted by symmetry
When a satellite is a distance R from the center of Earth, the force due to gravity on the satellite is F. What is the force due to gravity on the satellite when its distance from the center of Earth is 3R?

(1) F/9 (3) F
(2) F/3 (4) 9F

You should know that at the level you are working gravity acts via
an inverse square law. So:

F(kx)/F(x)=1/k^2,

In you case k=3, so F(3R)=F(R)/9.

RonL
• Jan 23rd 2007, 02:26 AM
symmetry
ok
Thanks! I did not know that formula.
• Jan 23rd 2007, 02:47 AM
CaptainBlack
Quote:

Originally Posted by symmetry
Thanks! I did not know that formula.

The question obviously presupposes that you know that gravity acts
via an inverse square law. The relation:

F(kx)/F(x)=1/k^2,

is an imeadiate consequence of the inverse square law.

RonL