babyjachy said:
ok..what does it mean by "what ratio Force must change"?could you please elaborate.
It is often convenient to solve problems that have some number of unknowns that you know remain constant by way of setting up ratios. You probably remember doing this with the ideal gas law, PV = nRT, where you'd set up a ratio to find out how, say, temperature changes when the pressure changed and the volume remained constant. You didn't have to find the values of n, R, or V because they canceled out in the ratio.
In this case it's the gravitational law that you use. The law of gravitation is an inverse square law. That is, the force between two bodies varies as the inverse of the square of the distance between them. If the masses of the two bodies (m1 and m2) remain the same and only the distance changes, and everything else is constants, then it's a candidate for the ratio approach.
The force due to gravity on a body of mass m at radial distance r
o from the center of the Earth (which has mass M) is:
F = G\frac{M m}{r_o^2}
Divide both sides by m to find the acceleration due to gravity at radial distance r.
g = \frac{G M}{r_o^2}
At some other radial distance, say r
1, the acceleration due to gravity would be:
g_1 = \frac{G M}{r_1^2}
Note that G and M are constants. Set up the ratio g
1/g
o and see what happens.
