# Homework Help: Question on gravitation(low-altitude satellite)

1. Oct 24, 2009

### karis

1. The problem statement, all variables and given/known data
A low-altitude satellite near the earth's surface has a speed of 7.90 km s-1. The radius of the earth is about 4 times that of the moon and the ratio of the average density of the earth to that of the moon is about 5:4. The speed of a low-altitde satellite near the moon's surface would be

A. 1.77km s-1
B. 2.21km s-1
C. 2.47km s-1
D. 3.57km s-1
E. 4.42km s-1

2. Relevant equations
v2 = GM/r

3. The attempt at a solution
i got the ans of 3.12km s-1
the ratio of the mass of earth to moon would be 80:1
n the ratio of radius of earth to moon is 4:1
so i guess it's 7.92/20 ?

Pls help, i dunno what i did wrong :(
Thz so much

2. Oct 24, 2009

### Fightfish

Look at your equation again. What variable does 7.9^2/20 represent?

3. Oct 24, 2009

### karis

7.9 is the speed of satellite near earth's surface, and 20 is the ratio of the mass and radius combining, (1/80)*4

4. Oct 25, 2009

### Fightfish

Not the individual terms, what variable does your answer aka 7.9^2/20 = 3.12 represent? Look carefully; is it the speed of the satellite?