# Homework Help: Law of Gravitation

1. Nov 27, 2005

### Dooga Blackrazor

Satellite A has a mass of 1000kg. Satellite B has a mass of 2000kg. BOth are in orbit 250km above the surface of the earth. Satellite A will have an orbital speed that is ____ satellite B.

1. the same as.
2. four times that of.
3. twice that of.

Not really sure on this. I originally said half that of and it was incorrect.

The gravitational force between two masses is 36 N. If the distance between the mass is decreased to one-half its original value, the force of gravity will be:

1. 144 N
2. 9 N
3. 18 N

My original answer was 72 N, and I forgot to square the radius if I remember correctly. So I am fairly confident the answer is 144 N.

Thanks

2. Nov 27, 2005

### Physics Monkey

A hint for your first question: does the trajectory of a particle in a given gravitational field depend on its mass? In other words, if a satellite is moving around the Earth at a given radius in a circular orbit, does its speed depend on its mass?

A hint for your second question: the gravitational force is inversely proportional to the square of the distance.

3. Nov 27, 2005

### finchie_88

Very Briefly, cos Ive gotta go in a minute.
$$F_g = \frac{GM_eM}{r^2}$$
$$F_c = \frac{Mv^2}{r}$$
Therefore...
$$v = \sqrt{\frac{GM_e}{r}}$$
Where M_e is the mass of the earth, and M is the mass of the object in orbit, so the velocity of the two will be the same as long as the radius of orbit is the same.