# Homework Help: Law Of Universal Gravition (am I Right?)

1. Oct 31, 2005

### heelp

A certain neutron star has five times the mass of our sun packed into a sphere about 10km in radius. estimate the surface gravity on this monster.

= (5)(1.99 *10^30 =m
10km =r
g = G mass of earth/r^2 = 6.67* 10^ -11 9.95* 10^30/10^2 = 6.64m/s

Calculate the acceleration of gravity 250km above the earth's surface in terms of g

ar = v^2/r I'm really stuck I would I go about solving this problem? your help will be appreciated.

2. Oct 31, 2005

### ranger

The acceleration of gravity is dependent upon the mass of the earth and the distance (d) that an object is from the center of the earth. Think a little about the inverse square law.

3. Oct 31, 2005

### ranger

The value of g on any other planet can be calculated from the mass of the planet and the radius of the planet. The g on the surface would be inversely proportional to the square of the radius and proportional to the Mass of the planet times the gravitational constant.

There was no need to use the mass of the earth. The question is asking you for the g of a star thats 5X the mass of our sun. The equation that you are using is right. You just inserted the wrong values.

Last edited: Oct 31, 2005
4. Oct 31, 2005

### heelp

I should enter the mass off the star? it wan't given

5. Oct 31, 2005

### ranger

Of course it was. The question says "A certain neutron star has five times the mass of our sun". The mass is indirectly stated. Just loook up the mass of the sun then times it by 5 thats all. Common you can do it

Last edited: Oct 31, 2005