# Gravity and superpostion of two spheres

1. Jul 19, 2015

### J-dizzal

1. The problem statement, all variables and given/known data
The figure shows a spherical hollow inside a lead sphere of radius R = 4.6 m; the surface of the hollow passes through the center of the sphere and “touches” the right side of the sphere. The mass of the sphere before hollowing was M = 245 kg. With what gravitational force does the hollowed-out lead sphere attract a small sphere of mass m = 20 kg that lies at a distance d = 14 m from the center of the lead sphere, on the straight line connecting the centers of the spheres and of the hollow?

2. Relevant equations

3. The attempt at a solution

when i find the mass of the "hollowed out" sphere its much larger than the larger sphere. I dont know how else to approach this problem.

Last edited: Jul 19, 2015
2. Jul 19, 2015

### Staff: Mentor

The mass of the sphere is inconsistent with its given diameter and the density of lead. I would ignore the given "lead" and calculate the density based on the given size and mass.

3. Jul 19, 2015

### J-dizzal

I updated my original photo with my calculations. Would the total force be the force of the full sphere minus the force of the "hollow" sphere? F=F1-F2? I got the wrong answer not sure where i went wrong.

4. Jul 19, 2015

### Staff: Mentor

Your volume and density calculations don't look right to me. Can you recheck them?

5. Jul 19, 2015

### J-dizzal

Thank you! I am having problem getting adjusted to an hp 300s+, I often have to double check its picking up all the buttons i press.

6. Jul 20, 2015

### Staff: Mentor

Right (if you take both F1 and F2 to be positive).
A quick check: is your mass ratio 1 to 8?
The ratio of forces should be a bit smaller than that (1 to 7.something or 6.something) as the smaller sphere is a bit closer. If that is not true, something is wrong.