# B GMm/r^2 for r = 0?

1. Jun 15, 2017

### JLT

This is probably a stupid question - but let's say you have a hollow Sun (M is hollow), with another mass in the center, so M and m share the same center of mass, the distance between the centers is zero.

Would GMm/r^2 be infinite in this case?

The force at the center of any object - if you define your system to be the center chunk of matter, and how that center chunk of matter sees the rest of the planet?

((o))

sorry... I did not get enough sleep last night, that's my excuse!

2. Jun 15, 2017

### Orodruin

Staff Emeritus
The 1/r^2 expression is only valid for the gravitational force on an object outside a spherically symmetric mass distribution.

Your expression would go to infinity, but it would not be describing the force between the objects.

3. Jun 15, 2017

### Ibix

The force is, in fact, zero. Look up the Shell Theorem.

Larry Niven wrote a novel called Ringworld which featured a ring around a star. He had to write a followup describing the giant ion engines the ring had to keep itself centred on its star because this fact was brought to his attention after publication...

4. Jul 3, 2017

### the_valence_electron

actually their will be no force acting because all the forces from different part of the sun will cancel out.