# GMm/r^2 for r = 0?

• B

## Main Question or Discussion Point

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!

Related Classical Physics News on Phys.org
Orodruin
Staff Emeritus
Homework Helper
Gold Member
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.

JLT
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...

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