# The attractive gravitational forces between two people separated by 1.5 meters

• robax25

#### robax25

Homework Statement
calculate attractive forces between you and your friend if your body weight is 80 kg and your friend weight is 70kg. The distance is 1,5m.

At which angular velocity could you spain about a common center such that the centrifugal forces don't drive you away from each other?

what is the maximum force if the distance was zero?
Relevant Equations
F= Gm1M2/r²
v=ω * r
F= Gm1m2/r² = 1.667*10^-7 N.

I don't understand How to calculate angular velocity?

• PhDeezNutz

Possibly start by finding the distance from each of you to the common center. Where will the center be?

0.75m. Do you think that centrifugal force is 1.667*10^-7 N?

Centrifugal force is F= mv²/r,
F= mω²r.

0.75m. Do you think that centrifugal force is 1.667*10^-7 N?

Centrifugal force is F= mv²/r,
F= mω²r.
If the two of you were to spin about that center, would your center of mass be moving?

Does that suggest that some other center might be a more proper choice?

Homework Statement:: calculate attractive forces between you and your friend if your body weight is 80 kg and your friend weight is 70kg. The distance is 1,5m.

At which angular velocity could you spain about a common center such that the centrifugal forces don't drive you away from each other?

what is the maximum force if the distance was zero?
Relevant Equations:: F= Gm1M2/r²
Isn't this formula only valid for point masses or spherically symmetric objects? That said, I assume it's what you are supposed to use.

If the two of you were to spin about that center, would your center of mass be moving?

Does that suggest that some other center might be a more proper choice?
yes. Center of mass will move.

• PeroK
yes. Center of mass will move.
That requires an external force acting on the two-body system, doesn't it?

Isn't this formula only valid for point masses or spherically symmetric objects? That said, I assume it's what you are supposed to use.
I need to calculate centrifugal force and angular velocity.
That requires an external force acting on the two-body system, doesn't it?
yes. It does.

I need to calculate centrifugal force and angular velocity.

yes. It does.
What provides the external force to move the centre of mass?

As far as, I understand that there is only gravitational force acts on them .I mean F= mg if they Spain, the force will be same. Obviously, the F=Gm1m2/r² will not be the force that I have thought the centrifugal force.

As far as, I understand that there is only gravitational force acts on them .I mean F= mg if they Spain, the force will be same. Obviously, the F=Gm1m2/r² will not be the force that I have thought the centrifugal force.
Right. Only gravity. Since that is an internal force rather than an external force, momentum must be conserved. The center of mass cannot move.

It follows that the two objects cannot both circle about their geometric center. What center can they circle about so that the center of mass does not move?

Isn't this formula only valid for point masses or spherically symmetric objects? That said, I assume it's what you are supposed to use.
Out of interest, I modeled the bodies as stacks of 11 perfect spheres of various radii, roughly forming the shape of people.
It cut the gravitational attraction by about 10% compared with a single sphere for each.

Last edited:
• PeroK and Bandersnatch
Out of interest, I modeled the bodies as stacks of 11 perfect spheres of various radii, roughly forming the shape of people.
It cut the gravitational attraction by about 10% compared with a single sphere for each.
Treat the system as a spherical 70kg chicken and a spherical 80kg chicken (in a vacuum) ...
E.g.

• wrobel