# Physics

A small sphere X of mass M is placed a distance d from a point mass. The gravitational force on the sphere X is 90 N. Sphere X is removed and a second sphere Y of mass 4M is placed a distance 3d from the same point mass. What is the gravitational force on sphere Y?

I did:
90 = GMm/d
Force on Y = G4Mm/3d

I thought it was 120 N but I was wrong.

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1. I assume the point Mass is M.

force= 90*4/3^2=40 N

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bobpursley
2. gravity follows an inverse-square relationship
... you didn't square the d

3 times the distance means 1/9 of the force

4 times the mass means 4 times the force

90 N * 4/9 = ?

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3. Aha, that makes sense. Thank you both.

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