Physics Review Question
Unit: Mechanics
Year: 1989 Question#: 18
Question: The diagram at the right shows spheres A and B with masses of M
and 3M, respectively. If the gravitational force of attraction of sphere A on sphere B is
2 Newtons, then the gravitational force of attraction of sphere B on sphere A is
(1 ) 9N
(2 ) 2N
(3 ) 3N
(4 ) 4N
What is this question really asking?
Explanation by: Anna R
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Answer 1
9N
INCORRECT
The gravitational attraction between two masses is equal both ways, the force of mass A on
mass B is equal and opposite to the force of mass B on mass A. 
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Answer 2
2 N
CORRECT
The gravitational attraction between two masses is equal both ways, the force of mass A on
mass B is equal and opposite to the force of mass B on mass A. 
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Answer 3
3 N
INCORRECT
The gravitational attraction between two masses is equal both ways, the force of mass A on
mass B is equal and opposite to the force of mass B on mass A. 
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Answer 4
4 N
INCORRECT
The gravitational attraction between two masses is equal both ways, the force of mass A on
mass B is equal and opposite to the force of mass B on mass A. 
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What's this question really asking?
How do objects relate to each other in terms of gravitational force? (What is Newton's
third law...law of force pairs!)
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