The gravitational force between two objects is 3600 N. What will be the gravitational force between the objects if the mass of each object is reduced to one-third of its original mass?

Gravitational force is proportional to the product of the two masses. Reducing each mass to one-third makes that product (1/3) × (1/3) = 1/9 of its original value. So the new force is 3600 ÷ 9 = 400 N, which is choice A, 400 N.

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