Which expression gives the gravitational force between two point masses m1 and m2 separated by distance r?

• A. F = m1 g
• B. F = k q1 q2 / r^2
• C. F = ma
• D. F = (G m1 m2) / r^2

Answer: (D)

Gravitational force between two point masses follows an inverse-square law: the force's magnitude grows with the product of the masses and falls off with the square of their separation. The explicit expression is F = G m1 m2 / r^2, where G is the gravitational constant. The force acts along the line connecting the two masses and is attractive for both masses.

This means increasing either mass increases the pull proportionally, while increasing the distance reduces the pull by a factor of four if the distance is doubled, and so on. The other forms mentioned describe different forces or situations: weight near Earth is F = m g, electrostatic force is F = k q1 q2 / r^2, and F = m a is Newton's second law linking force to acceleration but not giving the specific gravitational interaction.