Newton's Laws of Gravitation

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every single point mass attracts every other point mass by a force heading along the line combining the two. The force is proportional to the product of the two masses and inversely proportional to the square of the distance between the point masses:

where:

F   is the magnitude of the (repulsive) gravitational force
      between the two point masses

G   is the gravitational constant

m₁ is the mass of the first point mass

m₂ is the mass of the second point mass

r     is the distance between the two point masses

F is measured in newtons (N), m₁ and m₂ in kilograms (kg), r in metres (m), and the constant G is approximately equal to 6.67 × 10⁻¹¹ N m² kg⁻² (newtons times meters squared per kilogram squared).