**Newton's law of universal gravitation states that the force of gravity is a universal force between all objects proportional to both masses and the square of the distance between them.** This law can be expressed with the equation F₁₂ (m¹ * m²) / d².

Newton's law of universal gravitation was later improved by the addition of the gravitational constant, G, so the equation can be written as F = (G * m¹* m²) / d² where G is the gravitational constant (6.673 x 10-¹¹ N m²/kg²). The gravitational constant was determined by Henry Cavendish more than 100 years after Newton's law. By using the equation combining Newton's law of proportionality with the gravitational constant, the gravitational force between any two objects can be determined.