Physics – 6.1.2 The Solar System | e-Consult

The gravitational force between the Sun and a planet is given by Newton's Law of Universal Gravitation: F = Gm₁m₂/r², where F is the gravitational force, G is the gravitational constant, m₁ is the mass of the Sun, m₂ is the mass of the planet, and r is the distance between the centres of the Sun and the planet.

As the distance (r) between the Sun and the planet increases, the gravitational force (F) decreases. The planet experiences a net force towards the Sun, which is what causes it to orbit.

The orbital speed of a planet is determined by the balance between the gravitational force pulling it towards the Sun and its inertia (tendency to continue moving in a straight line). A stronger gravitational force requires a higher orbital speed to maintain a stable orbit. Conversely, a weaker gravitational force allows the planet to orbit at a slower speed. Therefore, as the distance from the Sun increases, the gravitational force decreases, and the planet's orbital speed also decreases. The planet needs less speed to counteract the weaker pull of gravity at a greater distance.