23.3 - Orbits

Gravity provides the force that creates orbits

• Planets orbit the Sun, and moons and artificial satellites orbit planets, in almost circular orbits.
• An object moving in a circle is constantly changing direction, so even at a constant speed its velocity is constantly changing.
• A changing velocity means the object is accelerating, which requires a resultant force.
• This force acts towards the centre of the circle.

Gravity provides the inward force

• For an orbiting object, this inward force is provided by gravity:
  • The Sun's gravitational pull keeps planets and comets in orbit.
  • A planet's gravitational pull keeps moons and artificial satellites in orbit.
• Because the object is already moving, the gravitational pull doesn't pull it straight in - it constantly bends its path into a circle.