6.14 - Orbits

Gravity keeps planets and satellites in circular orbits

• Planets orbit the Sun in almost circular orbits, and moons and artificial satellites orbit planets the same way.
• The force that keeps them in orbit is the gravitational force, which acts towards the centre of the orbit.

Why velocity changes while speed stays constant

• The gravitational force acts at right angles to the object's instantaneous velocity.
• This changes the object's direction of motion without changing its speed.
• Because direction changes constantly, the velocity is constantly changing and the object is accelerating towards the centre of the orbit.

Orbital speed and radius

• Gravitational force is stronger closer to a star or planet.
• To maintain a stable orbit at a smaller radius, an object must travel faster so it doesn't get pulled inwards.
• If an object's orbital speed changes, its radius must change too:
  • A faster object orbits at a smaller radius.
  • A slower object orbits at a larger radius.