Circular Orbits (AQA GCSE Physics): Revision Note

Orbital Motion

• There are many orbiting objects in our solar system

  • They each orbit a different type of planetary body

Orbiting bodies in the Solar System

• A smaller body or object will orbit a larger body

• In order to orbit a body such as a star or a planet, there has to be a force pulling things towards that body

  • Gravity provides this force

• The gravitational force exerted by the larger body on the orbiting object is always attractive

• Therefore, the gravitational force always acts towards the centre of the larger body

• The gravitational force is the centripetal force as it will cause the body to move and maintain a circular path

Gravitational attraction causes the Moon to orbit around the Earth

Circular motion in an orbit

• Planets travel around the Sun in orbits that are (approximately) circular

• Objects moving in circular orbits move at a constant speed, but their direction is constantly changing

• A change in direction indicates a change in velocity, and a change in velocity indicates acceleration

  • Therefore, if an object in orbit is constantly changing direction, then its velocity is constantly changing, and hence, it must be accelerating

• A resultant force is needed to cause an acceleration

• This resultant force is gravity, which must act at right angles to the instantaneous velocity of the object to create a circular orbit

  • This is always towards the centre of the orbit

  • The instantaneous velocity of the object is the velocity at a given time

The direction of the instantaneous velocity and the gravitational force at different points of the Earth’s orbit around the sun

Circular Orbits

Planets

• There are several similarities in the way different planets orbit the Sun:

  • Their orbits are all slightly elliptical (stretched circles) with the Sun at one focus (approximately the centre of the orbit)

  • They all orbit in the same plane

  • They all travel the same direction around the Sun

• There are also a few differences:

  • They orbit at different distances from the Sun

  • They orbit at different speeds

  • They all take different amounts of time to orbit the Sun

Orbit of planets around the Sun

Moons

• Moons will orbit planets in a circular path

• Some planets will have more than one moon

• The closer the moon is to the planet:

  • the shorter the time it will take to orbit

  • the greater the speed in the orbit

Artificial satellites

• A satellite needs to travel at a specific speed to maintain a circular orbit at a particular distance from the object it is orbiting

• If the speed of the satellite is too large:

  • the radius of the orbit will increase, and the satellite will move into a higher orbit, further from the object it is orbiting

  • the satellite may leave the object's gravitational field altogether, but only if the speed is fast enough, or a continuous force acts on it, such as propulsion

• If the speed of the satellite is too small:

  • the radius of the orbit will decrease, and the satellite will move into a lower orbit, closer to the object it is orbiting

  • the satellite may spiral inwards towards the object, but only if a continuous force acts on it, such as atmospheric drag

The speed of an artificial satellite is affected by its orbital radius

• If an artificial satellite in a stable orbit is to change the radius at which it is orbiting, then its speed must change

• If the satellite is moved further away from the object to an orbit with a larger radius:

  • the force of gravity decreases

  • the orbital speed decreases

  • the orbital period increases

• If the satellite is moved closer to the object to an orbit with a smaller radius:

  • the force of gravity increases

  • the orbital speed increases

  • the orbital period decreases