# 8.1.6 Circular Orbits

### Orbital Motion

• There are many orbiting objects in our solar system
• They each orbit a different type of planetary body

Orbiting Objects or Bodies in Our Solar System Table • 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 in 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 in circular orbit are travelling at a constant speed
• The orbit is a circular path, therefore the direction in which the object is travelling will be constantly changing direction
• A change in direction causes a change in velocity
• Acceleration is the rate of change of velocity, therefore if the object is constantly changing direction then its velocity is constantly changing and so the object in orbit is accelerating
• A resultant force is needed to cause an acceleration
• This resultant force is gravity and it 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

#### Exam Tip

When you are describing the motion of an object in orbit make sure you describe the correct terminology. Speed is a scalar quantity, it has a magnitude (size) only. Velocity is a vector quantity, it has both a magnitude (size) and a direction.

Higher Tier Only

### 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
• If the speed of the satellite is too big:
• The radius of the orbit will increase and the satellite will spiral into space
• This is because the gravitational attraction cannot provide enough force to keep it in orbit
• If the speed of the satellite is too small:
• The radius of the orbit will decrease and the satellite will move towards the object it should be orbiting
• This is because the gravitational attraction is too strong to maintain a constant orbital radius Diagram showing how the speed of an artificial satellite affects its orbit

• If an artificial satellite is to change the radius at which it is orbiting then the speed at which it is travelling must change
• To maintain a stable orbit:
• If the speed increases the radius must increase
• If the speed decreases the radius must decrease ### Author: Katie

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.
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