Circular Orbits (Edexcel GCSE Physics)

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Orbiting Bodies

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

Orbiting Objects or Bodies in Our Solar System Table

Orbiting Objects, downloadable IGCSE & GCSE Physics revision notes
  • smaller body or object will orbit a larger body

    • For example, a planet orbiting the Sun

  • In order to orbit a body such as a star or a planet, there has to be a force pulling the object 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 will cause the body to move and maintain in a circular path

gravity-&-orbits, IGCSE & GCSE Physics revision notes

Gravitational attraction causes the Moon to orbit around the Earth

Orbits of 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, downloadable IGCSE & GCSE Physics revision notes

The orbits of planets around the Sun

Orbits of Moons

  • Moons orbit planets in a circular path

  • Some planets 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 of the orbit

Comets

  • The orbits of comets are very different to those of planets:

  • Their orbits are highly elliptical (very stretched) or hyperbolic

    • This causes the speed of the comets to change significantly as their distance from the Sun changes

    • Not all comets orbit in the same plane as the planets and some don’t even orbit in the same direction

comet-orbit, IGCSE & GCSE Physics revision notes

The elliptical orbit of a comet

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 large:

    • 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 low:

    • 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, downloadable IGCSE & GCSE Physics revision notes

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 decrease

    • If the speed decreases the radius must increase

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Circular Orbits

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

    • Objects in circular orbit are travelling at a constant speed but a changing velocity

  • In a circular path, 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

  • 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

Motion in an orbit, downloadable IGCSE & GCSE Physics revision notes

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

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Ashika

Author: Ashika

Expertise: Physics Project Lead

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.