Edexcel IGCSE Physics: Double Science 复习笔记:8.1.3 Orbital Motion

Edexcel IGCSE Physics: Double Science 复习笔记:8.1.3 Orbital Motion

Orbital Motion


  • 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


  • smaller body or object will orbit a larger body
    • For example, a planet orbiting a Sun


  • 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 will cause the body to move and maintain in a circular path



Gravitational attraction causes the Moon to orbit around the Earth




  • Gravity causes different astronomical orbits to orbit certain others
  • In general:
    • Planets orbit the Sun
    • Moons orbit planets
    • Comets orbit the Sun
    • Artificial satellites to orbit the Earth or any body in the Solar System



Differences in Orbits


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 (different orbital radius)
    • They orbit at different speeds
    • They all take different amounts of time to orbit the Sun


  • The further away a planet is from the Sun, the slower it travels and therefore the longer it takes to orbit


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






  • 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


  • As the comet approaches the sun, its speed increases
  • As it moves further away from the sun, its speed decreases



The elliptical orbit of a comet