# 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

#### 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

• 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