Gravitational Effects on Orbits (Cambridge O Level Physics)

• The strength of gravity on different planets after an object's weight on that planet
• Weight is defined as:

The force acting on an object due to gravitational attraction

• Planets have strong gravitational fields
  • Hence, they attract nearby masses with a strong gravitational force

• Because of weight:
  • Objects stay firmly on the ground
  • Objects will always fall to the ground
  • Satellites are kept in orbit

The Effect of Gravity on Earth

Objects are attracted towards the centre of the Earth due to its gravitational field strength

• Both the weight of any body and the value of the gravitational field strength g differs between the surface of the Earth and the surface of other bodies in space, including the Moon because of the planet or moon's mass
  • The greater the mass of the planet then the greater its gravitational field strength
  • A higher gravitational field strength means a larger attractive force towards the centre of that planet or moon

• g varies with the distance from a planet, but on the surface of the planet, it is roughly the same
  • The strength of the field around the planet decreases as the distance from the planet increases
• However, the value of g on the surface varies dramatically for different planets and moons

• The gravitational field strength (g) on the Earth is approximately 10 N/kg
• The gravitational field strength on the surface of the Moon is less than on the Earth
  • This means it would be easier to lift a mass on the surface of the Moon than on the Earth

• The gravitational field strength on the surface of the gas giants (eg. Jupiter and Saturn) is more than on the Earth
  • This means it would be harder to lift a mass on the gas giants than on the Earth

Values of Gravitational Field Strength

Value for g on the different objects in the Solar System

• On such planets such as Jupiter, an object’s mass remains the same at all points in space
• However, their weight will be a lot greater meaning for example, a human will be unable to fully stand up

Comparison of g on Earth & Jupiter

A person’s weight on Jupiter would be so large a human would be unable to fully stand up

• There are many orbiting bodies in the Solar System which can be defined by the object that they orbit around

Orbiting Bodies in the Solar System

Orbiting body	What it orbits
planet	the Sun
the Moon	planet
comet	the Sun
asteroid	the Sun
artificial satellite	any body in the Solar System (apart from the Sun)

• A 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

• Therefore, it is said that the force that keeps a planet in orbit around the Sun is the gravitational attraction of the Sun
• 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
• Therefore, the force that keeps an object in orbit around the Sun is the gravitational attraction of the Sun and is always directed from the orbiting object to the centre of the Sun

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

Orbital Motion of the Moon

Gravitational attraction causes the Moon to orbit around the Earth

Non-Circular Orbits

• Orbits of planets, minor planets and comets are elliptical
  • An ellipse is just a 'squashed' circle
• Planets, minor planets and comets have elliptical orbits
  • However, the Sun is not at the centre of an elliptical orbit
  • This is only the case when the orbit is approximately circular

An elliptical orbit

• As the distance from the Sun increases:
  
  • The strength of the Sun's gravitational field on the planet decreases
  • The orbital speed of the planet decreases

• To keep an object in a circular path, it must have a centripetal force
  • For planets orbiting the Sun, this force is gravity

• Therefore, the strength of the Sun's gravitational field in the planet affects how much centripetal force is on the planet
  • This strength decreases the further away the planet is from the Sun, and the weaker the centripetal force

• The centripetal force is proportional to the orbital speed 
  • Therefore, the planets further away from the Sun have a smaller orbital speed
  • This also equates to a longer orbital duration 

Orbital Speed & Distance

How the speed of a planet is affected by its distance from the Sun

• This can be seen from data collected for a planet's orbital distance against their orbital speed 
  • For example, Neptune travels much slower than Mercury

Orbital distance, period & speed of the planets in the Solar System

Planet	Orbital distance from Sun / million km	Orbital period	Orbital speed / km/s
Mercury	57.9	88 days	47.9
Venus	108.2	225 days	35.0
Earth	149.6	365 days	29.8
Mars	227.9	687 days	24.1
Jupiter	778.6	11.9 years	13.1
Saturn	1433.5	29.5 years	9.7
Uranus	2872.5	75 years	6.8
Neptune	4495.1	165 years	5.4