Gravitational Effects on Orbits (CIE IGCSE Physics)

• The strength of gravity on different planets affects 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

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

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

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

• 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

• 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

Gravitational attraction causes the Moon to orbit around the Earth

EXTENDED

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

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 
  • E.g. Neptune travels much slower than Mercury

Table of Orbital Distance, Speed and Duration

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

EXTENDED

• An object in an elliptical orbit around the Sun travels at a different speed depending on its distance from the Sun 
• Although these orbits are not circular, they are still stable
  • For a stable orbit, the radius must change if the comet's orbital speed changes
• As the comet approaches the Sun:
  • The radius of the orbit decreases
  • The orbital speed increases due to the Sun's strong gravitational pull
• As the comet travels further away from the Sun:
  • The radius of the orbit increases
  • The orbital speed decreases due to a weaker gravitational pull from the Sun

Conservation of Energy

• Although an object in an elliptical orbit, such as a comet, continually changes its speed its energy must still be conserved
  • Throughout the orbit, the gravitational potential energy and kinetic energy of the comet changes 
• As the comet approaches the Sun:
  • It loses gravitational potential energy and gains kinetic energy
  • This causes the comet to speed up
  • This increase in speed causes a slingshot effect, and the body will be flung back out into space again, having passed around the Sun
• As the comet moves away from the Sun:
  • It gains gravitational potential energy and loses kinetic energy
  • This causes it to slow down
  • Eventually, it falls back towards the Sun once more
• In this way, a stable orbit is formed