Physics Notes – Astrophysics
Definitions
The universe is a large collection of billions of galaxies.
A galaxy is a large collection of billions of stars.
Our solar system is in the Milky Way galaxy
Gravity
What is mass?
The amount of matter in something
What is gravity?
Gravity is a long range attractive force between two objects with mass
We call gravitational field strength “g” and on earth it has a value of 10N/kg
Weight vs Mass
Earth’s gravitational field strength is 10N/kg. 1Kg mass is pulled downwards by a force of 10N.
Weight (N) = Mass (kg) x Gravitational Field Strength (N/kg)
Gravitational Field on Earth
It is a uniform field close to the surface
It is a radial field from a distance
What Gravity does in stellar proportions
It is responsible for orbits of stars, comets
, Orbits
An orbit is the elliptical course of an object or spacecraft about a star or planet
Orbits calculations
Use – Speed = Distance/Time
V = 2πr/T - 2πr is the circumference of the planet (a circle)
Satellite orbit
There are different types:
Low Earth (Manned imaging, communications, weather)
Medium Earth
High Earth (GPS)
Geostationary (GPS – need to be in the same place all the time)
Polar (an example of low earth)
Centripetal Force - not needed
Definition
A force that causes an object to travel in a circle
Acceleration
A force causes acceleration
Acceleration is a change in velocity
Velocity is a vector quantity therefore has a direction as well.
Therefore Acceleration is either a change in speed or a change in direction or both.
On earth
The earths gravity produces a centripetal force on the moon
This causes the moon to accelerate towards the Earth
This acceleration is due to the moon always changing direction, not due to it changing speed which is
constant.
The force is dependant on the mass of the planet/star
Definitions
The universe is a large collection of billions of galaxies.
A galaxy is a large collection of billions of stars.
Our solar system is in the Milky Way galaxy
Gravity
What is mass?
The amount of matter in something
What is gravity?
Gravity is a long range attractive force between two objects with mass
We call gravitational field strength “g” and on earth it has a value of 10N/kg
Weight vs Mass
Earth’s gravitational field strength is 10N/kg. 1Kg mass is pulled downwards by a force of 10N.
Weight (N) = Mass (kg) x Gravitational Field Strength (N/kg)
Gravitational Field on Earth
It is a uniform field close to the surface
It is a radial field from a distance
What Gravity does in stellar proportions
It is responsible for orbits of stars, comets
, Orbits
An orbit is the elliptical course of an object or spacecraft about a star or planet
Orbits calculations
Use – Speed = Distance/Time
V = 2πr/T - 2πr is the circumference of the planet (a circle)
Satellite orbit
There are different types:
Low Earth (Manned imaging, communications, weather)
Medium Earth
High Earth (GPS)
Geostationary (GPS – need to be in the same place all the time)
Polar (an example of low earth)
Centripetal Force - not needed
Definition
A force that causes an object to travel in a circle
Acceleration
A force causes acceleration
Acceleration is a change in velocity
Velocity is a vector quantity therefore has a direction as well.
Therefore Acceleration is either a change in speed or a change in direction or both.
On earth
The earths gravity produces a centripetal force on the moon
This causes the moon to accelerate towards the Earth
This acceleration is due to the moon always changing direction, not due to it changing speed which is
constant.
The force is dependant on the mass of the planet/star
