IGCSE Physics notes module 8: Astrophysics

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