Astrophysics
The following sub-topics are covered in this section.
(a) Units
(b) Motion in the universe
(c) Stellar evolution
(d) Cosmology
(a) Units
8.1 use the following units: kilogram (kg), metre (m), metre/second (m/s), metre/second2 (m/s2
), newton (N), second (s), newton/kilogram (N/kg)
(b) Motion in the universe
8.2 know that
• 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.
8.3 understand why gravitational field strength, g, varies and know that it is different on other
planets and the Moon from that on the Earth
Gravitational field strength depends on density and radius. A greater density and greater radii
means a stronger gravitational field.
Between any two objects there is a force of attraction. This is called the gravitational force and it
depends on the mass of the two object and the distance between the two masses.
So earth is bigger therefore has a greater gravitational field but as you leave earth's atmosphere it
gets weaker. The moon is smaller (less dense, smaller radii) and therefore has a weaker gravitational
field strength.
On the Earth it is about 10N/kg
Weight varies on planets because g varies
8.4 explain that gravitational force
● Gravity causes moons to orbit planets
● Gravity causes artificial satellites to orbit the Earth
● Gravity causes planets and comets to orbit the sun
,If an object is within the field of another objects gravitational force then it will travel around it in
a path known as an orbit. For example the Earth orbits the sun, this means that the sun's
gravitational force pulls it inwards so it orbits the sun in an almost circular orbit
• causes moons to orbit planets
• causes the planets and comets to orbit the Sun :
because the strength of the gravitational field remains the same when it goes around the sun in an
elliptical shape. The mass and distance is constant so the force is constant. The planets orbit the sun
at a relatively steady speed.
comets orbit the sun at varying speeds because comets move past the sun i.e. change their distance
towards sun. They will move faster when further away from the sun as the gpe (gravitational
potential energy) is weaker and it can therefore speed up. When the Comet is closer to the sun this
is stronger and it slows down. A comet's tail will always point away from the sun as it consists of rock
containing melted ice which is blown away from the sun but it’s solar wind.
Kinetic energy → GPE and back
• causes artificial satellites to orbit the Earth
Artificial satellites are launched into space by rockets
Artificial satellites are used for
1. Communication (broadcasting and relaying telephone calls)
2. Navigation, map making
3. Collecting information to help with weather forecast
4. Spying
5. Scientific surveys of the earth’s surface
There are two kinds
Polar satellites: lower orbit so faster
Satellites in low polar orbits pass over the poles, they take about 90 minutes for each orbit
The Earth spins beneath the satellite as it moves, so the satellite can scan the whole surface of the
Earth.
uses
● monitoring the weather
● observing the Earth’s surface
● military uses including spying
Geostationary satellites: further away from earth so slower
Orbit above the equator, have a different trajectory (path) to polar satellites
, they stay in a fixed position over the Earth’s surface. This means they always stay in the same
position above the earth’s surface. This means that they always have a stable connection to a point
on earth.
uses
● communications - including satellite TV
● global positioning or GPS - which is used for sat navs (satellite navigation systems)
How speed affects orbit:
● an object moving too quickly( at too great a speed) will continue moving in a straight line
past another object (eg. Planet) without deflecting towards it. It will not orbit.
● an object moving too slowly(its speed is not great enough) will find that the gravitational
attraction is too strong and therefore the object is pulled into the planet. It will not orbit.
● If the an object moves at the right speed, it stays in a steady slightly elliptical but basically
circular orbit.
8.5 describe the differences in the orbits of comets, moons and planets
They all have elliptical orbits. However many orbits are almost circular.
Comets orbit at varying speeds. Because they move past the object so they change their distance
towards mass. Comets have very elongated orbits.
In moons and planets the orbit speed is constant because the strength of the gravitational field does
not vary as the orbit is Circular . Mass and distance is constant so force is constant.
planets go around a star; moons go round planets.
The earth’s orbit is slightly elliptical therefore it is sometimes closer and sometimes further away
from the sun. When it is closer it moves slower as it experiences a greater gravitational force.
( kinetic energy → GPE ) When the earth is further away it moves less slowly. However this is much
more pronounced in comets.
8.6 use the relationship between orbital speed, orbital radius and time period:
2πr = circumference
The following sub-topics are covered in this section.
(a) Units
(b) Motion in the universe
(c) Stellar evolution
(d) Cosmology
(a) Units
8.1 use the following units: kilogram (kg), metre (m), metre/second (m/s), metre/second2 (m/s2
), newton (N), second (s), newton/kilogram (N/kg)
(b) Motion in the universe
8.2 know that
• 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.
8.3 understand why gravitational field strength, g, varies and know that it is different on other
planets and the Moon from that on the Earth
Gravitational field strength depends on density and radius. A greater density and greater radii
means a stronger gravitational field.
Between any two objects there is a force of attraction. This is called the gravitational force and it
depends on the mass of the two object and the distance between the two masses.
So earth is bigger therefore has a greater gravitational field but as you leave earth's atmosphere it
gets weaker. The moon is smaller (less dense, smaller radii) and therefore has a weaker gravitational
field strength.
On the Earth it is about 10N/kg
Weight varies on planets because g varies
8.4 explain that gravitational force
● Gravity causes moons to orbit planets
● Gravity causes artificial satellites to orbit the Earth
● Gravity causes planets and comets to orbit the sun
,If an object is within the field of another objects gravitational force then it will travel around it in
a path known as an orbit. For example the Earth orbits the sun, this means that the sun's
gravitational force pulls it inwards so it orbits the sun in an almost circular orbit
• causes moons to orbit planets
• causes the planets and comets to orbit the Sun :
because the strength of the gravitational field remains the same when it goes around the sun in an
elliptical shape. The mass and distance is constant so the force is constant. The planets orbit the sun
at a relatively steady speed.
comets orbit the sun at varying speeds because comets move past the sun i.e. change their distance
towards sun. They will move faster when further away from the sun as the gpe (gravitational
potential energy) is weaker and it can therefore speed up. When the Comet is closer to the sun this
is stronger and it slows down. A comet's tail will always point away from the sun as it consists of rock
containing melted ice which is blown away from the sun but it’s solar wind.
Kinetic energy → GPE and back
• causes artificial satellites to orbit the Earth
Artificial satellites are launched into space by rockets
Artificial satellites are used for
1. Communication (broadcasting and relaying telephone calls)
2. Navigation, map making
3. Collecting information to help with weather forecast
4. Spying
5. Scientific surveys of the earth’s surface
There are two kinds
Polar satellites: lower orbit so faster
Satellites in low polar orbits pass over the poles, they take about 90 minutes for each orbit
The Earth spins beneath the satellite as it moves, so the satellite can scan the whole surface of the
Earth.
uses
● monitoring the weather
● observing the Earth’s surface
● military uses including spying
Geostationary satellites: further away from earth so slower
Orbit above the equator, have a different trajectory (path) to polar satellites
, they stay in a fixed position over the Earth’s surface. This means they always stay in the same
position above the earth’s surface. This means that they always have a stable connection to a point
on earth.
uses
● communications - including satellite TV
● global positioning or GPS - which is used for sat navs (satellite navigation systems)
How speed affects orbit:
● an object moving too quickly( at too great a speed) will continue moving in a straight line
past another object (eg. Planet) without deflecting towards it. It will not orbit.
● an object moving too slowly(its speed is not great enough) will find that the gravitational
attraction is too strong and therefore the object is pulled into the planet. It will not orbit.
● If the an object moves at the right speed, it stays in a steady slightly elliptical but basically
circular orbit.
8.5 describe the differences in the orbits of comets, moons and planets
They all have elliptical orbits. However many orbits are almost circular.
Comets orbit at varying speeds. Because they move past the object so they change their distance
towards mass. Comets have very elongated orbits.
In moons and planets the orbit speed is constant because the strength of the gravitational field does
not vary as the orbit is Circular . Mass and distance is constant so force is constant.
planets go around a star; moons go round planets.
The earth’s orbit is slightly elliptical therefore it is sometimes closer and sometimes further away
from the sun. When it is closer it moves slower as it experiences a greater gravitational force.
( kinetic energy → GPE ) When the earth is further away it moves less slowly. However this is much
more pronounced in comets.
8.6 use the relationship between orbital speed, orbital radius and time period:
2πr = circumference
