IGCSE PHYSICS – TOPIC 8
ASTROPHYSICS
a) MOTION IN THE UNIVERSE
• EARTH – our planet, the Earth, is the third of eight planets in our
Solar System
• THE MILKY WAY GALAXY – our solar system is in the Milky Way galaxy
– a galaxy is a large collection of billions of stars
• GALAXY – a galaxy is a large collection of billions of stars
• UNIVERSE – a large collection of billions of galaxies, containing the
Milky Way and billions of other similar grouping called galaxies
• An objects gravitational field strength (g) depends on:
o The MASS OF THE OBJECT (the more the mass of something, the
stronger the gravity acting on it is)
o The DISTANCE FROM THE OBJECT (gravity gets weaker with
distance)
o This means that the value of the gravitational field strength (g) on
a planet or moon will differ
• For an object to orbit something there has to be a pulling force
towards that body – this is the GRAVITATIONAL ATTRATION of the
larger object. Gravitational force causes:
o Moons to orbit planets
o Planets to orbit the Sun
o Causes artificial satellites to orbit the Earth
o Causes comets to orbit the Sun
• PLANETS orbit the Sun in a SLIGHTLY ELLIPTICAL/NEARLY CIRCULAR
orbit.
o SIMILARITIES: They all orbit in the same plane. They all travel in
the same direction around the sun.
o DIFFERENCE: They orbit at different distances and different
speeds. They all take different amounts of time to orbit the Sun
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, • The ORBIT OF COMETS is different to planets:
o There orbits are HIGHLY ELLIPTICAL (stretched)
o This causes the speed of the comets to change as the distance
form the Sun changed
o Comets do not orbit in the same plane or direction as planets
and can collide with planets or other comets as they travel at
different speeds
As the comet gets CLOSER
TO THE SUN, the force on it
due to gravity INCREASES so
it travels at a GREATER
SPEED. Also, as it gets closer
to the sun, its gravitational
potential energy
DECREASES by being
transferred to kinetic
energy. The INCREASE IN
KINETIC ENERGY means
there is an INCREASE IN
SPEED.
Orbital Speed = 2Pi x Orbital Radius/Time period
v = 2 x Pi x r/T
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ASTROPHYSICS
a) MOTION IN THE UNIVERSE
• EARTH – our planet, the Earth, is the third of eight planets in our
Solar System
• THE MILKY WAY GALAXY – our solar system is in the Milky Way galaxy
– a galaxy is a large collection of billions of stars
• GALAXY – a galaxy is a large collection of billions of stars
• UNIVERSE – a large collection of billions of galaxies, containing the
Milky Way and billions of other similar grouping called galaxies
• An objects gravitational field strength (g) depends on:
o The MASS OF THE OBJECT (the more the mass of something, the
stronger the gravity acting on it is)
o The DISTANCE FROM THE OBJECT (gravity gets weaker with
distance)
o This means that the value of the gravitational field strength (g) on
a planet or moon will differ
• For an object to orbit something there has to be a pulling force
towards that body – this is the GRAVITATIONAL ATTRATION of the
larger object. Gravitational force causes:
o Moons to orbit planets
o Planets to orbit the Sun
o Causes artificial satellites to orbit the Earth
o Causes comets to orbit the Sun
• PLANETS orbit the Sun in a SLIGHTLY ELLIPTICAL/NEARLY CIRCULAR
orbit.
o SIMILARITIES: They all orbit in the same plane. They all travel in
the same direction around the sun.
o DIFFERENCE: They orbit at different distances and different
speeds. They all take different amounts of time to orbit the Sun
51
, • The ORBIT OF COMETS is different to planets:
o There orbits are HIGHLY ELLIPTICAL (stretched)
o This causes the speed of the comets to change as the distance
form the Sun changed
o Comets do not orbit in the same plane or direction as planets
and can collide with planets or other comets as they travel at
different speeds
As the comet gets CLOSER
TO THE SUN, the force on it
due to gravity INCREASES so
it travels at a GREATER
SPEED. Also, as it gets closer
to the sun, its gravitational
potential energy
DECREASES by being
transferred to kinetic
energy. The INCREASE IN
KINETIC ENERGY means
there is an INCREASE IN
SPEED.
Orbital Speed = 2Pi x Orbital Radius/Time period
v = 2 x Pi x r/T
52
