Satellites and Projectile Motion
Newton’s Thought Experiment
Newton’s thought experiment allowed us to
understand satellite orbits.
If a projectile is launched with sufficient horizontal
velocity, it will travel so far that the curvature of the
Earth must be taken into account.
Now imagine a projectile launched with such a great
horizontal velocity that it never reaches the ground! It
will continue to circle the Earth until its horizontal
velocity decreases.
The satellite orbits around the Earth because it is in
constant free-fall due to the Earth’s gravity.
Apparent Weightlessness
Astronauts in orbit around the Earth are in a constant state of free-fall. The spaceship, the
astronauts and everything inside are all accelerating towards the Earth due to gravity.
This is known as apparent weightlessness. The effects are because of gravity, and not because they
have escaped from the gravitational field of the Earth.
Period of a Satellite
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The period of a satellite is the time it takes to complete one orbit.
The higher the satellite orbit the longer it takes to orbit.
Orbits, heights, periods and functions of satellites in orbit around the Earth
Name
Orbit
Height
Orbital
Period
Function
LANDSAT 7
Polar
725 km
103 min
Mapping
TIROS
Polar
7000 km
256 min
Weather
Echostar 3
Around
Equator
36,000
km
24 hours
Communication
The Moon
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The Moon is a natural satellite of the Earth
Orbital height is around 385 000 km
Orbital period is around 27 days
Geostationary Satellite

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A satellite that stays above the same point on the Earth at all times.
Its period is 24 hours.
Intercontinental Communication
A



A signal is sent from A to the satellite.
The satellite amplifies the signal and sends the signal to station B.
Curved Reflectors are used at each stage
B
Satellite Communication – Parabolic (Curved) Reflectors
Receiving the signal
receiving
aerial
curved
reflector
Curved reflectors are used to
increase the strength of a
received signal from a satellite or
other source. The curved shape
of the reflector collects the signal
over a large area and brings it to
a focus. The receiving aerial is
placed at the focus so that it
receives a strong signal.
Transmitting the signal
transmitting
aerial
curved
reflector
Curved reflectors are also used
on certain transmitters to
transmit a strong, parallel signal
beam. In a dish transmitter the
transmitting aerial is placed at
the focus and the curved shape
of the reflector produces a
parallel signal beam.