Transcript Satellites - HRSBSTAFF Home Page

Planetary and Satellite Motion
12.2
Man-made Satellites
• To successfully launch a satellite,
physicists must be very precise in terms of
speed, direction, and force.
What is an orbit?
• An orbit is a controlled freefall that
provides an illusion of weightlessness due
to relative motion
• Newton was the first to figure this out
Newton’s Mountain
• His idea: Put a cannon on the top of a
very high mountain. Shoot a cannon ball
horizontally. There are two options:
• A) The cannon ball falls to the ground
• B) The cannon ball goes far enough to
actually start travelling
around the Earth.
Newton’s Mountain
• Objects at rest fall 4.9 m in the first second
under the influence of gravity (on Earth).
• If they travel at 8km/s horizontally, they will
fall 4.9 m, matching the curvature of the
Earth which also “falls” 4.9m every 8km.
Where does this math come from?
• Think back to grade 11 physics
(kinematics)
1 2
d  vi t  at
2
• d = 0 + ½ (-9.81m/s2)(1 s)2
• d = -4.9m
Only one problem…
• Air resistance!
• This can be overcome by being on a truly
high mountain (where the air is so thin that
air resistance is negligible).
So how do we launch satellites into
orbit?
• We need to perform calculations to decide
how fast we need the object to travel
• We need to take into account weather
(which affects our air resistance)
• We need to take into account how close to
Earth the satellite will be orbiting.
• It isn’t launched horizontally 
• https://www.youtube.com/watch?v=8t2eyE
Dy7p4
Geostationary Orbit
• Satellites in geostationary orbit stay the same
distance from Earth and so travel in a circular
orbit
• They appear to hover over one spot on Earth’s
surface so that we can receive signals from
them constantly
• This occurs because they are orbiting once
around the Earth every 24 hours (just like one
day on Earth)
• T = 24 hours
• Examples: GPS, cellular phone, TV satellites
Example 1
• What altitude is required for geostationary orbit?
•
•
•
•
What do we already know?
T = 24 hours
Mass of Earth = 5.98 x 1024 kg
Radius of Earth = 6.38 x 106 m
• What are we looking for?
• Altitude = radius from earth’s centre to satellite
subtract the earth’s radius
What is the force holding the
satellite in orbit?
• Fc = Fg
• 35900 km above Earth’s surface
Example 2
• What velocity must a satellite be travelling
at to stay in orbit?
• 3070 m/s
• Page 591
• Questions 15 to 17