• Can be categorised as natural or man-made
  • When a satellite is in orbit, the centripetal force is provided by gravity
  • The gravitational force causes the satellites to fall continuously towards the Earth
  • However, it falls to the Earth at the same rate that the surface of the earth curves away from the satellite
  • Therefore, the satellite never hits the Earth
  • Satellite motion is a specific example of projectile motion
  • If the ground slopes at the same rate the projectile falls, it will never hit the ground

Orbit Types

Geosynchronous (Geostationary) Orbits
  • Remain above one point on the Earth
  • Follow an equatorial path
  • Have a period of 1 day, same as Earth’s rotation
  • Radius constant (35 780 km above Earth, see derivation on notes)
  • Used for communication, television broadcasting and weather forecasts
Low Earth Orbits
  • Orbit Earth between 1000-2000 km above Earth’s surface
  • Travel fast to counteract gravity, T=90 min is common
  • Cannot go below 160 km because of atmospheric drag, so velocity decreases, Fg has more effect, it crashes and burns
  • Can have circular or elliptical orbits
  • Have many different directions
  • Used to take high resolution images of the Earth
Highly Elliptical Orbit
  • Move much faster near the perigee, but slower at the apogee
  • Therefore, has a long dwell time at the apogee, and short at the perigee
  • GPS use several satellites in different planes to provide complete coverage

elliptical orbit.fw

Satellite Uses

  • Earth observation
  • Communication
  • Navigation
  • Weather
  • Research
Global Positioning System
  • Satellite navigation system
  • Needs 4 GPS systems to locate accurately
  • Maintained by the US Gov’t

Kepler’s Laws

  1. The planets move in elliptical orbits with the Sun at one focus
  2. The line connecting a planet to the Sun sweeps out equal areas in equal intervals of time
  3. For every planet, the ratio of the cube of the average orbital radius to the square of the period of revolution is constant:

kepler law


  • Apparent weight comes from the normal force
  • You feel weightless when no normal force is applied
  • Therefore, W = mg = Fnet ; Fn = 0


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