Physics P5

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SATELLITES & GRAVITY

  • Geostationary satellites take 24 hours to orbit the earth, above the equator. 
  • This means they always stay above the same point on earth
  • A gravitational force keeps a satellite in orbit
  • The height and period of a satellited orbit depends on its job
  • Any object moving in a circle needs a force towards the centre of the circle to maintain its circular path
  • This is called centripetal force
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SATELLITE COMMUNICATION

  • Some radio waves are reflected by the ionosphere
  • High frequency radio waves (microwaves) pass through it to reach an orbiting satellite
  • Long wavelength radio waves are easily diffracted around hills
  • Short wavelength microwaves only diffract a small amount
  • The amount of diffraction depends on the size of the gap or obstacle
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SCALES & VECTORS

  • A scalar quantity (e.g. speed) has size only
  • A vector quantity (e.g. velocity) has size and direction
  • momentum = mass x velocity
  • Momentum is a vector which is always conserved
  • Changes in temperature and volume cause changes in pressure
  • Rate of change of momentum creates a force on the walls of the container
  • Rockets expell a large number of particles at high speed
  • Projectiles : 1) have a constant horizontal velocity. 2) have a trajectory that is parabolic. 3) accelerate towards the ground at 10 m/s2
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WAVES

  • Light travels in straight lines
  • Diffraction and interference of light can only be explained by a wave model
  • Polarised light has oscillations in one plane only
  • A convex lens makes a beam of light converge
  • Convex lenses are used in cameras and projectors to produce real, inverted images and as a magnifying glass to produce a virtual image that is the right way up
  • Magnification = image size / object size
  • Interference occurs when two waves overlap. They can reinforce or cancel
  • This results in louder and quiter areas in sound and bright and dark areas in light
  • The type of interference depends on the path difference between the two waves
  • Refraction occurs whn light passes from one medium to another. 
  • As the speed of light changes, its direction can change
  • refractive index = speed of light in a vacuum / speed of light in a medium
  • Light sometime produces a rainbow, or spectrum of colours - dispersion
  • Total internal reflection occurs when light goes from a dense to a less dense medium.
  • The critical angle is the angle of incidence for which the angle of refraction is 90'
  • At greater angles of incidence all the light is totally internally reflected
  • This happens in an optical fibre
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