# 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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