Friction
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Friction is a universal effect that causes a system to lose energy
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For example, kick a soccer ball:
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In grass, the ball slows down pretty quickly
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On pavement, the ball rolls a long time
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Wave systems also have a friction effect, called damping
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Pluck a guitar string:
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The vibrations slowly diminish in amplitude (size)
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The sound becomes softer with lower amplitude
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Drop a rock into a pool of water:
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The waves propagate out from the splash
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After some distance, the size of the waves is much smaller
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The initial force (kicking the ball, plucking the string, dropping the rock) puts energy into the system (ball, string or water).
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Friction causes the energy in the system to dissipate over time.
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If the system has a continuing source of energy, the loss of energy due to friction can be overcome. For example, gravity can be a source of energy:
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For the same force in your kick, kicking the ball down a hill will cause it to roll further than on flat ground.
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The hill needs to be steeper to maintain the motion of the ball, if the ball is on grass than if the ball is on pavement.
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