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AP1200_Ch3_Waves-2Principles-2008

# AP1200_Ch3_Waves-2Principles-2008 - AP1200 3 Waves Concepts...

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1 AP1200 – 3. Waves Concepts AP1200 – 3. Waves Concepts 3.2 Wave principles 3.2 Wave principles Michel A. Van Hove Michel A. Van Hove WARNING: PRINTING THIS DOCUMENT WILL USE MUCH INK AND WILL NOT SHOW ANIMATIONS.

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2 Human waves in sports stadium Human waves in sports stadium The wave “La Ola” 8 this is one wave pulse
3 Let’s perform the Let’s perform the CityU wave pulse CityU wave pulse ! ! time Perform this up-down motion once : first all together to synchronize up-down speed 1 sec up + 1 sec down = 2 sec period then with 2 sec delay (start when neighbor reaches bottom) observe speed of propagation along chain then with 1 sec delay (start when neighbor reaches top) change in speed of propagation along chain? then with 1 sec delay , but ½ + ½ = 1 sec period change in speed of propagation along chain? 7 1 person beats 1 sec intervals, 1 person measures travel time, 1 person measures period

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4 period T Perform this up-down motion repeatedly = periodically : first all together to synchronize up-down speed 2 sec up + 2 sec down = 4 sec period then with 1 sec delay (start when neighbor midway up) observe speed of propagation along chain then with 2 sec delay (start when neighbor at top) observe speed of propagation along chain then with short delay (natural reaction time) observe speed of propagation along chain then with 4 sec delay (start when neighbor reaches bottom) what happened? delayed by one full period: neighbors are in phase! Effect of periodic motion on speed of propagation? time 8 1 person beats 1 sec intervals, 1 person measures travel time, 1 person measures period Let’s perform the Let’s perform the CityU wave train CityU wave train ! !
5 A human wave pulse A human wave pulse (1 sec delay) (1 sec delay) What or who is the source? Can anyone act as a source? Yes! Because everyone simply copies the neighbor’s motion This is Huygens’ Principle: in a wave any point acts as source for its continuation space 6

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6 period T L period T delay D delay D delay D delay D Wave pulse speed: v = L/D independent of period T!
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AP1200_Ch3_Waves-2Principles-2008 - AP1200 3 Waves Concepts...

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