why are wind tubines soo heavy and they can still be span by the wind ?????????

Question: why are wind tubines soo heavy and they can still be span by the wind ?????????
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  energy
  
  wind
Asked by sammybarmmy13 to Matt H on 25 Jun 2013.
- Matthew Hudson answered on 25 Jun 2013:
  
  They are so heavy because they are so large. They need to be big to get the most efficiency out of the wind that passes them. It may look like they only rotate slowly, but that is an optical illusion from how big they are… in fact, when they are turning the tips of the blade are often approaching the speed of sound!

Question: why are wind tubines soo heavy and they can still be span by the wind ?????????