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A rock is dropped from an 80- meter clif...

A rock is dropped from an 80- meter cliff. How long does it take to reach the ground ?

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Since the rock's displacement is down, we call down the positive direction, so a = +g. We're given `v_(0) s`, and a , and asked for t. So v is missing, it isn't given and it isn't asked for, and we use Big Five #2.
`Delta s= v_(0) t+(1)/(2) at^(2) implies Delta s = (1)/(2) a t^(2) (" since" v_(0)=0)`
`t= sqrt((2Delta s)/(a))`
`sqrt((2Delta s)/(+g))=sqrt((2(+80m))/(+10 m//s^(2)))=4.0s`
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