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As a rock of mass 4 kg drops from the ed...

As a rock of mass 4 kg drops from the edge of a 40-meter-high cliff, it experiences air resistance, whose average strength during the descent is 20 N. at what speed will the rock hit the ground?

A

8m/s

B

10m/s

C

12m/s

D

20m/s

Text Solution

Verified by Experts

The correct Answer is:
D
Apply conservation of mechanical energy (including the negative work done by `F_(r)`, the force of air resistance).
`K_(i)+U_(i)+W_(r)=K_(f)+U_(f)`
`0+mgh-F_(r)h=(1)/(2)mv^(2)+0`
`v=sqrt((2h(mg-F_(r)))/(m))`
`sqrt((2(40m)[(4kg)(10N//kg)-20N])/(4kg))`
`=sqrt((2(40m)[20N])/(4kg))`
`=sqrt(2(10)(20)(m^(2))/(s^(2)))`
`=20m//s`
Again, `W_("total")=DeltaK` could also be used.
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