Question

Water going over a fall splashes on rocks 20 m below. If 20 kg of water flows over the fall per second, how much force do the rocks exert on the falling water?

Answer 1

using work energy theorem

Work done by the force = change in kinetic energy

W = 0.5*m*v^2

But Work done is W = F*S

F*S = 0.5*m*v^2

but v is the speed of water at the rock = sqrt(2*g*h) = sqrt(2*9.81*20) = 19.8 m/s

then

F*S = 0.5*m*v^2

F*20 = 0.5*20*19.8^2

F = 196 N