This one is a doozy . . . A fresh water pond that is 15.7 m deep is contained on one side by a cliff. The water has eroded a nearly horizontal "tube" through a bed of limestone, which allows the water to emerge on the other side of the cliff. If the "tube" has a diameter of 4.05 cm, and is located 5.6 m below the surface of the pond, what is the frictional force between the "tube" wall and a rock that is blocking the exit? I've spent a bit too long on this one, and I am getting nowhere . . . any assistane would be greatly appreciated. The sooner the better as well lol. Thanks for your help in advance.