REBECCA BLACKWELL/AP Images Review. Figure P5.15 shows a worker poling a boat-a very effi- cient mode of transportation- across a shallow lake. He pushes parallel to the length of the light pole, exerting a force of magnitude 240 N on the bottom of the lake. Assume the pole lies in the verti- cal plane containing the keel of the boat. At one moment, the pole makes an angle of 35.0 with the vertical and the water exerts a hor- izontal drag force of 47.5 N on the boat, opposite to its forward veloc- ity of magnitude 0.857 m/s. The mass of the boat including its cargo and the worker is 370 kg. (a) The water exerts a buoyant force vertically upward on the boat. Find the magnitude of this force. (b) Model the forces as constant over a short interval of time to find the velocity of the boat 0.450 s after the moment described. Figure P5.15