Ch3_2 #2

Part A:
See the sketch on Ch 3 Sheet 16''' on the right side of the sheet (there is no video for that sheet): in your problem Mary heads the boat straight across the river, which flows east and thus pushes Mary downstream. (The solid vectors give the velocity vectors relative to the ground, i.e. Vbg is the velocity vector pointing where Mary actually goes - not where she heads the boat which is the direction of the dashed velocity vector Vbw). Identify the 2 given velocity vectors in your problem, make the sketch as on Sheet 16''' and place the velocity values given into the sketch. Notice that d is the change of position in the y-diredtion. Thus you need to use the velocity in the y-direction to get the crossing time.
Part B:
Notice that the distance downstream is the change of position in the x-diredtion. Thus you need to use the velocity in the x-direction to get the distance downstream.