Sometimes, an object moves within a medium which is moving with respect to a fixed observer. Here you have a boat sailing straight right. in a current. Therefore the magnitude of the velocity of the boat with respect to a fixed observer on land will not be the one indicated by the speedometer on-board.
This simulation is an application of the velocity composition law:
The velocity of the boat relative to the ground (the only one necessary to determine the sailing route) is the addition of the two others.
These considerations would be the same for an airplane which encounters a wind.
Click on the grid to modify the water current.