Sometimes objects move within a medium that is moving with respect to an observer. For example, an airplane usually encounters a wind - air that is moving with respect to an observer on the ground below. As another example, a motorboat in a river is moving amidst a river current - water that is moving with respect to an observer on dry land.
In these examples, the magnitude of the velocity of the moving object (whether it be a plane or a motorboat) with respect to the observer on land will not be the same as the speedometer reading of the vehicle. That is to say, the speedometer on the motorboat might read 20 km/h; yet the motorboat might be moving relative to the observer on shore at a speed of 25 km/hr. Motion is relative to the observer. The observer on land would measure the speed to be different than that of the person on the boat. The observed speed of the boat must always be described relative to who the observer is.
The following simulation shows the motion of a boat in a current.
Things to try:
The following file contains activities and problems associated with the simulation.
The University of Queensland