Imagine (Or build) a hovercraft fan fastened to the front of a car facing foward, and a small contraption that will drop table tennis balls into the airstream from the fan. A sensor also attatched to the car measures the speed of the pingpong balls in the airstream when the fan is running to be 40 miles per hour.
Now start the car and drive forwards at 40 miles per hour( against the direction of the fan).
What speed does the sensor measure the balls to be travelling?
What speed does a sensor fastened to the ground measure the speed of the balls to be?