Final answer:
The coffee mug appears to be standing still because motion is relative and depends on the observer's frame of reference.
Step-by-step explanation:
The reason why the coffee mug appears to be standing still to you even though it is moving at 25 mi/hr north is because of motion relativity. Motion is relative, meaning that the perception of motion depends on the observer's frame of reference. In this case, you are using the Earth as your frame of reference, and since you and the coffee mug are both moving at the same speed and direction, it looks like it is standing still to you.
An example of this can be seen when you are in a moving vehicle and you look out the window. Objects that are stationary relative to the vehicle appear to be moving, while objects that are moving at the same speed as the vehicle appear to be stationary. This is because your perception of motion is based on your frame of reference.