Final answer:
A car's odometer records the total distance traveled, not displacement. The speedometer shows the instantaneous speed. Calculating the average speed from odometer data involves dividing the total distance by the trip time, whereas average velocity is zero if starting and ending points are the same.
Step-by-step explanation:
A car's odometer records the total distance traveled. This is distinct from displacement, which is the straight-line distance from a start point to an endpoint. When using the odometer's data to calculate the average speed of a car trip, you take the total distance traveled and divide it by the trip duration. If the path taken is straight without any change in direction, then the average speed and magnitude of the average velocity are the same. However, average velocity is displacement divided by time, so it considers the direction and can be different from average speed, particularly in scenarios involving a return to the starting point.
For instance, if you drive to a store and return home, the odometer will display the cumulative distance, which might be interpreted as your average speed being nonzero. Yet, the average velocity would be zero since you ended up at the starting point after the round trip. The speedometer, conversely, measures instantaneous speed, indicating the speed of the vehicle at any particular moment, without accounting for direction.