Final answer:
An odometer measures distance traveled while a speedometer measures speed. The average speed is calculated by dividing distance by time and is equal to the magnitude of average velocity only when motion is in a straight line. Instantaneous speed is the magnitude of instantaneous velocity.
Step-by-step explanation:
A car's odometer measures the distance traveled, not displacement. Distance is a scalar quantity that only considers the total path covered, irrespective of direction. On the other hand, a car's speedometer measures speed, and not velocity. Speed is a scalar quantity, whereas velocity is a vector quantity that includes both magnitude and direction.
When you divide the total distance traveled on a car trip (as determined by the odometer) by the time for the trip, you are calculating the average speed. The average speed and the magnitude of the average velocity are the same when the trip involves movement in a straight line without changing direction. In situations where direction changes, average speed remains the measure of total distance covered over time, while average velocity is the total displacement divided by the total time.
Instantaneous velocity and instantaneous speed relate to each other as they are both measures at a specific point in time. However, instantaneous speed is just the magnitude of the instantaneous velocity and does not take direction into account.