Final answer:
In Physics, speed is a scalar quantity showing how fast an object moves, while velocity is a vector including both speed and direction. The car's speedometer measures speed. Average speed differs from magnitude of average velocity, except for motion in a straight line without direction changes.
Step-by-step explanation:
The subject of this question is Physics, specifically relating to the concepts of speed and velocity. To clarify the distinction, consider the following: speed is a scalar quantity that represents how fast an object is moving and is solely concerned with magnitude, whereas velocity is a vector quantity that includes both the speed (magnitude) of motion and the direction in which the object is moving.
For instance, if a car is reported to have a speed of 60 mph, this tells us how fast the car is going but not the direction. Conversely, if we describe a car as having a velocity of 60 mph south, this not only informs us of the car's speed but also the direction it is traveling in. Therefore, when a car's speedometer reads 60 mph, it measures speed, not velocity. This is because the speedometer cannot detect the direction of the car's movement.
By dividing the total distance displayed on a car's odometer by the elapsed trip time, one calculates the average speed, not the magnitude of average velocity. In scenarios where a car moves in a straight line without changing direction, the average speed and the magnitude of average velocity are identical. However, if the car's journey involves directional changes, these two quantities will differ.