Final answer:
The speedometer reading of 25 miles/hr does not guarantee that the car has a constant velocity because it measures instantaneous speed without accounting for direction. Constant velocity requires both unchanging speed and direction, and a speedometer alone cannot determine if these conditions are met.
Step-by-step explanation:
If the speedometer of your car reads 25 miles/hr (40.2 km/hr), you cannot say with 100% certainty that the car has a constant velocity. The speedometer measures instantaneous speed, which is the speed of the car at a specific instant in time. It does not account for the direction of travel, which is a necessary component of velocity. For velocity to be constant, both speed and direction must remain unchanged.
It's possible that the car's speed varies during a trip; the speedometer only tells you the speed at the moment you look at it. Changes in direction can occur without affecting the speedometer reading, as it cannot measure changes in the vector component of velocity. Only if the car travels in a straight line with an unchanging speed, it would have a constant velocity.
For example, if you drive to a store and return home, your car's odometer would record the total distance traveled, resulting in an average speed calculation if divided by the total time of the trip. However, the average velocity for this round trip would be zero since the initial and final positions are the same, resulting in a zero displacement.