Final answer:
A photo tachometer measures an object's speed without contact. An odometer records distance traveled, while a speedometer shows instantaneous speed. Average speed is calculated by dividing the distance traveled by time.
Step-by-step explanation:
A photo tachometer is a device that measures the speed of an object without contact with the object. A car's odometer measures the distance traveled, not the displacement, because it records the cumulative distance the car has moved regardless of direction. Conversely, a car's speedometer measures speed, not velocity, because it shows how fast the car is moving at any given moment without regard to direction, known as instantaneous speed.
If you calculate the total distance traveled on a car trip using the odometer reading and divide it by the time for the trip, you are calculating the average speed. The quantities of average speed and the magnitude of average velocity are the same when the motion is in a straight line without changing direction.