Automobile speedometers give an instantaneous velocity. It is important to remember that because the velocity has a direction, a change in either the speed or the direction results in a change in velocity. This leads to an interesting fact, a car going around a curve can have a constant speed and yet still have a changing velocity. The car in the following diagram is traveling around a curve. Suppose the driver looks at his speedometer when the car is at points "A", "B", and "C". At each of these points he notices that the speedometer reads 30km/h. His speed is a constant 30km/h as he rounds the curve. Now look at what is happening to his direction of motion. As he goes around the curve, his direction changes from East to South. Because of this change in direction, we have to conclude that his velocity is changing even though his speed is constant.
