ACCELERATION IS A VECTOR
 

Acceleration is a vector quantity. Sometimes the magnitude of the acceleration vector is called acceleration, and is symbolized by lowercase italic letters such as a. Technically, however, the vector expression should be used; it is symbolized by lowercase boldface letters such as a. In the example of a car driving along a highway, suppose that the speed is constant at 25 m/s (Fig. 1). The velocity changes when the car goes around curves and also if the car crests a hilltop or bottoms out in a ravine or valley (although these can’t be shown in this two-dimensional drawing). If the car is going along a straightaway and its speed is increasing, then the acceleration vector points in the same direction that the car is traveling. If the car puts on the brakes, still moving along a straight path, then the acceleration vector points exactly opposite the direction of the car’s motion.
Acceleration vectors can be illustrated graphically as arrows. In Fig. 1, three acceleration vectors are shown approximately for a car traveling

Fig. 1. Acceleration vectors x, y, and z for a car at three points (X, Y, and Z) along a road. Note that y is the zero vector because there is no acceleration at point Y.

along a curving road at a constant speed of 25 m/s. Three points are shown, called X, Y, and Z. The corresponding acceleration vectors are x, y, and z. Acceleration only takes place where the car is following a bend in the road. At point Y, the road is essentially straight, so the acceleration is zero. This is shown as a point at the origin of a vector graph.