As you sit there, gravity pushes you into the comfortable leather seat. The car accelerates and and an additional force pushes you into the seat back. You round a curve, and yet another force pushes you toward the outside of the curve.
Definition sketches for linear and circular motion.
Let us examine the idea of acceleration more closely. Considering first acceleration in one dimension, the left panel of figure 6.1 shows the position of an object as a function of time, . Velocity is increasing in AB, and the acceleration is positive there. Velocity is constant in BC, which means that the acceleration is zero. Velocity is decreasing in CD, and the acceleration is negative.