**4.4 Newton’s SECOND LAW**

Newton’s first law, we have seen that when a body is acted on by no force it moves with constant velocity and zero acceleration. In Fig. 4-9a a sliding to the right on wet ice, so there is negligible friction. There are forces acting on the puck; the downward force of gravity and the upward exerted by the ice surface sum to zero. So the net force I if acting on the puck has zero acceleration, and its velocity is constant in newton second law of motion.

Figure 4-9c shows another experiment, in which we reverse the direction force on the puck so that in the direction opposite to v. In this case as puck has an acceleration; the puck moves more and more slowly to the right. If ward force continues to act, the puck eventually stops and begins to move more rapidly to the left. The acceleration ii in this experiment is to the left, in the same as As in the previous case, experiment shows that the acceleration is if constant. We conclude that the presence of a net force acting a body causes the body The direction of the acceleration is the same as that of the net force magnitude of the net force is constant, as in Figs. 4-9b and 4-9c, then so is the of the acceleration. These conclusions about net force and acceleration also apply to a body along a curved path. For example, Fig. 4-10 shows a hockey puck moving’ circle on an ice surface of negligible friction. A string attaching the p ice exerts a force of constant magnitude toward the center of the circle. The acceleration that is constant in magnitude and directed toward the center of the The speed of the puck is constant, so this is uniform circular motion, as di Section 3-5. Figure 4-11a shows another experiment to explore the relationship acceleration of a body and the net force acting on that body. We apply a co force to a puck on a horizontal surface, using the spring described in Section 4-2 with the spring stretched a constant amount. As in Fi and 4-9c, this horizontal force equals the net force on the puck. If we change of the net force, the acceleration changes in the same proportion. net force doubles the acceleration (Fig. 4-11 b), halving the force halves the (Fig. 4-11 c), and so on. Many such experiments show that for any given body of the acceleration is directly proportional to the magnitude of the net f on the body. For a given body the ratio of the magnitude 11:11 of the net force to the a of the acceleration is constant, regardless of the magnitude of the net call this ratio the inertial mass, or simply the mass, of the body and denote it figure.