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LAW OF ACCELERATION, PART 2 The second part of this law tells us that the direction of the acceleration is in the direction of the total applied force. For instance, if you roll a giant snowball to the south, it will travel ... to the south. If the snowball is already moving, a slight nudge to the southwest will alter the snowball's direction. This portion of the law might evoke you to blurt out, "No, duh." But consider a parallel universe that offered the opposite: you attempt to fl ex a client's shoulder and, shockingly, instead it abducts or extends. Again, science fi ction chaos. Let's return to the golf course and see this law in action as we replace the golf ball with a bowling ball. This extra mass will require much more force in your stroke to deliver the same acceleration. Return to hitting regular-sized golf balls, but now exert the same force as you did with the bowling ball (Image 6). Such a large force applied to a small mass will cause greater acceleration (and put you in the rough). If you'd like to see Newton's Second Law come to life in your tissues, just wait for a commercial break during a television program. As you rise from the couch to a standing position, your muscles (several forces) pull your bones and fasciae (mass) to generate movement in your limbs (acceleration). The second part of the law is what ensures that the direction of your acceleration will send you where you want to go—the bathroom and not the kitchen. LAW OF ACTION-REACTION You launch a canoe onto a lake. The surface is like smooth glass, and with every stroke of your oar, a ripple of water passes the boat (Image 7). You glide out to a small, fl oating dock in the middle of the lake. Since neither the boat nor the fl oating platform are attached to something steady (like the earth), they shift away from each other as you awkwardly climb from one to the other (Image 8). Safely on deck, you hear a loon pass overhead. If that bird were the ghost of Newton, he might have said, "for every action there is an equal and opposite reaction." And this is exactly the premise of his Third Law of Motion—the Law of Action-Reaction. In other words, whenever one object pushes on a second object, the second one pushes back on the fi rst by the same amount. The strength of the action and reaction are equal and occur in opposite directions. This law surfaces a few times in the above storyline. First, the action of your paddle against the water is met by the equal, but opposite, reaction of the water on your paddle. (If the water offered no reaction, you'd still be on the shore.) In a sense, the water and you (who generated the force) are pushing against each other, with you gliding forward and the water swirling backward. This law also came to life when you attempted to climb onto the dock. At a critical moment, you had one foot in the boat and one on the fl oating island (Image 8). As you pushed off the boat with your foot, the canoe—much lighter than the dock—fl oated away. The defensive line getting the job done. 5 STORY 3 A paddle in the water. 7 This might just hurt. 6