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STORY 2 LAW OF ACCELERATION, PART 1 It's snowing heavily and your car is stuck in a ditch. Fortunately, another vehicle stops and three big guys jump out to help. While you steer, they slowly push your little Honda back onto the road (Image 3). You thank the fellows and drive safely to your destination. You may also want to express your gratitude to Newton's Second Law of Motion. Known as the Law of Acceleration, it has nothing to do with your gas pedal, but focuses on the relationship between force, mass, and acceleration. This law states that the acceleration of an object is directly proportional to the force acting on it and is inversely proportional to its mass. The direction of the acceleration is in the direction of the applied force. What the heck does that mean? Some guys pushed ( force) your car (mass) and it moved (acceleration) in the direction that they pushed it. It's snowing heavily and your car is stuck in a inversely proportional to its mass. The direction of the Three guys successfully pushing out a Honda. Let's go a bit deeper. If we recall, force is any influence that causes an object to undergo a certain change. Thus, the three guys created force and it caused your car to move. Mass—the quantity of matter in an object— pertains to your car. Luckily for the guys, it was small and contained little mass. Acceleration—the rate at which the velocity of a body changes with time—was evident as they shifted your car's position from ditch to road. Now, let's see what happens if we change some of the story's variables. For instance, let's say we switch out your little Honda for a 2-ton Hummer (Image 4). The three guys produce the same amount of force, but now there's much more mass. For all of their pushing, they barely shift the rig. Thus, the acceleration of the vehicle diminishes. In other words, if the same force is applied (three guys) and the mass increases (the Hummer), acceleration decreases. But now a UCLA football van pulls up and 10 massive linemen jump out ready to push (Image 5, page 78). Even against the Hummer's large mass, they are able to produce a great enough force to send the vehicle accelerating back onto the road. Here we see how an increase in applied force increases acceleration. pertains to your car. Luckily for the guys, it was small and contained little mass. which the velocity of a body changes with time as they shifted your car's position from ditch to road. story's variables. For instance, let's say we switch out your little Honda for a 2-ton Hummer (Image 4). The three guys produce the same amount of much more the rig. Thus, the other words, if the same force is applied (three guys) and the mass increases (the Hummer), acceleration decreases. massive linemen jump out ready to push (Image 5, page 78). Even against the Hummer's large mass, they are able to produce a great enough force to send the vehicle accelerating back onto the road. Here we see Three guys failing to push out a Hummer. 3 4 Under more serious circumstances, such as the sudden stopping (or hurtling) of an automobile, the realities of Newton's First Law can produce injurious effects on the cervical spine. For instance, if you were rear-ended while stopped in traffi c, the fl inging motion of your whiplashed head would demonstrate how an object at rest tends to stay at rest. Unfortunately, in those few milliseconds that your head was at rest, your torso was pressed forward by the car seat (left). These different inertias (torso moving, head stationary) send your cranium fl inging into extension and then (as we'll see in the Third Law of Motion) swing it forward into fl exion (right). . . . hyperfl exion of the cervical vertebrae. The First Law of Motion on display in a whiplash injury, hyperextension followed by . . .