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76 m a s s a g e & b o d y w o r k j a n u a r y / f e b r u a r y 2 0 1 5 LAW OF INERTIA Imagine you're running steadily down a trail. You turn a blind corner and there—standing absolutely still in your pathway—is a massive elk. ( Just go with me here.) You "apply the brakes" by clamping your muscles and digging in your heels, but you can't avoid plowing right into the huge mass of fl esh and fur. He wanders off (mumbling about how odd humans can be) and you check yourself over for bruises and broken bones (Image 1). resists a change in motion and explains why you didn't knock the elk off its hooves upon collision. (Since the elk has more mass and thus more inertia than you, the force of your decelerating body had marginal effect.) Now, let's replay the above scenario, but let's replace you with J. J. Watt (giant American football player) and the elk with a baby goat. With the relative inertias switched, upon contact, Mr. Watt would send the wee goat airborne. In another example of this law, line up two golf balls and gently tap Ball #1 onto the green. While Ball #2 stays in place (an object at rest stays at rest), the force of the club causes Ball #1 to roll toward the hole (an object in motion tends to stay in motion). If we removed resistive forces like gravity, friction, and air resistance, Ball #1 would never stop traveling (Image 2). Newton's First Law may seem so utterly obvious that you might wonder why it even needs to be discussed. Yet, this simple concept pretty much explains mobility and stability. After all, fl ip the law on its head and imagine the kind of action you would have if an object at rest was inclined toward motion and vice versa. Science fi ction chaos. Running into an elk. Tapping Ball #1 onto the green. 1 2 m a s s a g e & b o d y w o r k j a n u a r y / f e b r u a r y 2 0 1 5 1 2 Ooof! STORY 1 Running into an elk. 1 Ooof! This goofy tale illustrates Newton's First Law of Motion, known as the Law of Inertia. To paraphrase, it states that an object at rest tends to stay at rest, while an object in motion tends to stay in motion. Inertia means that an object will keep on doing what it's doing. In this case, you were in motion while the elk was at rest. What's more, to overcome an object's inertia, a force is required. This force will cause the object to stop, move, or change direction. In your case, your mobility was altered by the stationary force of the elk. (It stopped you cold.) This highlights how inertia