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A person stands on a scale in an elevator (true, it does not happen everyday but it is the sort of insanity that happens on the MCAT!). She notices that the scale is reading lower than her normal weight. Which of the following most reasonably describes the motion of the elevator?
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Correct Answer: C. It is moving up and slowing down.
Let’s randomly assign the upwards direction as positive. If up is positive, keeping in mind that her weight is directed down (i.e. towards the center of the Earth), the scale pushes up so that she does not go through the scale, and we know that the sum of forces is equal to ma from Newton’s 2nd Law, we get:
Fscale – weight = ma
Examine the preceding equation again keeping in mind the following interpretation: The scale reading is less than her normal weight, so ma and thus the acceleration must be negative; i.e., down. Downward acceleration means either: (1) something speeds up while moving down, or (2) something slows down while moving up (i.e. upward deceleration).
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