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Homework Help: Weight in an elevator

  1. Oct 24, 2013 #1
    1. The problem statement, all variables and given/known data

    I need to find the weight of a person who has mass 71 kg, and is riding an elevator, at different time intervals. The intervals are 1,5, and 9 seconds. I was given a velocity vs time graph, so I'll try to describe as best i can: The graph starts at (0,0), the slope is 4/1, however when it reaches (2,8)- 2 seconds and 8 m/s - it flatlines for about 4 seconds, then the slope changes to -2/1 and stops when it reaches (0,10) - 10 seconds and 0 m/s.

    2. Relevant equations

    No relevant equations

    3. The attempt at a solution

    I tried to find the acceleration, which I know is the slope of the graph, then multiplying it by the mass, but obviously that didnt work, also my answer has to have two sig figs, and of my course my attempted answers give me 3 or 4. I think if i could form an equation for the graph and then find the differential, then substitute in the time intervals, that would give me a better number to multiply the mass by
  2. jcsd
  3. Oct 24, 2013 #2
    Hi rysezhae. Welcome to Physics Forums.

    Let me guess. You are trying to find the reading that a spring scale would register with a mass of 71 kg sitting on the scale. This is not always the person's weight. His weight is defined as the force that gravity exerts on him, mg, and this doesn't change even though the reading on the spring scale may vary with the acceleration of the elevator. If the acceleration of the elevator equals zero, the spring scale will register his actual weight. So the reading registered by the spring scale is not just the mass times the acceleration; otherwise, the scale would register zero if the elevator were not accelerating.

    Back to your acceleration calculations. If I understand you correctly, the acceleration is 4m/s^2 from 0 to 2 seconds, 0 m/s^2 from 2 to 6 seconds, and -2 m/s^2 from 6 seconds to 10 seconds. Is that correct. Now focus on the person as a free body, and identify the two forces acting on him. Then write down (symbolically) the Newton's 2nd law force balance on this free body. This should give you enough to get what you need.
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