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Newton's Law

  1. Sep 24, 2008 #1
    1. The problem statement, all variables and given/known data

    "A student whose weight is 500 Newtons stands on a scale in an elevator and records the scale reading as a function of time. The data is shown in the graph above. At time t=0, the elevator is at displacement x=0 with velocity v=0. Assume that the positive directions for displacement, velocity, and acceleration are upward.

    Graph (scale reading 'newtons' vs time):
    0-5 seconds --> 500 Newtons
    5-10 seconds --> 700 Newtons
    10-15 seconds --> 500 Newtons
    15-20 seconds --> 300 Newtons"

    3. The attempt at a solution

    a) Free Body Diagram w/ all forces on the student at t=8

    Normal Force going up
    mg going down

    b) Calculate the acceleration of the elevator for each 5 second interval

    0 m/s^2 because force stays the same meaning its at rest since velocity is 0 at 0seconds

    a= 3.92m/s^2

    force is back to 500 meaning constant velocity
    a= 0m/s^2

    a= -3.92 m/s^2

    c) It asks for velocity when t= 5,10,15,20

    v= 0 m/s

    v= 19.6 m/s

    v=19.6 m/s
    v=0 m/s

    Now I have to plot this on a velocity vs time graph so I have it at:
    0 velocity from 0-5s
    line from (5,0) to (10,19.6).
    horizontal line from (10,19.6) to (15,19.6)
    line from (15,19.6) to (20,0)

    Just want to see if I am doing this right... Thank you.
  2. jcsd
  3. Sep 24, 2008 #2
    Just from a quick skim, it looks like you're doing it correctly.
  4. Sep 24, 2008 #3
    thank you.
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