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Scale/elevator question

  1. Apr 6, 2015 #1
    1. The problem statement, all variables and given/known data

    A person stands on a scale in a elevator at rest. The scale reads 900N.
    1) what is the persons mass
    2) the elevator accelerates up at 2.5m/s^2. What does the scale read now ?
    3)The elevator then continues to move upwards with a steady speed of 4m/s for 5 seconds. What does the scale read during this period
    4)the elevator now decelerates at 1.8m/s^2 until it reaches zero velocity. What does the scale read during this period



    2. Relevant equations : F=ma and W=mg


    3. The attempt at a solution
    1) rearrange w=mg for m = 900/9.81 = 91.7kg

    2) a= 9.81+2.5 = 12.31 so f=ma .... f=91.7*12.31 = 1128.8N

    3) I AM COMPLETELY STUCK ON THIS PART ? -if it is moving upwards at 4m/s for 5 seconds. Then there would be no acceleration as a=0
    so the scale would read the same 1128 N
    would i need to show any working out instead of w=mg = 91.7* 9.8 ?

    4) COMPLETELY STUCK - I UNDERSTAND A= -1.8M/S^2 as it is decelerating
     
    Last edited: Apr 6, 2015
  2. jcsd
  3. Apr 6, 2015 #2
    Question 3) if it is moving upwards at 4m/s for 5 seconds. Then there would be no acceleration as a=0

    so the scale would read the same 1128 N

    would i need to show any working out instead of w=mg = 91.7* 9.8
     
    Last edited: Apr 6, 2015
  4. Apr 6, 2015 #3
    Do you draw a free body diagram and sum your forces? That would make it considerably simpler. Your solutions for 1 and 2 are indeed correct. However, I cannot think of a physics teacher that would not take off points on a test for the lack of free body diagram and summing of forces.
     
    Last edited: Apr 6, 2015
  5. Apr 6, 2015 #4
    how would i draw a free body diagram and sum forces ? is there any tutorial or website i could reference off ?
     
  6. Apr 6, 2015 #5
    http://www.physicsclassroom.com/Class/newtlaws/U2l2d.cfm might help. And here is a decent video . Make sure you understand Newton's three laws. The mathematics are resultant from his postulations about motion. I would recommend taking an introductory physics course from a certified teacher, honestly. There are some free textbooks here on PF, but most of them seem to deal with more advanced topics. It's worth a look though.
     
  7. Apr 6, 2015 #6
    Really, that video is hardly a substitute for a good lecture.
     
  8. Apr 6, 2015 #7
    thanks for your help. i am understanding the concepts slowly. Where do i search on this website for textbooks. Would you know of any textbook that explains these concepts but are not to technical to understand.
     
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