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Accelerations effect on force

  1. Mar 1, 2015 #1
    1. The problem statement, all variables and given/known data
    man's weight is 83.8 kg
    1lb=4.48N
    what would the scale read (in N) if he went from 0m/s to 7m/s up in 6.56 seconds?

    2. Relevant equations
    F=ma
    a=v/t


    3. The attempt at a solution
    a=7/6.56
    a=1.07
    f=(83.5)(1.07)
    f= 89.3N
     
  2. jcsd
  3. Mar 1, 2015 #2

    BvU

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    Hello Shoebox (?),

    Do you have question? (I suppose so, or you wouldn't post, right :wink:). I sure have one:

    What would the scale read if he isn't accelerated at all, just standing still and upright ?
     
  4. Mar 1, 2015 #3
    it would read 821 N
     
  5. Mar 1, 2015 #4
    What does a weighing scale usually read, the weight of something else actually? Know that and you will get your answer!! :wink:
     
  6. Mar 2, 2015 #5
    it reads pounds. so it would be 183.26 pounds. but i still dont understand how the acceleration would affect his weight
     
  7. Mar 2, 2015 #6

    BvU

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    Take a scale, go to the nearest skyscraper, step into the elevator, stand on the scale .
    If your weight is 83.8 kg then the scale reading (in N, it's borrowed from a physicist) initially is 822 N.
    The only force you feel is the force of the scale that keeps you in place.

    Now press the button for the top floor. The elevator accelerates from 0 m/s to 7 m/s in 6.56 seconds.

    The man doesn't stay on the ground floor but is accelerated with the elevator cage.
    The man also accelerates from 0 m/s to 7 m/s in 6.56 seconds
    The only force he feels is the force of the scale pushing him up.

    Do you think the scale reading during those 6.56 seconds
    1. stays the same
    2. is lower than 821 N
    3. is higher than 821 N
    After these 6.56 seconds, the elevator zooms up with constant speed. What is the acceleration ? What's the scale reading ?

    Then it brakes at the 84th floor, two floors below the destination top floor. It decelerates from 7 m/s to 0 m/s in 3.5 s.
    What is the acceleration ? What's the scale reading ?


    Physicists scales are in N. Normal developed civilizations (and countries that were conquered by Napoleon) have scales in kg. Some remote tribes on faraway islands have scales in stones.
     
    Last edited: Mar 2, 2015
  8. Mar 3, 2015 #7
    I'm not referring to the unit (pounds). As BvU said, the unit of the scale in in N (Newtons). I mean what quantity does it measure? Draw the free body diagram of the man. The forces acting on him in a stationary elevator will be (1) His weight and (2) Normal reaction from the weighing machine. The weighing machine actually measures the normal reaction on it by the man, which is equal and opposite to the normal reaction exerted by the machine on the man (By Newton's 3rd Law).
     
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