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Homework Help: Forces : Man in a lift on a scaler

  1. Nov 9, 2009 #1
    1. The problem statement, all variables and given/known data
    A man of mass 70 kg stands in a lift on a set of weighing scales. The scales are calibrated in newtons. Assuming that g=10N kg -1
    What weight will the scales register when the :

    a) lift is static
    b)lift accelerates down at 10 ms-2
    c)lift accelerates up at 10 ms-2

    2. Relevant equations


    3. The attempt at a solution

    F= 70 x 0

    b) accelerates down at 10ms -2

    F=70 /10


    c) accelerates up at 10 ms -2

    70 + 7 = 77

    Is this correct ?

  2. jcsd
  3. Nov 9, 2009 #2
    Can someone please help me ?
  4. Nov 9, 2009 #3

    Doc Al

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    Staff: Mentor

    No, your answers are not correct. First identify all the forces acting on the man. (One of those forces will equal the scale reading.) Then apply ΣF = ma to each case.

    Don't forget to give your answers in Newtons, not kg.
  5. Nov 9, 2009 #4
    I don't understand you, sorry. Can you explain again please ?

    Is F=ma all I need here ?

    I am not sure, however I really want to solve it :)
  6. Nov 9, 2009 #5

    Doc Al

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    Staff: Mentor

    What forces act on the man? (Hint: Two forces act on him.) What direction do they act?

    Yes. Realize that F stands for the net force.
  7. Nov 9, 2009 #6
    So I think it would be gravity acting down and also the normal force pushing him up, correct ?

    Knowing that in the first case I subtract and in the second case I add the forces right ?

    However I still do not get the connecting between 'g' and the question. :(
  8. Nov 9, 2009 #7

    Doc Al

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    Staff: Mentor

    Exactly. And realize that the normal force is what the scale measures.

    Just use up as + and down as -. Using that convention, ΣF = N - W.

    You are given the mass but need the weight (to apply F=ma). That's where 'g' comes in.
  9. Nov 9, 2009 #8
    Ill try do a then :

    if the GF is pushing him down and NF is pushing him up then his weight is 70N cause the gravity force is greater by 70 N

    Second one:
    If the lift is going down then the gravity force must he smaller by 10 :

    70-10 = 60 N

    Third one :

    vice versa

    70+10=80 N

    Am I thinking correct at this time ?

    By the way I really appreciate your help and patience here.
  10. Nov 9, 2009 #9

    Doc Al

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    Staff: Mentor

    First things first: If the man's mass is 70 kg, what is the GF on him? (The gravitational force is the same in all three cases, as gravity isn't changing.)
  11. Nov 9, 2009 #10
    it must be zero cause hes not going up, right ?
  12. Nov 9, 2009 #11
    The man is 70 kg, and there's a gravitational force acting down on him, with this in mind F=m(man)*a(???) even though the elevator isn't accelerating what other acceleration is there?
    Keep in mind gravity is in m/s^2.
  13. Nov 9, 2009 #12

    Doc Al

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    Staff: Mentor

    No. I asked about the gravitational force acting on the man. That's certainly not zero! How do you compute the force of gravity? (That's where you use 'g'.)
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