Forces : Man in a lift on a scaler

  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. Can someone please help me ?
  4. Doc Al

    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. 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. Doc Al

    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. 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. Doc Al

    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. 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. Doc Al

    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. it must be zero cause hes not going up, right ?
  12. 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. Doc Al

    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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