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Homework Help: Comapring acceleration of person in elevator

  1. Jan 25, 2015 #1
    1. The problem statement, all variables and given/known data

    Two persons, each having mass 70 kg, stand in identical elevators. One stand normally, one hangs in the ceiling of elevator. How is the acceleration of them when the elevators move?

    a. Equal in magnitude and direction

    b. Equal in magnitude and opposite in direction

    c. Different in magnitude but equal in direction

    d. Different in magnitude and direction

    e. Not enough information

    2. Relevant equations
    F = m.a

    3. The attempt at a solution
    I assume the second person hangs on a rope attached to the ceiling of the elevator so there are two forces acting on the man, weight and tension. There are also two forces acting on the man who stands in the elevator, weight and normal force.

    Assuming the elevators move upwards, the acceleration experienced by each man is:

    a = (T - W) / m and a = (N - W)/m

    We don't know the exact value of T and N, so the answer will be "not enough information"?

  2. jcsd
  3. Jan 25, 2015 #2
    Who says?

    T = W + ma

    N = W + ma

    T = N

  4. Jan 26, 2015 #3
    T = N is only when the acceleration of the two elevators is the same. Does the phrase: "identical elevators" imply that those elevators will move with same acceleration or just imply that the elevators have the same mass?

  5. Jan 26, 2015 #4
    It's open to interpretation. It doesn't really matter does it? The important thing is that you analyzed the situation correctly.

  6. Jan 26, 2015 #5


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    Gold Member

    It is normal in such introductory problems to make simplifying assumptions such as "identical elevators" => elevators are accelerating at the same rate and in the same gravitational field. That is, the elevators are identical in all respects that are significant to the problem at hand.
  7. Jan 26, 2015 #6
    OK. Thanks a lot for all the explanation
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