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Mass and pulley in an elevator

  1. Feb 4, 2017 #1
    For this problem, use
    g = 10 N/kg.

    3-p-101.png


    As shown in the figure above, a pulley is mounted to the ceiling of an elevator. The elevator is initially traveling with a constant velocity of 1.40 m/s, directed up. The rope (which we assume to have no mass) passing over the pulley has block A tied to one end and block B tied to the other. The mass of block A is 3.80 kg. Block A remains on the floor of the elevator, at rest with respect to the elevator. The mass of block B is 1.40 kg, and block B is also at rest with respect to the elevator, hanging from the rope.


    (a) Calculate the magnitude of the tension in the rope.
    This is the only part of the problem I got correct. My answer: 14 N

    I used the equation ...

    ma - mg = -T
    (-1.4)(10) = -T
    14 N = T

    to find the tension in the rope attached to weight B, since I figured that only this block would be giving the rope a tension.


    (b) Calculate the magnitude of the force exerted on block A by the floor of the elevator.

    (c) The total mass of the pulley and its support is 0.700 kg. Calculate the magnitude of the force exerted by the ceiling of the elevator on the pulley support.

    At some later time, the elevator is moving, and it has an acceleration of 1.60 m/s2, directed down. The blocks remain at rest with respect to the elevator.

    (d) Calculate the magnitude of the tension in the rope now.


    (e) Calculate the magnitude of the force exerted on block A by the floor of the elevator now.

    I'm having difficulty solving for remaining portions of this problem. Please provide guidelines and an explanation for how I can go about answering the different parts of this question; I would really like to understand.

    Thank you!
     
  2. jcsd
  3. Oct 23, 2017 #2
    Something to consider:
    For a, b and c, the elevator is moving at constant velocity, it is not accelerating. Therefore, according to Newton's second law of motion, the movement of the elevator is irrelevant. It is a distraction. After that the rest of the problem simplifies to Newtons third law.

    For d and e the elevator is falling, exerting a downward force on the floor and therefore an equal but opposite upward force on the blocks lessening the tension by the same amount.
     
  4. Oct 23, 2017 #3

    haruspex

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    Nonetheless, you need to post some attempt and your thinking so far.
     
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