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Homework Help: Elevator Help

  1. Sep 29, 2005 #1
    Im having trouble with a part of a problem. The problem is:

    There is an elevator with a mass of 1200[tex]kg[/tex] accelrating upward at 2.1[tex]m/s^2[/tex]. Find T.

    For that what I did was [tex]1200 \times 9.8m/s^2[/tex] which gave me 11,760. I then plugged this into [tex]F=ma[/tex] and got 14,280.

    The second part of the equation says:

    The elevator now moves with a constant upward velocity of 10[tex]m/s[/tex]. Find T.

    This is where I get stuck. I dont know how to factor in velocity.
    Last edited: Sep 29, 2005
  2. jcsd
  3. Sep 29, 2005 #2
    what you've done is plug mg = ma, that isnt right
    the gravity is not pulling the elevator down, the elevator is moving upward
    that means thhere is some force being exerted by the tension in the cable

    taking the upward direction to be postiive
    T - mg = ma = Net Force
    T = ma + mg

    When there is constnat velocity what is the accleration? So what is the tension using the above equation?
  4. Sep 29, 2005 #3
    Velocity doesn't affect the force. What you want to think about is acceleration. If it's at a constant velocity, what does that tell you about the net force?
  5. Sep 29, 2005 #4
    my mistake, your answer is correct.. its jsut the way you said it sounded like you did something wrong. The first part is correct, follow the advice of finding the accleration in second part to figure out the tension
  6. Sep 29, 2005 #5


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    Here is the approach I would take:

    1. Construct free body diagram of the elevator. Cable tension T is up and elevator weight mg is down

    2. Write the equation of motion [itex]\Sigma F = m a[/itex]

    3. Case 1: a = fixed value

    4. Case 2: if velocity is constant, what can you say about acceleration?
    Last edited: Sep 29, 2005
  7. Sep 29, 2005 #6
    ok so if there is a constant velocity there is no acceleration? Which would just make my answer 11760?
  8. Sep 29, 2005 #7

    Doc Al

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    Staff: Mentor

    I assume that T is the tension in the cable that pulls the elevator up.

    To apply F = ma properly, you need to use the net force on the elevator. There are two forces acting on the elevator: The cable tension pulling up and the weight (mg) pulling down. Find the net upward force (in symbols) and set that equal to ma. Then you can solve for T.

    Hint: If the elevator moves at a constant velocity, what must be the net force on it?

    Looks like several others beat me to it! :smile:
    Last edited: Sep 29, 2005
  9. Sep 29, 2005 #8


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    As long as your units are consistent, I'd say yes.
  10. Sep 29, 2005 #9
    Yea the answer was 11760. Thanks for the help.
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