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Homework Help: Is this answer rigth

  1. Nov 21, 2008 #1
    The tension on a string from which?
    The tension on a string from which a 4.0 kg object is suspended in an elevator is equal to 44.0 Newton.

    What is the acceleration of the elevator?

    Is this answer rigth

    44 N - (4.0 kg X 9.8) = a (4.5 + 4) I think the 4.5 is mass of elevator and 4 the mass of the object which is suspended.

    Acceleration = 0.57 m/s^2
  2. jcsd
  3. Nov 21, 2008 #2


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    Staff: Mentor

    The mass of the elevator is not important. You are starting in the right direction. (4kg X 9/8 m/s^2) is the normal weight that would produce tension in the string if there were no vertical acceleration. The difference between this tension and the actual tension in the accelerating elevator is due to the elevator's acceleration. Please try writing the equation(s) again, being more careful to separate out the parts...
  4. Nov 21, 2008 #3
    No, the left side of the equation is right, however, the right side is not. There is no way the mass of the elevator is 4.5, but regardless, that is irrelevant. For this question, you do not need to know the mass of the elevator, just the object suspended from the string.
  5. Nov 21, 2008 #4
    I can understand that
    44 N - (4.0 kg X 9.8) = a X4
    a= 1.2 m/s^2
    is it ok now
  6. Nov 21, 2008 #5


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    Staff: Mentor

    So tell us in your own words why you think that is the right answer.
  7. Nov 21, 2008 #6
    I think that was not right. because I am asked to find the elevator's acceleration.

    Mass of elevator is 44N / 9.8 = 4.5

    44 N - (4.0 kg X 9.8) = a X 4.5
    a = -1.1 m/s^2
  8. Nov 21, 2008 #7
    You were right before, now it just seems you are becoming more confused. In this case, when the elevator accelerates, that which is in it, accelerates too.
  9. Nov 21, 2008 #8
    I am really confused.

    44-(4X9.8)= 4 X a
    a= 1.2 m/s^2 now I think that right because as you said the elevator has acceleration as the spring.
  10. Nov 21, 2008 #9
    Yes, that is right, but do you understand why?
  11. Nov 21, 2008 #10
    Yes because the acceleration of the spring should be the same with the elevator.

    is the direction right that the acceleration is +1.2 m/s^2 that means that the elevator accelerates up?
  12. Nov 23, 2008 #11


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    Science Advisor
    Homework Helper
    Gold Member

    Acceleration is always in the direction of the net force.
    If the net force acts up on the object, then the object is accelerating up.
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