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Homework Help: Find minimum velocity?

  1. Mar 31, 2005 #1
    Help Please!!

    I have 2 questions that i need help with:

    1) A girl has a mass of 60 kg. She wishes to slide down a rope made of nylon stockings from a second story window. The stockings will break if the tension in then exceeds 400 newtons. How should she slide down the rope so that she lands safely at a minimum velocity?

    The next question I tried....but I'm not sure if i got the right answer.

    2) Andrew is on an elevator. Before it begins to move, he pulls a newton force scale out of his pocket and attaches it to the elevator ceiling. He then hangs his 10.0 kg backpack on the scale and notes a reading of 98 newtons. When the elevator finally moves the scale adjusts to read 93 newtons. Find acceleration.

    Here's my solution....tell me if its right :biggrin:

    Before the elevator moves, the force of gravity acting on the backpack is 98 N...so since there is no motion, the forces are balanced, so the force pulling upwards is also 98 N. When the elevator moves, the scale reads 93 N --> (98 - 5). Since the downward force lost 5 newtons...I added 5 newtons to the upward force...so 98 + 5 = 103 N...so if u take up as positive and down as neg. the net force would be 103 - 93 = 10. Since Fnet = ma...you know the mass is 10 kg...so you get 10 = 10a....solving for a you get 1.00 m/s squared. Is that right??
  2. jcsd
  3. Mar 31, 2005 #2
    Use Newton's Third Law: [tex]F_{net} = ma[/tex]

    In this case, the tension is the net force on the rope, and you know her mass. So solve for the acceleration.

    EDIT: This question asks for the minimum velocity that will land her safely... how fast is safely?
    Last edited: Mar 31, 2005
  4. Mar 31, 2005 #3
    I'm not sure what the minimum velocity would be...if it helps, there's someone at the bottom of the rope that's supposed to catch her. And the question just asks for the method that she sould used to slide down the rope.
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