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Homework Help: Newton's law related question...

  1. Nov 3, 2016 #1
    1. The problem statement, all variables and given/known data

    A man weighing 980 N slides down a rope that can support a weight of only 755 N.

    a) How is this possible?
    b) What is the least acceleration he can have without breaking the rope?
    c) What will his minimum speed be after sliding down 8.0 m?

    2. Relevant equations
    Not specified, but I think it's ΣF = ma, and W = mg.

    3. The attempt at a solution
    For a) I detail here that because of the speed at which the person is moving, he weighs less than he would if he were stationary, is this correct?

    For b) For this, I used "W = mg". I figured that the gravity is known and so is his weight, so I transposed this and found his mass to be 100 kg. (980/9.8). So now that I had the mass of the person, I put into the equation "W = mg", that the weight must = 755 and the mass = 100 kg and I got that -7.55 m/s2 was the speed, is this correct?

    For c) I wasn't sure how to solve this question, could someone help me out?

    Thank you.
  2. jcsd
  3. Nov 3, 2016 #2
    Have you drawn a free body diagram of the man, showing the forces acting on him?
  4. Nov 3, 2016 #3
    I attempted to draw one, but I wasn't sure how to do so or how that would help in this case.
  5. Nov 3, 2016 #4
    Let's see what you drew. The first step in any mechanics problem should be to draw a free body disgram.
  6. Nov 3, 2016 #5


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    Homework Helper
    Gold Member

    For a) any person or body has the same weight regardless of the speed or acceleration that it has, only in speeds close to the speed of light the weight changes but this is relativity theory and I am sure the problem doesn't want us to go into relativity theory

    What you trying to say in a) is that because of the acceleration (and not because of the speed) that the person has, the force of tension from the rope to the person, is less than 980N (more specifically it has to be less than 755N). We use newton's second law on the person with the two forces acting on it, the weight W=980N and the tension T<=755N from rope. We ll have ##W-T=ma## and ##T<=755N## its just math from now on to solve for ##a>=??##.
  7. Nov 3, 2016 #6
    @Delta2: It is contrary to Physics Forums rules and guidelines to reveal the complete solution to a problem, which is in essence what you have done here. I was hoping that the OP would be able to work all this out on his own, once he had been given some hints and been shown how to develop and properly apply a free body diagram. I think that would have be much more beneficial for his learning process than just being told how to do the problem. Please, in the future, refrain from revealing so much detailed information. Thanks.
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