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Application of Newton's Laws

  1. Dec 3, 2005 #1
    Hey, new member here, and I'm stumped on a physics problem.
    Two weights are tied onto each end of a piece of rope and set on a table. The rope has a scale on it. The two weights are hanging over the edge of the table - one has Fg of 100 N, the other has Fg of 50 N. What does the scale read?
    My physics teacher presented this problem the other day, and I've been obsessed with it ever since... but I don't quite know how to achieve an answer. He gave us the hints that the answer is not 50, 75, or 100, but it's still in that range (50 < x < 100). If anybody could just put me on the right track to solving this problem, I'd be much obliged.
     
  2. jcsd
  3. Dec 3, 2005 #2

    lightgrav

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    If the system is symmetric, except for the amount of weight on each side,
    my hint would be that there are two forces which keep the system from accelerating sideways. The two forces are probably not equal ... .
     
  4. Dec 3, 2005 #3
    I'm not sure what you mean by the rope having a scale "on it."
     
  5. Dec 3, 2005 #4
    Last edited: Dec 3, 2005
  6. Dec 3, 2005 #5

    lightgrav

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    NumberLover,

    Your diagram shows 100 N applied upward to the bottom of the left-hand piece of metal ... WHAT?

    You're not likely to answer interesting Force questions with careless diagrams.

    The whole thing WOULD accelerate leftward, not rightward ...
    except for 2 important Forces which keep the whole thing still ...

    Unless this is an unrealistic problem where it DOES accelerate leftward .
     
  7. Dec 3, 2005 #6
    I'm sure those arrows are only pointing towards the blocks giving their weight. The diagram was to show the general layout and I doubt was intented to be a FBD.
     
    Last edited: Dec 3, 2005
  8. Dec 3, 2005 #7

    lightgrav

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    The point is that a neat diagram of applied Forces is the FIRST thing to do,
    (while you're still reading the question) with those Forces labeled by source.

    (Setting the Sum of Forces equal to ma is next.)
     
  9. Dec 3, 2005 #8
    Yeah that was it, but I'll be more careful when doing drawings in the future. The weights are hanging off the table, so I believe the only Force affecting each individual weight would be gravity, yes?
     
  10. Dec 3, 2005 #9

    lightgrav

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    Doesn't the string contact each cylinder?

    Does the string contact the table?

    Aren't there any upward Forces?

    Does the whole thing accelerate leftward?
     
  11. Dec 3, 2005 #10
    The ends of the string each have a weight attached to them, the string is on the table. I don't believe there are any upward Forces, and based on the information given, the whole thing will probably accelerate to the left.
     
  12. Dec 3, 2005 #11
    I'm not fully comprehending how acceleration can aid in solving this problem, though my teacher hinted that it might.
     
  13. Dec 3, 2005 #12
    If there is not other forces like friction, the rope will accelerate and keep moving to the heavier side. And ultimately will hit the ground. That's it.
     
  14. Dec 3, 2005 #13
    But the question is asking what the scale reads at this moment in time.

    (I bet that wasn't too clear either - I apologize. I've been working probabilities of poker in my head all night, so I'm a little exhausted.)
     
  15. Dec 3, 2005 #14
    OK, wait, I think I've got it now. Thanks.
     
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