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Two questions that I don't really don't understand

  1. Oct 25, 2013 #1
    1. The problem statement, all variables and given/known data
    Two 5-N boxes are attached to opposite ends of a spring scale and suspended from pulleys as shown. In my drawing, the xxxxx represents the scale in the midde.

    ...|o\-------xxxxxx-------/o|
    ...|...\........................./...|
    ...|....\______________/ ....|
    ...|....|_____________|.....|
    [5N]...............................[5N]

    What is the reading on the scale?
    a) 0N
    b) 2.5N
    c) 5N
    d) 10N
    e) 25N

    Relevant equations
    F = Ma

    The attempt at a solution
    Well since there is 5N on both sides I would expect that the reading on the scale is zero because i would assume that it stays at equilibrium. Apparently I'm wrong on that so any hints would be nice

    2. The problem statement, all variables and given/known data
    A 4kg block is connected by means of a massless rope to a 2-kg block as shown in the figure. Complete the following statement: If the 4kg block is to begin sliding, the coefficient of static friction between the 4kg block and the surface must be

    .............[....].-------oo
    ________[4kg]_____/...|
    |..........................|...|
    |________________|..[2kg]

    a) less than zero
    b) greater than 2
    c) greater than 1, but less than 2
    d) greater than 0.5, but less than 1
    c) less than 0.5, but greater than zero

    Relevant equations
    F = Ma
    Fs = uN

    The attempt at a solution
    Since both blocks are connected I would assume that they accelerate together at the same rate
    So f = (4kg + 2kg)a
    F = 6a

    if the 4kg block begins sliding then the system is achieving an acceleration that would overcome the force of static friction, but Fsmax would equal uN
    Fs = uN
    Fs = u(mg)
    Fs = u(6x9.8)
    Fsmax = u58.8

    and now here i'm stuck and i don't know what to do
     
  2. jcsd
  3. Oct 25, 2013 #2

    CWatters

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    Science Advisor
    Homework Helper

    The strings are connected to each end of the spring in the scale. The spring isn't accelerating so the forces acting on it must sum to zero (f=m*a but a=0) BUT that doesn't mean they can't stretch the spring.
     
  4. Oct 25, 2013 #3

    CWatters

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    Science Advisor
    Homework Helper

    For question 2...

    Forget what happens after it starts moving. The question asks about the coefficient of static friction.

    For the 4kg block to move the force produced by the 2kg block must overcome Fsmax the max force due to friction.
     
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