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Pulley type probelm

  1. Sep 13, 2016 #1
    1. The problem statement, all variables and given/known data

    2. Relevant equations
    Sum of Force
    g = 32.174 ft/ s^2

    3. The attempt at a solution
    I consider the right hand side weight,

    T = -mg = - 10*32.174 = 321.74 lb
    Why is the answer 3.33, which seems to be even smaller than the mass??
    Should I consider the momentum, or something with the 5 lb mass??

  2. jcsd
  3. Sep 13, 2016 #2


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    The tension is equal to the weight only if the velocity of the mass is constant. Here there is acceleration and you need to take that into account.
  4. Sep 13, 2016 #3


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    Also check your units. Is 10 lb x 32.174 ft/s2 really equal to 32.174 lb?
  5. Sep 13, 2016 #4
    I see,
    I see (not familiar with US Unit) my thought was T = mg , so if it is metric unit T (N) = m (kg) * g (m/s^2)
    so in this case maybe T (lb*ft/s)

    And how do you find the acceleration? thanks
  6. Sep 13, 2016 #5


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    First the units. When you see a number with lb. next to it, that means weight which is a force. To find the mass (the unit is slugs) you divide the weight in lb. by 32.174 ft/s2. The number you get should be used wherever "m" appears. If g appears next to m, as in mg, you just use the number for weight in lb.

    Now for the acceleration. Newton's Second law says that the um of all the forces is equal to mass times acceleration.
    1. What is the sum of all the forces acting on the mass on the table?
    2. What is the sum of all the forces on the hanging mass?

    Once you have the answer to these two questions, you can set each equal to the appropriate mass times the acceleration.
  7. Sep 13, 2016 #6

    jack action

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    About the US units:

    'lb' is already a unit of force. In the US system, we measure a weight in 'lb' and in the SI system we measure a mass in 'kg'. And - of course - weight = mass X g, in either system. The weight in the SI System has the basic unit of Newton and the basic unit for mass in the US system is the slug.

    By definition, 1 lbf = 1 slug.ft/s² (note subscript 'f' for 'pound-force'). So the correct way to use ##W=mg## with your numbers is to find the mass in slug which is 10 lbf / (32.2 ft/s²) = 0.31056 slug.

    By definition, 1 lbf = 32.2 lbm.ft/s² (note subscript 'm' for 'pound-mass'). This means that 1 slug = 32.2 lbm, such that 0.31056 slug X 32.2 lbm/slug = 10 lbm. So you can see that 10 lbf is equivalent to 10 lbm under the earth acceleration. This way, there is no need to do a conversion (at least numerically).
  8. Sep 13, 2016 #7
    1. x direction: Tension y direction: Normal force, and Weight
    2. x direction: none y direction: Tension and weight

    So I set F_x1 = F_x2?
    and same for y dir?
  9. Sep 13, 2016 #8


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    Consider the mass on the table first. There is only one force in the x-direction which means that the net force in the x-direction is just T. So, according to Newton's Second law, in the horizontal direction
    1. T = ??
    In the vertical direction, the mass just slides on the table without jumping off it or sinking into it. What does that make the vertical acceleration equal to?
    2. avertical= ??

    Now for the hanging mass. Nothing is going on in the horizontal direction. What is the net force in the vertical direction?
    3. Fnet,y= ??

    Finally, set the net force in each direction equal to the (appropriate) mass times acceleration in the same direction. You also need to consider whether the two masses have the same acceleration and why or why not.
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