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Homework Help: Momentum to overcome inertia

  1. Apr 20, 2010 #1
    i need help getting started on a fluid mechanics problem, i am not looking for a worked solution just a nudge to get me started on the problem, i am rather stumped.

    i need to determine the volume flow rate Q required to overcome the inertia of a homogenous rectangular block with weight N.

    the block of d(x), w(y), h(z) stands at rest on the x-y plane, the fluid acts on the centroid of the block.

    - negligable distance from nozzle to block
    - steady flow
    - incompressible fluid

    the part of my memory that is trying to make it's way to the surface tells me to quantify the inertia(?) of the homogenous block at rest and calculate the amount of momentum required to tip the block from it's resting position.

    based on other problems i have completed i don't expect to have any problems calculating the required flow rate to produce such a momentum.

    it really seems as though i am missing a basic relationship/principle to get me started on the problem.

    Attached Files:

    Last edited: Apr 21, 2010
  2. jcsd
  3. Apr 20, 2010 #2
    Does this problem come with a picture? Because at least for me I can't figure out what the problem looks like based on your description.
  4. Apr 21, 2010 #3
    added a graphic, it's a bit basic but the best i could do with 'paint'
  5. Apr 21, 2010 #4
    Are you giving us the entire problem? What does it say about friction? Are we to assume the water won't be able to overcome the static friction, and we need to calculate the required torque to tip it over?

    If we are to assume we need to tip it over, then you need to calculate the torque caused by gravity and the torque caused by the water flow about its axis of rotation. And the axis of rotation will be the corner edge of the box (sort of where you drew the 'w' in the figure). Then just equate them to find the minimum water flow needed to push it over.
  6. Apr 21, 2010 #5
    Yup, thats all the problem.

    i believe your assumption is correct, that the block will not slide along the x plane overcoming friction (no friction data is provided) rather pivoting on the outer edge of the x-y plane, as you said ' near the w '.

    i understand the equating of forces, i only want to find the minimum volume flow rate required to tip the block.

    so, to equate these forces, the problem requires me to find the torque required to push the block over (assume acting through the centroid) equivalent to the force of gravity acting through the centroid.

    thanks for the help 'talking' it out has really helped, (i'll be back if i get stuck ;) )