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How do I determine Force or time from an impact?

  1. Dec 6, 2011 #1
    Here's my situation...

    I have a loader dumping rock onto a proposed screening rack to separate stone sizes.
    The typical load is estimated at 4100kg and falls about 1m above the screen.

    My velocity at impact works out to about 4.4m/s just before impact (determined by SQRT(2 x 9.81m/s^2 x Height of fall).

    From this my impulse works out to (mass x change in velocity) 18,040 kg-m/s

    How do I determine the Impact Force? I don't know the deflection, or the change in time to bring the load to rest - these are both unkown.

    Any pointers?
  2. jcsd
  3. Dec 7, 2011 #2
    If this is a practical problem and you need an estimate I would make a reasonable guess for the time of the collision... 0.1 sec,? 0.5sec? 1sec?
    Then force = impulse/time
  4. Dec 7, 2011 #3
    It is a practical problem that I'm trying to solve.

    I thought that there was a way to integrate something (impulse or momentum?) to determine the time. It's been years since I've done this sort of calculation and it has escaped me.

    Are you sure that it can't be done without estimating a time?
  5. Dec 7, 2011 #4
    No, you will need to estimate the time since it depends on the elastic properties of the screen and the rocks, the taughtness of the screen, how the rocks interact with each other once they hit the screen etc...
  6. Dec 7, 2011 #5
    Huh... then there's no real point of trying to calculate it - is there?

    For every tenth, hundredth, thousandth, etc... of a second that I'm off - it significantly affects the resulting Force by the inverse magnitude - which becomes quite large.
  7. Dec 8, 2011 #6
    While you do need to make a time estimate, you might be able to determine some reasonable bounds....

    I think another real issue, even more difficult, is that all the rock (pieces) does not hit the screen at the same time...nor does it likely hit in a uniform pattern....so in practical terms you don't know the impulse either...
  8. Dec 8, 2011 #7
    That's true... but at least the impulse can be calculated for a worst case scenario (all at once).

    I might be able to perform an iterative calculation by assuming an initial structural design, then determining the deflection based on MAX yield stress, and use that deflection as the design parameter for the first problem. The idea works in my head, not sure on paper. What do you think?
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