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Horizontal force calculation

  1. Nov 3, 2016 #1
    1. The problem statement, all variables and given/known data

    "A stream of sand moving at 4 m/s is directed horizontally eat against a wall at a rate of 120 kg per minute. If the sand falls vertically downward after impact, find the horizontal force exerted by the wall on the sand, and the horizontal force exerted by the sand on the wall".

    2. Relevant equations

    ΣF = ma, W = mg,

    3. The attempt at a solution

    I'm really lost on how to solve this problem. You aren't given the acceleration or the time that it took, so I can't work out the mass. I'm not sure what I'm missing but any help is appreciated, thanks.
  2. jcsd
  3. Nov 3, 2016 #2


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    Staff: Mentor

    Hint: How are momentum and force related?
  4. Nov 3, 2016 #3


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    Science Advisor

    Different hint... Flow problems like this can be hard if you cannot bring an appropriate intuition to bear. Try making the continuous nature of the flow irrelevant.

    Imagine a long uniform stream containing exactly 120 kg of sand. Imagine that the stream is already in motion and the front end is just starting to impact the wall. If it helps your intuition along, imagine the grains of sand in the stream sliding across an icy surface at constant velocity. After one minute, the tail end of the stream hits the wall and the scenario is over.

    Can you determine the average force of the wall on this truncated stream during the 60 seconds it is impacting?
    Can you see that making the stream longer or shorter (and making the time interval match) will not change the answer?
    Can you see that an infinitely long continuous stream will result in the same force?
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