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Homework Help: Block on a slope, friction related

  1. Jan 16, 2013 #1
    A 300kg block is sitting on a slope of 20 degrees, the coeff of friction is 0.75, if the block is connected to a mass hanging vertically via a pulley, what is the mass of the hanging block such that it will overcome friction but not move the block

    F = mgsin20 for the force acting down the slope
    F = mgcos20 for the force acting perpendicular to the slope

    Have attached a PDF of my attempt, but basically I found the force acting down the slope and then used this as the force in my next equation to calculate the mass,

    Getting a result of 384kg, I now realise I havent properly taken friction into account and im unsure of how to do this.

    Attached Files:

  2. jcsd
  3. Jan 16, 2013 #2
    In your attempt you've added the two vector components together as if they are scalar quantities ie 1006 + 2765 = 3771. I suggest you draw a free body force diagram for the 300kg block. Are we to assume it is on the point moving upwards?
  4. Jan 16, 2013 #3
    Indeed, got another solution ill upload in a minute
  5. Jan 16, 2013 #4
    For some reason the scanner isnt working, but basically i found the force required to in effect keep the block on the slope still, using

    F = mgsin20 + umgcos20

    and from this force i used m=F/g to get the mass of the block, 314kg.... this seem right ?
  6. Jan 16, 2013 #5
    Could someone maybe run through this and tell me if they get a matching 314kg for the other block? thanks
  7. Jan 16, 2013 #6
    Have uploaded a new file with my method, anyone comment please?

    Attached Files:

  8. Jan 16, 2013 #7
    Well...you should draw all the forces and components acting on the block and also show clearly your reasoning. Otherwise your calculations are correct.:smile:
    Last edited: Jan 16, 2013
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