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Trying to find a normal force

  1. Feb 21, 2006 #1
    Problem: You want to nail a 1.1 kg board onto the wall of a barn. To position the board before nailing, you push it against the wall with a horizontal force F to keep it from sliding to the ground. If the coefficient of static friction between the board and the wall is 0.71, what is the least force you can apply and still hold the board in place?

    Thoughts: fsmax= coeffcient of friction*N.

    I'm having trouble finding N because I'm used to it being parallel to W and in this case it's perpendicular. Without knowing the F the han dis exerting on the board I'm not sure how to go about finding N.

    What I have tried is simply doing N as mass*gravity to get 10.79 then multiplying by the .71 but the online homework system is saying that answer is wrong.

    Obviously there's something I'm not understanding correctly. Any help would be appreciated.
  2. jcsd
  3. Feb 21, 2006 #2
    Because the wall is vertical, so if you act on the board a horizontal force[tex]\vec{F}[/tex], the wall also act on it a normal force [tex]\vec{N}[/tex] and [tex]|\vec{N}|=|\vec{F}|[/tex].
    To keep the board form falling, the friction force must satisfy the condition
    [tex]F_{fr}\geq P[/tex]
  4. Feb 21, 2006 #3
    How can I find the force if the object doesnt have any acceleration?
  5. Feb 21, 2006 #4
    Newton's second law maybe?
  6. Feb 21, 2006 #5
    Just using my equation, you can find the answer. Try...
  7. Feb 22, 2006 #6
    You need to use Newton's second in two directions. In the horizontal direction we have two forces...the force on the nail by you (holding it) and the normal force on the nail from the wall. In the vertical direction we have the weight of the nail acting downward. In order for the nail to be stationary, there has to be a force acting upward...that's your static friction force. So set your net force component in each direction to zero and solve the system.

  8. Feb 22, 2006 #7


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    Staff Emeritus
    Science Advisor

    The gravitational force on any object is its weight.

    What is the weight of a 1.1 kg object? (No, it's NOT "1.1 kg"- that's mass, not weight!)
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