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Homework Help: Object against wall with horizontal force acting on wall

  1. Oct 7, 2009 #1
    1. The problem statement, all variables and given/known data

    A horizontal force F of 16 N pushes a block weighing 6.0 N against a vertical wall (Fig. 5-47). The coefficient of static friction between the wall and the block is 0.61 and the coefficient of kinetic friction is 0.41. Assume that the block is not moving initially.

    (b) In unit-vector notation, what is the force exerted on the block by the wall?

    ______i + ______j
    2. Relevant equations


    static friction formula

    3. The attempt at a solution

    i have no idea what to do. don't really understand this concept and its vectors. Please help.
  2. jcsd
  3. Oct 7, 2009 #2
    i and j are the unit vector so their magnitude is 1 therefore when you multiply a force by the unit vector you will get the direction the force is acted while the magnitude of the force is stil the same. For example you apply a force F in the x direction and we use i as the unit vector for the x axis so the force experience by the object will be Fi. This goes the same for the y axis and in your case the unit vector seems to be j. As for friction, always remember that frictional force=uN where u is the coefficient of friction and N is the normal force experience by the object.
  4. Oct 7, 2009 #3
    Sorry, I didnt make myslef clear, but I dont understand the components and how static friction keeps the block against the wall. I understand the vector units, but I dont understand the physics behind this problem. I know that static friction must equal the weight of the block and that the wall is pushing back on the block, which is the normal force.

    The answers I put in ( which is 16 for the i direction and 9.76 for the j direction) are not right.
  5. Oct 7, 2009 #4
    If the force pushing against the block is the normal force then how is weight of the block related to the friction? Remember frictional force is uN and the N need not be weight.
  6. Oct 7, 2009 #5
    I was told that the weight of the block is related to friction by the fact that mg is puling down and the friction between the wall and the block is pulling it up.

    I sort of understand what you said, but I still dont understand the physics behind the question.

    If the supplied horizontal force is 16 N, and it is not the normal force, then what is the normal force?
  7. Oct 7, 2009 #6
    Erm nope i din't say the force applied is not the normal force i am only saying weight need not be the normal force is all cases. I guess the reason why you are told that weight is related to friction is because you are looking at things when the object is on the ground and a force is applied to the horizontal direction. If you were to push a box across a distance x and you were to push the same box with someone sitting on it which case do you think more work is done? Since x is the same that would mean more force is required and why is more force required? Is it due to fricition? In that case wouldn't that mean uN is greater but u is same so its the N that changes. After going through this scenario we can conclude that friction does not depend on the weight but rather the normal force. :biggrin: Sounded abit long-winded haha
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