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Block on Wall-Does it Fall?

  1. Oct 18, 2006 #1
    Block on Wall--Does it Fall?

    A 15 N horizontal force F pushes a block weighing 3.0 N against a vertical wall (Figure 6-25). The coefficient of static friction between the wall and the block is 0.71, and the coefficient of kinetic friction is 0.51. Assume that the block is not moving initially.
    (a) Will the block move?
    (b) In unit-vector notation, what is the force exerted on the block by the wall?

    I drew a diagram with a block against a wall. F acts on the block and points toward the wall. N also acts on the block and points away from the wall. mg acts on the block and points down; friction acts on the block and points up.
    I also said that N=-F

    The block will move if mg>static friction
    ==> mg>(mu_s)(N)
    ==> mg>(mu_s)(F)
    ==> 3.0>(0.71)(15)
    ==> 3>10.65
    since mg<static friction, the block will not move.

    But for part (b), when I said that N=-15i+0j, WebAssign said that the y-component was wrong. is friction a force exerted on the block by the wall? But WebAssign won't accept 10.7 as the y-component, either...
  2. jcsd
  3. Oct 18, 2006 #2


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    It should accept 3 as the y -component. The wall exerts both a normal and a friction force on the block. At points of contact, you must consider forces in all directions . The 15N normal force acting left. The friction force acting up. But the friction force is not 10.7. It can only be equal to the blocks weight, from equilibrium considerations in the y direction. Remember, the static friction force is less than or equal to
    (mu_s)(N). It is only equal to (mu_s)(N) just at that instant the block starts to move.
  4. Oct 18, 2006 #3
    Thank you!
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