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Homework Help: Ladder leaning against a wall

  1. Mar 20, 2008 #1
    1. A ladder leans up against a wall. If the ladder is not to slip, which one of the following must be true?
    a.friction between the ladder and wall must not be zero
    b.friction between the ladder and floor must not be zero
    c. both a and b
    d. either a orb
    e. neither a or b

    2. net force =0, net torque = 0

    3. i think it should be b. Can someone concur or correct me?
  2. jcsd
  3. Mar 20, 2008 #2
    i beleive you are correct and the answer is b. the ladder and floor must have friction or the ladder falls.

    as for a the ladder is perfect in ballance and no forces are acting upon it the ladder would not slip and would stay up.
  4. Mar 20, 2008 #3
    I think it would be d. I mean, if you let the friction coefficient at the wall approach infinity, mustnt it be enough to stop the ladder from sliding down, no matter the mass on the ladder or the leaning angle?
  5. Mar 20, 2008 #4
    I dont think it can be d, since the ladder is not weightless, the weight applied on the frictionless floor will cause the ladder to come off from the wall and onto the floor.
  6. Mar 20, 2008 #5

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    The frictional force at the wall is there because there is normal reaction N at the point of contact. If the frictional force at the floor is zero, then there is nothing to balance the N at the wall, and ladder would be pushed away from the wall just enough to make N zero, and then it would slide on the floor, as mentioned by DBXO at the bottom. So, there has to be frictional force at the floor.

    The correct answer would be (b).

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