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static friction question

Discussion in 'MCAT Study Question Q&A' started by TypeSH07, May 16, 2008.

  1. TypeSH07

    TypeSH07 Junior Member
    7+ Year Member

    Mar 20, 2004
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    When a question asks how much force is needed to get an object moving is the answer "at least the value of the static force" or would it be greater than the static force? I thought it would be the latter, but some example problems seem to indicate otherwise. I thought if the force exerted equaled the maximum static there would no net force so there would be no movement.

    An example of this is 2.12a in the Berk Review physics book. Their answer choice is at least 12 for a 15lb pack with coefficient for static friction being .8. I would've thought it would be some value greater than 12. Thanks.
  2. RySerr21

    RySerr21 i aint kinda hot Im sauna
    10+ Year Member

    Dec 22, 2007
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    Fellow [Any Field]
    common sense tells me to agree with what you are saying. that in order for the block to move it would have to be greater than the static force. but i dunno man, this is why i hate physics.
  3. MilkmanAl

    MilkmanAl Al the Ass Mod
    Physician 7+ Year Member

    Mar 23, 2008
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    Resident [Any Field]
    You're correct. You would need a force greater than the force of friction to move the object. And don't hate physics. Love the physics!
  4. physics junkie

    5+ Year Member

    Nov 20, 2006
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    I can't really say for sure if the following reasoning is correct but if the berkeley review prep solution is correct then I think I can explain it. What the question is testing is if you know that the coefficient of static friction is always greater than the coefficient of kinetic friction. If you apply infinitesimally more force than the max. force due to static friction then the constant you want to compute the friction changes and the box will start moving. Because the coefficient changes abruptly at the boundary so does the amount of force that goes towards accelerating the pack. For example, assume the coeff. of kinetic friction is 0.6 in the problem above. Then, let's say you applied 12.00001 ft*lb of force you would end up with (0.8-0.6)12.00001 ft*lb of force going to accelerating the box.

    What happens at the boundary value is kind of vague and you would probably need some more advanced physics to describe it.
  5. BerkReviewTeach

    BerkReviewTeach Company Rep & Bad Singer
    Vendor 10+ Year Member

    May 25, 2007
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    The point of this question is not only the topic, but also test logic. It's one of those best, but not perfect examples.

    The solution in the book states, "If she is to move the backpack, then the force she uses must be greater than the largest possible value of the static friction." So you are correct in your physics. The mathematical solution also shows that same thing, that Fapplied > 12lbs. BUT, the answer choices are as follows:

    • A) Less than 12 lbs.
      B) At least 12 lbs.
      C) At least 15 lbs.
      D) More than 15 lbs.

    So here is where you have to shift into test-taking mode. The absolute correct answer is not a choice. So, you have to choose the best answer. Choice B is the best of the four choices.

    I'm not sure which version you have, because in my older one, the answer stops there. In my newer one, there is the following line following the question:

    "Often on the MCAT, you have to choose the best answer. Given that the answer must be slightly greater than 12 lbs., the closest answer is at least 12 lbs. The force cannot go much beyond 12 lbs., because once the backpack breaks free, the resistive force will decrease, due to reduced friction. Static friction exceeds kinetic friction, so choices C and D cannot be correct. Choice A is too small, so only choice B remains."
  6. WinterLights

    Sep 15, 2007
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    What are the units for the force? 12 apples? 12 pylons? In any case, I am pretty sure that you need a force greater than the maximum static frictional force in order to move the object.

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