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Newton's Laws: Sharp vs. Dull Knives

  1. Oct 13, 2008 #1
    1. The problem statement, all variables and given/known data
    Why does a sharp knife cut better than a dull knife? Explain using Newton's Laws.


    3. The attempt at a solution
    1st Law: Since the sharper knife has less surface area, it keeps more better than the dull knife. The friction is less.

    2nd Law: Less surface area on knife means more acceleration means more force.

    3rd Law: Less opposing force because of less surface area?

    Please correct me if I'm missing something. Thanks.
     
  2. jcsd
  3. Oct 14, 2008 #2
    First: List newton's laws and include them in yuor answer (chances are if this is graded, the grader wants to see these laws explicitly stated). What does the first law say? How does this relate to EACH knife, or what, if anything, does it have to say about the properties of the forces exerted by or on each knife? then move on to the second....

    In each part: always specify WHICH knife you are talking about, and specifically compare it to the other knife.... right now your answer is (especially in parts 2 and 3), to tell the truth, pretty unclear about which knife you are discussing, and then in part 1, you seem to be missing a word or two (it keeps more _____ better?).

    Also: I end to personally say the problem has less to do with friction (which is best to think of in terms on sliding ON a surface or resisting sliding, in the case of kinetic friction) and more to do with what might be better called "resistance" (like air resistance, except in this case, it a mass other than an air mass). Air resistance does tend to relate to area. Friction perhaps to your surprise does not relate to area.
     
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