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Homework Help: Friction questions

  1. Jan 3, 2008 #1
    I am having trouble with two concept questions.

    We did a lab measuring static and dynamic friction for cardboard, rubber, sandpaper, and cork with a wood block.
    Calculations show that the coefficient of static friction is always greater that that of kinetic friction.

    The questions are:
    a) How do the coeffients of static and kinetic frictions compare? Explain.
    My answer was that the coeff. of static friction is always greater, reason being that it takes a greater force to get an object to start moving from rest (acceleration), which is when static f. was measured, versus just keeping an object moving (a = 0).

    b) Do "smoother" surfaces always have less friction than "rougher" ones?
    Ans: No, because it depends on the other surface they are interacting with??

    I have tried to answer them, but if someone could just check and point out what is wrong I would appreciate it. I think i got the first question right but i'm pretty sure the second one is wrong.I really don't know how to answer b.
    What are the concepts behind the second one? Because i truly don't even know where to start with that.
  2. jcsd
  3. Jan 3, 2008 #2


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    Homework Helper

    You must have learned from the lab, right? A rough surface can have a smaller coefficient of friction than a smooth one. The ultimate source of friction is adhesion between groups of molecules and a smooth surface can put a lot more molecules in close proximity than a rough one. There are, of course, exceptions to every rule, but to say ALWAYS is just plain wrong. Your answer is right.
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