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How do i find the coefficient of static friction?

  1. Dec 5, 2017 #1
    1. The problem statement, all variables and given/known data
    A coffee cup in your dashboard slides forward when you decelerate from 20 m/s to rest in 3.5 s or less, but now if you decelerate for a longer time. What is the coefficient of static friction between your coffee cup and your dashboard?

    2. Relevant equations
    a = delta a / delta t, Ff = uFn (i think)

    3. The attempt at a solution
    I found out the maximum deceleration needed so that the cup doesn't move (a is < or = to -5.71) but i don't know how to find the coefficient of static friction
     
    Last edited: Dec 5, 2017
  2. jcsd
  3. Dec 5, 2017 #2

    Merlin3189

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    1 - I'm not sure why you introduce falling over. The question talks about sliding or not falling..
    2 - When you say "I found the minimum acc..." it would be helpful to show how you did that.
    3 - So, you calculated the acceleration for which the cup slides. Is the acceleration more or less, if you decelerate for a longer time?
    4 - Can you calculate the force on the cup needed to produce this acceleration?
     
  4. Dec 5, 2017 #3

    haruspex

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    It can be important to get this equation exactly right. In fact, there are two, one for static and one for kinetic. Note the difference:
    ##F_{kinetic}=\mu_kF_N##
    ##F_{static}\leq\mu_sF_N##
     
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