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Homework Help: Dynamics: Newtons laws of motion

  1. Oct 30, 2008 #1
    1. The problem statement, all variables and given/known data
    73) A cyclist is coasting at a steady speed of 12m/s but enters a muddy stretch where the effective coefficient of friction is 0.60. Will the cyclist emerge from the muddy stretch without having to pedal if the mud lasts for 11 meters?. If so, what will be the speed upon emerging.

    I have a test tomorrow and this was a question at the end of the chapter that I'm having trouble on. I have no idea how to go about this. Ive showed it to at least 5 other classmates who are all stumped. The answer in the back is "Yes, 3.8m/s"

    How would i go about arriving at that answer. Ive tried applying every formula in the chapter without success.

    2. Relevant equations
    Dont even know where to go with that in this equation

    3. The attempt at a solution
    Me and 4 other students spend a half hour yesterday trying this problem out without any success.
  2. jcsd
  3. Oct 30, 2008 #2
    It would be helpful to try doing a 'before' and 'after' sketch including eveything you already know about the problem (except for the answer!).

    Then talk us through what's going on (in words rather than formulae) so we can see what kind of thought processes you're using.

    Once you've worked out what you need to do (by talking it through) you can start thinking about how to do it - which formulae to use.
  4. Oct 30, 2008 #3
    Well he his initial velocity before he hits the mud is 12m/s. So i tried figuring out the force he would be driving into the mud with. And from there I was going to calculate the effect of the coeffecient from his normal force. But since his mass wasnt given Im completely clueless. I have absluteley no clue where to go from here.
  5. Oct 30, 2008 #4
    OK... though probably better of thinking about that as the force that the mud exerts on the cyclist, as it's his motion you're interested in.

    The method is spot on. Don't worry about not knowing the cyclist's mass - just call it m for the moment. When you're dealing with forces, the mass of the object often divides out once you set up your equations. So have a go at what you've suggested and see what happens.
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