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Homework Help: Newton's Laws with Friction

  1. Sep 28, 2008 #1
    I have two separate problems that I think require essentially the same equations and thought process. So if I can get one I should be able to get the other:

    First problem
    1. The problem statement, all variables and given/known data
    A bat crashes into the vertical front of an accelerating subway train. If the coefficient of friction between bat and train is 0.89, what is the minimum acceleration of the train that will allow the bat to remain in place?

    2. Relevant equations
    kinematics equations

    3. The attempt at a solution
    No idea how to even start this one.

    Second problem
    1. The problem statement, all variables and given/known data
    At the end of a factory production line, boxes start from rest and slide down a 30 degree ramp 5.7 m long. If the slide is to take no more than 3.5 s, what is the maximum allowed frictional coefficient?

    2. Relevant equations
    same as first problem, i think?

    3. The attempt at a solution
    First I thought that I need to find the acceleration of the box over 5.7 meters in 3.5 seconds. I got about .931 m/s^2. I'm not sure where to go from there. One big question I have is how does the 30 degree angle play into the question?

  2. jcsd
  3. Sep 28, 2008 #2
    In both of these situations, it would probably be helpful to draw a free body diagram for the bat in #1 and for the box in #2. Then use the equations that you have identified.

    Also, You will find it helpful to set up and manipulate your equations with variables first before trying to substitue in any values. You will discover some things you think you need to know will divide out of the equations...
  4. Sep 28, 2008 #3
    Thanks for the reply. I drew the free body diagrams but I'm still not sure what to do and how to manipulate newton's law.
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