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Homework Help: Work and coefficient of friction

  1. Apr 29, 2008 #1
    1. The problem statement, all variables and given/known data

    This is in my work unit. Can someone lead me in the direction of finding the value of the coefficient of friction? What we have is a box being pushed up an incline plane. There is an applied force parallel to the horizontal as well as the force of gravity given. The dimensions of the plane are given but nothing else. How can I calculate the value of the coefficient of friction?
  2. jcsd
  3. Apr 29, 2008 #2
    What you've given is not enough to solve the problem. Is the box accelerating up the plane? Is it moving up with constant speed? Give the problem statement verbatim, and post any attempts to solve the problem you've done.
  4. Apr 29, 2008 #3
    A worker pushes a crate weighing 93N up an inclined plane, pushing horizontally parallel to the ground. It then has a digram giving the dimensions of the inclined plane so that an angle can be calculated and work can too.

    c) THe coefficient of friction is .2. How much work is done by friction? (be careful of signs)

    My teacher says that there is something wrong with this question (ie. there is an error). The only thing I can see possibly wrong is that the coefficient is wrong.
  5. Apr 30, 2008 #4
    Perhaps my teacher is wrong and there is nothing wrong with this question.
  6. May 1, 2008 #5
    You really haven't provided enough of the question for people to help. I'm learning this stuff right now, too, and could provide things from my text, except it's not super clear what the question is, what information you've been given, etc. You are supposed to follow the basic setup provided when you ask your question--the full question, the equations involved and your attempt. People will have a hard time helping you if you only do part of it.
  7. May 1, 2008 #6
    I have already stated previously what are givens in the question. There is a box being pushed up an inclined plane by an applied force that is parallel to the horizontal. We are also given the box's mass and the dimensions of the triangle.
  8. May 1, 2008 #7
    What is the acceleration of the box? You need to know that before you can find the coefficient of friction.
  9. May 1, 2008 #8
    It doesn't tell you the acceleration. My teacher challenges that you can still find the coefficient of friction.

    Knowns: applied force; dimensions of triangle; force of gravity component
  10. May 1, 2008 #9
    Wait, now I'm confused. What exactly are you trying to find?
  11. May 1, 2008 #10
    Read the entire thread. This question is out of the textbook, but my teacher says that there is an error with the coefficient of friction (which the book claims to be .2). It is obvious that there is an error with the coefficient of friction, but I just don't know how to calculate it. The reason I know that there is an error with it is because when I calculated the coefficient if the system were to be in equilibrium it was .16. As a increases, the coefficient decreases, so it would be impossible for it to be .2.
  12. May 1, 2008 #11
    If it were in equilibrium, then all forces would be equal, so there would be no acceleration. So you have to make the assumption that [tex]v_{i} = v_{f}[/tex]
    [tex]KE_{i} + PE_{i} + F_{fric} \cdot d = KE_{f} + PE_{f} [/tex]
    Reduces to
    [tex]PE_{i} + F_{fric} \cdot d = PE_{f} [/tex]
    Taking the bottom of the ramp to be at 0 potential, you finally get
    [tex]F_{fric} \cdot d = PE_{f}[/tex]
  13. May 2, 2008 #12
    I can't see what's wrong with the question where they ask for the amount of work done.
    If the coefficent of friction is bigger than .16 the box won't slide down on it's own, but you can still push it upwards. To calculate the amount of work done by friction you don't need to know how fast the box is pushed upwards, just the length and angle of the slope. (which dranseth claims were given, altough he didn't give them to us)
  14. May 2, 2008 #13
    If you calculate the coefficient of friction that is when it will be its biggest. When you do this, the answer is .16. Therefore, .2 is wrong... It is impossible for it to be .2
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