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Friction - assistance needed

  1. Sep 17, 2005 #1
    Here's my first question:

    A 3.3 kg block is pushed along a horizontal floor by a force F of magnitude 20 N at an angle = -40°. The coefficient of kinetic friction between the block and floor is 0.25.

    (a) Calculate the magnitude of the frictional force on the block from the floor.
    (b) Calculate the magnitude of the block's acceleration acceleration.

    This problem is adapted from a book problem. The figures in the book are: m=3.5 kg, angle = -40; ck=.25. I worked that one and got answers that agreed with the book (a) 11 N (b) .14 m/s2. Plugging the different number into the same problem does not give me answers that are correct! Here's what I did for the book problem:

    (a) I first found T using equation substitution like in problem 6-2. I
    used T to find Fn and Fn to find fk. For the book problem, I got 11 N,
    which is correct per the book. For the homework assignment, I got 10 N,
    which is not correct. I checked my math, and I'm not doing anything
    differently. What am I missing?

    (b) I drew a diagram to show the different forces and decided there's only
    acceleration along the x axis, so I only needed to consider horizontal
    forces: F and fk. I calculated Fx = F cos A. I figured a = (Fx - fk)/m.
    I got .14 m/s2, which is what the book has. I did the same calculations
    for the online assignment and got 1.5, which is not correct. I again
    checked my math, and I'm not doing anything differently. But what am I
    doing wrong?

    the spelling-challenged fiestytig
  2. jcsd
  3. Sep 17, 2005 #2
    My second question that I need help with:

    In a pickup game of dorm shuffleboard, students crazed by final exams use a broom to propel a calculus book along the dorm hallway. If the 3.5 kg book is pushed from rest through a distance of 0.90 m by the horizontal 25 N force from the broom and then has a speed of 1.60 m/s, what is the coefficient of kinetic friction between the book and floor? Book says answer is .58. I can't get that.

    What I tried: I figured I could probably find acceleration and then divide that by gravity to get the coefficient. So I plugged in: 1.6^2 = 2*.9 * a, a=1.42. I tried dividing that by gravity, got .14 - no good. I decided I needed to use the m value and the F value given, so thought maybe I needed to go about it like this:
    F + fk = ma = 25 + ck*9.8 = 3.5 * 9.8 * ck, ck = .98. Wrong again. Looked through 2 Physics books for other things to try, but got nothing. What am I doing wrong here?
  4. Sep 17, 2005 #3
    Nobody wants to play with friction tonight? :(

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