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Homework Help: Frictional forces

  1. Oct 15, 2007 #1
    1. The problem statement, all variables and given/known data

    a block is at rest on the incline shown in the figure. the coefficients of static and kinetic friction are s = 0.42 and k = 0.36 respectively. The acceleration of gravity is 9.8 m/s^2. the angle is 19 degrees.

    what is the frictional force acting on the 43 kg mass? answer in units of N.

    3. The attempt at a solution

    I tried brake it up into components and still got it wrong, really have no idea how to do this problem.
  2. jcsd
  3. Oct 15, 2007 #2
    The friction coefficient is denoted by: μ
    The Normal force is: N=mgcos(θ)
    The frictional force is: μN=μmgcos(θ)
  4. Oct 15, 2007 #3
    yea, I get 167.3454417 as my answer (dont have to put it into significant figures for the homework) and its wrong...
  5. Oct 15, 2007 #4
    the angle of inclination is on the left, the object would slide to the left if it wasnt for the friction...if it makes a difference but I also tried to submit my answer as -167.3454417 and that was wrong too...
  6. Oct 15, 2007 #5
    What is the correct answer?
  7. Oct 15, 2007 #6
    I dont know the correct answer because I need to submit the homework online and it gives u like 7 chances, it only says if ur right or wrong
  8. Oct 15, 2007 #7
    Well, if it's looking for an answer in terms of N, and the frictional force is μN, and if μ=0.42, then in terms of N, the answer is 0.42N.
  9. Oct 15, 2007 #8
    its been a long time since i did these types of problems but i think at rest the frictional force would be F=(mew*mg)-mgsin(theta)
  10. Oct 15, 2007 #9


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    This is a duplicate post. I suggest calculating the value of the component of the gravity force acting down the plane. Then apply Newton 1 for bodies at rest to calculate the friction force. HINT: You won't need to know mu.
  11. Oct 15, 2007 #10
    so is this problem equilibrium or not?
  12. Oct 15, 2007 #11


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    It is given that the block is at rest; hence, yes, it is in equilibrium.
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