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Homework Help: Calculate Friction Force + Coefficient

  1. Oct 13, 2008 #1
    1. The problem statement, all variables and given/known data
    A force of 30 N accelerates a 5.0-kg block at 5.0 m/s2 along a horizontal surface.

    How large is the frictional force?
    (b) What is the coefficient of friction?


    2. Relevant equations

    I am not relly sure of the equations used..

    3. The attempt at a solution

    Well first i calculated the F Net which would be the mass * 9.8 in this case it was 5.0kg * 9.8 which is 49N, then i did 30N/49N which was .6122 apparently that is the the coefficient, and calculating (a) I have no idea
     
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  3. Oct 13, 2008 #2

    nicksauce

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    What are the things you know?

    ->The normal force is mg=49N.
    ->The friction force is the coefficient of friction times the normal force, [itex]49\mu[/itex].
    ->From Newton's second law, m*a=30 - force of friction.

    Can you solve the problem with this information?
     
  4. Oct 13, 2008 #3
    So with 49N I would subract 30
    49N-30N=19N
    The force of friction would be 19N?

    Then for the coefficient it would be 19/49? ~.3877?

    Apparently I am still doing something wrong,
     
  5. Oct 13, 2008 #4

    nicksauce

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    Don't ignore the fact that the block is accelerating! F=ma ! The force on the block is the Pulling force minus the friction force. ma is 5kg * 5m/s^2.

    On a different note, did you have some sort of external confirmation that the coefficient of friction is .6122? I do not get the same answer.
     
  6. Oct 13, 2008 #5
    well since this is a webassign and we have 50 chances to submit the correct answer, i been trying but somehow still hasnt turned up with the correct value.

    Would you have to consider the acceleration when calulating the friction force? I thought it was extra information
     
  7. Oct 13, 2008 #6

    nicksauce

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    You must use F = ma. F = Pulling force - friction force. ma = 5kg * 5m/s^2. There is no other way to find the force of friction.

    The only other information you have is that normal force = 49N, and friction force = coefficient of friction * 49N. This information alone cannot determine the force of friction or the coefficient of friction.
     
  8. Oct 13, 2008 #7
    So your saying this problem cannot be solved?
    doesnt seem likely
     
  9. Oct 13, 2008 #8

    nicksauce

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    The problem can be solved. As I have said before, you have to use F = ma = Pulling force - Friction force. You know m, you know a, you know the pulling force, and you don't know the friction force. One equation, one unknown, can be solved.

    Friction force = Pulling force - ma.
     
  10. Oct 13, 2008 #9
    hmm
    49N is the Force
    ma = 25N
    so 49N-25N=24N

    so the friction force should be 24N but it is saying it is wrong
     
  11. Oct 13, 2008 #10

    nicksauce

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    As stated in the problem, the pulling force is 30N. 49N is the weight / normal force.
     
  12. Oct 13, 2008 #11
    now i got it, the friciton force = 5N which was the pulling force =30N - (mass5 * acceleration5)
    =30 -25 so it is 5N

    now to find the coefficient of friction
     
  13. Oct 13, 2008 #12

    nicksauce

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    How does the force of friction relate to the normal force? What is the value of the normal force?
     
  14. Oct 13, 2008 #13
    O i see, its more clearer when someone else tells it
    so the 5N/49N making it .102
    Resolved!
     
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