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Net Force and Friction

  1. Feb 26, 2008 #1

    I'm new, so I'd like to say thanks in advance for helping me. The site wants all answers to the 4th decimal place. Idk how stuff normally works around here, but a small explanation with the answer would help me understand this stuff better.

    1. Three forces act on a particle that moves with an unchanging velocity v = (6 m/s) i - (7 m/s) j. Two of the forces are FA = (4 N) i + (5 N) j + (-2 N) k and FB = (-5 N) i + (8 N) j + (-4 N) k. What is the third force?

    _____N i + _____N j + _____N k

    2. A baseball player with mass m = 76 kg, sliding into second base, is retarded by a frictional force of magnitude 500 N. What is the coefficient of kinetic friction µkin between the player and the ground?


    2. I'm unsure because I missed class with an awesome case of food poisoning. -_-

    3. I was sick in class, so I'm unsure.
  2. jcsd
  3. Feb 26, 2008 #2
    1. [tex] F_A+F_B+F_C=0[/tex]
    2. [tex]f=\mu_kN[/tex]
  4. Feb 26, 2008 #3


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    Ah. Life is suffering. And that's the good part.

    Well, I guess the equation you missed which relates weight to frictional force would be
    F_{\rm fric}=\mu_{\rm kin} m g

    and for the first question the equation you missed was Newton's First (or is it the 2nd?) Law: A body in uniform motion remains in motion unless acted upon by a (net) Force. Thus the third force you seek must be that which cancels completely the other two.
  5. Feb 26, 2008 #4


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    P.S. I think this thread should be in the Introductory Physics forum.
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