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Homework Help: Static friction

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

    I have a general question about circular motion of a car on a flat curve.
    What must the static friction koefficient be if the car can make the swing by 60km/h?

    Thank you,

    2. Relevant equations

    - We have to follow newtons 2nd law.

    - Fcentripetal = m*(v^2/r)

    known stats:
    r= 150m
    v= 16,67m/s
    g= 9,81

    3. The attempt at a solution

    mg = v2/r /g

    m= v2/r*g

    m= ((16,67*16,67)/(150*9,81))

    m= 0.189

    Need probably help here, not sure if I did do the equations right.
    Last edited: Mar 2, 2007
  2. jcsd
  3. Mar 1, 2007 #2
    Surely they must also have given you the radius of the track?

    Have you any idea how to start the problem?
  4. Mar 1, 2007 #3

    the radius is 150m.

  5. Mar 1, 2007 #4
    Can someone help me please, need help as soon as possible.

    , thanks!
  6. Mar 1, 2007 #5

    So, frictional force equals to centripetal force, and also equals to that coefficient times normal force.

    Find centripetal, and thus that's the frictional force (it's keeping the car in that circle)
    and then
    Fn.coefficient = Centripetal force
    mass cancels out..
    g.coefficient = centripetal acc..

    hoping this would help
    Last edited: Mar 1, 2007
  7. Mar 1, 2007 #6

    thanks for the help.
    But can't you help me a little closer, I am pretty new to this and I am also a newbee.

  8. Mar 1, 2007 #7
    Centripetal = v^2/r
    v=60 km/hr -- convert this is m/s
    r=150 m

    g.Answer = (v^2/r)
    Answer = (v^2/r).g

    sounds like a weird answer.. perhaps me wrong
  9. Mar 1, 2007 #8
    Can somebody help us here, please honestly!!
  10. Mar 1, 2007 #9
    Thats it, just make sure you convert units and then divide by g
  11. Mar 1, 2007 #10
    My english are abit bad, can you please write down for me how to do this?

    use this as a helping figure.
    Last edited: Mar 1, 2007
  12. Mar 1, 2007 #11
    hmm.. ite

    the answer for your question is = (v^2/(r.g))
    v=16.67 m/s
    r=150 m
    g= 9.81

    and so answer = 0.189
  13. Mar 2, 2007 #12
    Can you show me the calculations how u did come
    to (v^2/(r.g)) ?

    Sorry I am a very newbee, and need everything in with a big fork:grumpy:

    please help someone.:frown:
  14. Mar 2, 2007 #13


    User Avatar
    Staff Emeritus
    Science Advisor

    Since this is a homework question, noone will calculate the answer for you. In fact, harmeet has given you lots of help, considering the fact that you have done nothing yet.

    How about filling in numbers 2 and 3 in the homework posting template. What do you know? e.g. Do you know the equation to calculate centripetal force?
  15. Mar 2, 2007 #14
    You are right.

    There, i did filling in, and did an attempt.

    Was it right or?

    thanks for any help,
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