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Homework Help: Make my day, please?

  1. Nov 8, 2006 #1
    Make my day, please? :)

    You and a bunch of your friends are trying to figure out what the force exerced on your bike's tires are by your brakes.
    Here are the things that you have to help you figure out this problem:
    -The road you're riding your bike down is completely horizontal
    -You know your own, and your bike's mass (the mass is what you decide)
    -You know your initial speed (None was given)
    -The final speed before you slam on your breaks is at the max before the bike would slide (meaning you are not permitted to slide at all)
    -You are given a tape measure.

    This is all the info I have, and trying to figure out this problem is being difficult. :/

    Maybe I'm over thinking things? I feel dumb. >.>

    Edit: I have no calculations already done because I do not know where to start.
    Last edited: Nov 8, 2006
  2. jcsd
  3. Nov 8, 2006 #2


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    Don't feel dumb. Are you familiar with the work-energy theorem, or the motion equation that relates distance, speed, and accelertaion?
  4. Nov 8, 2006 #3
    So, you are supposed to derive the algebraic formula for the force? What does each part tell you?

    -No extra forces to consider
    -You know the inertia
    -You know the intial speed
    -You know that the static friction between the tire and the road is always the case
    -You can determine the distance traveled with some intial velocity and some acceleration from the breaking force

    Piece them all together, or if this is the part you are lost at then which piece of the overall puzzle gives you the most trouble?
  5. Nov 8, 2006 #4
    I know a few work-energy formulas. (Sorry if this is incorrect, but I study in French :blushing: ) :

    Potential Energy and Ec (don't know the name in english)...

    Ec1 + Ep1 = Ec2 + Ep2

    1/2mv1^2 + mgh1 = 1/2mv2^2+mgh2
    I know W = F*D or W=deltaEc= 1/2m(V2^2 - v1^2)
    Is that the work-energy theorem you're talking about? Hmm, maybe if I find work then use the basic formula W=F*D, and use a distance that I choose?

    But then I still wouldn't know what the max speed is before I would slide. Unless there's a formula for that, or I just don't know, plain and simple. :s
  6. Nov 8, 2006 #5
    I guess it's the part where I have to figure out what the max speed is before I would slide.
  7. Nov 8, 2006 #6
    You know that between the tires and road, static friction must always exist (or else the tires slide). I think that you are supposed to just make some relation yourself that there is limiting force when the kinetic friction of the tires overcomes the static friction of the tires. You're right though, you aren't really given much to work with.
  8. Nov 8, 2006 #7
    Alright then, thanks. :)
  9. Nov 8, 2006 #8


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    No, they don't want you to figure out the max speed, you assume it is given. The reason why the sliding is mentioned is that as Mindscrape noted, you don't want kinetic friction of sliding, which will slow the car, to overtake the static friction between wheel and road which is driving the car forward. Or else the force required to stop the car would be partly contributed to by the sliding before the brakes take over. Bottom line is that your work energy theorem work = F.d = delta Ec looks good en francais, but don't forget that v1 is just a given v_max, and v2 is 0!! (work comes out negative).
  10. Nov 8, 2006 #9
    This also helps out a lot! I thank you both for taking the time to help me out.
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