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Homework Help: Quick Question

  1. Nov 29, 2004 #1
    Here's the Prob:

    What is the smallest raduis of an unbanked curve around which a bicyclist can travel if her speed is 18 mph and the coefficient of static friction between the tires and the road is .32?

    This was the response I got:

    For the second one: draw a freebody diagram, label all the forces, then use the centripital force equation F=(mv^2)/r to find the radius.

    Now...here's my question:

    Thanks, but wait, I'm still a little confused on the bike problem. We need to know the mass of the bike and the rider, and that's something we don't know yet. How do you find the mass? wait...tell me if I'm doing this right.....

    g=gravity? So to find the mass would you use the equation g=G*me/re^2?? If not...still, how is the mass found??
  2. jcsd
  3. Nov 29, 2004 #2


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    Homework Helper

    Just use the symbol m for mass and you will find that it cancels out in the end. Also, g is the local acceleration due to gravity which you can look up (certainly your textbook displays its value).

    Now go back and try to follow the original advice!
  4. Nov 29, 2004 #3
    The mass will drop out once you combine equations.

    This whole mass thing is a common problem. You have two choices: write out the equations in terms of a variable m for mass and see that they cancel eventually, or make up a value of mass and trust me that they cancel. I don't think I'd trust me if I were you.

  5. Nov 29, 2004 #4
    Thanks Soooooooo Much!!!
  6. Nov 29, 2004 #5

    I don't know if your prof/teacher is stressing that everything should be done in symbols until the last step. Even simple things like F=ma stuff. Once you get good at notation and solving in symbols you'll understand the units and whats going on more. You'll also be able to quicklly find if and where you made mistakes in canceling or simple algebra. (It also makes following your work eaiser to others i.e. tutors)

  7. Nov 29, 2004 #6
    uh, yeah. He kind of makes us do all the symbolism junk. It confuses me more than it should. Thanks for the advice. It'll come in handy later.
  8. Nov 29, 2004 #7
    Symbols are a godsend to somebody reading over your work. It saves them the trouble of having to guess what you did and then having to actually go and calculate it out to see if that's what you actually did. In particular, if one of the steps is wrong, you could confuse the reader so much that all he can say is "I have no idea how you got this number."

    It also helps because calculating numbers takes extra time and extra tools (calculator?) than just reviewing symbols. I think you'll find that many people are more amenable to going over your work if they're not made to do it all over again, too. Although a good tutor really will work the problem on his own beforehand, I doubt you'll find many people doing it for free that are so dedicated to work that isn't their own.

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