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Bicyclist coasting down hill (Newton's Laws)

  1. Oct 27, 2011 #1
    1. The problem statement, all variables and given/known data
    "A bicyclist can coast down a 5° hill at a constant 7.0 km/h. Assume the force of friction (air resistance) is proportional to the speed v so that Fair = cv."

    (a) Calculate the value of the constant c.
    (b) Calculate the average force that must be applied in order to descend the hill at 22 km/h. The mass of the cyclist plus bicycle is 79 kg.


    2. Relevant equations

    Newton's Laws

    3. The attempt at a solution

    Since a=0, Ʃforce must=0.

    Fair=Fg

    Fg=mgsin(5°)=Fair

    c=mgsin(5°)/v

    I just can't figure out how to work around not having mass in this problem.

    Any help would be greatly appreciated :)
     
  2. jcsd
  3. Oct 27, 2011 #2

    jambaugh

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    What do you mean not having mass? You say the mass of the cyclist + bike is 79kg!
     
  4. Oct 27, 2011 #3

    PhanthomJay

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    your equation is correct...the mass is given in part b. Now try part b.
     
  5. Oct 28, 2011 #4
    I understand that the mass is given in part B. But not having the mass in the initial problem implies that mass is not needed to solve part A. I was hoping that someone could tell me how to solve part A without mass. Sorry for the confusion
     
  6. Oct 28, 2011 #5

    PhanthomJay

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    The mass applies to both parts. You can't solve part 'a' numerically without knowing the mass, and you can't solve part 'b' numerically without knowing the numerical result of part 'a'.
     
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