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Homework Help: Direction of frictional force in a bicycle

  1. Feb 16, 2012 #1
    1. The problem statement, all variables and given/known data

    During paddling of a bicycle, the force of friction exerted by the ground on the two wheels is such that it acts

    (A) Front Wheel -----> Backward | Rear Wheel ------> Forward
    (B) Front Wheel -----> Forward | Rear Wheel ------> Backward
    (C) Front and Rear Wheels -----> Backward
    (D) Front and Rear Wheels -----> Forward

    2. Relevant equations

    Pure thinking

    3. The attempt at a solution

    I thought it like this. We have a bicycle and it is moving towards +ve x-axis. Now at the bottom part of the wheel(which is in contact with the road and where friction is acting) is moving clockwise. The bottom part will then have a motion towards left and in order to avoid slipping, friction must act in the forward direction.
    As in the case of rear wheels, friction should act in the backward direction in order to help the motion of the bicycle to move towards +ve x-axis.
    But my answer is not correct. Its says just opposite to my thinking.
    Last edited by a moderator: Feb 17, 2012
  2. jcsd
  3. Feb 16, 2012 #2
    I think that you forgot to consider the fact that the bicycle pedal drives the rear wheel and not the front wheel:smile:
  4. Feb 16, 2012 #3
    i think that you are talking about static friction and not about rolling friction...What really happens is that we are making the rear wheel(by applying force on the pedal) to push the ground backward and hence the ground will exert a frictional force on the rear wheel in the forward direction.The front wheel is just like a passive component which is pushed in forward direction against ground due to the rear wheel's rotation. So , the frictional force on the front wheel due to ground will be in the opposite dirction to that of the bicycle's motion ...
    Last edited by a moderator: Feb 17, 2012
  5. Feb 17, 2012 #4
    Thanks. Got it now....
  6. Mar 30, 2012 #5
    As soon as the back tire moves even a little, the front tire has to move as well. Since both tires move at the same speed and are made of the same material, the frictional force on the tires is equal and opposite. So where is the force that is pushing the cycle forward??
  7. Mar 30, 2012 #6

    Doc Al

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    Staff: Mentor

    Why do you think this?
  8. Mar 30, 2012 #7


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    hi abelthayil! :smile:
    you're forgetting that this is static friction, and static friction can be anything between 0 and µN …

    there's no reason for the forces to be equal
  9. Mar 31, 2012 #8
    Oh right! Frictional force has nothing to do with speed but isn't the coefficient of friction equal for the two tires?
    I assumed that the normal force on both tires would be equal. I guess it isn't. So does that mean the position of the seat determines how easy it is to cycle?(to overcome the friction)
  10. Mar 31, 2012 #9


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    yes, but the static friction divided by the normal force isn't (usually) the coefficient of static friction …

    static friction can be anything between 0 and µN​
    the ratio of the two normal forces depends only on the position of the centre of mass

    if the brake only operates one wheel, then it would help (for emergency braking in a straight line) for the normal force on that wheel to be as great as possible (so that µN is greatest)

    but if the brake operates both wheels, then it makes no difference to the total normal force, nor to the total µN

    ( but i don't know what the best position is to avoid sideways skidding while turning :confused:)
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