Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Direction of frictional force in wheels

  1. Jan 6, 2012 #1
    1. if a ball rolling towards east frictional force also acts towards east...why is it so??(i mean to say doesnt frictional force act in opposite to the direction of motion?)

    2. if a bicycle is being pedaled, then the frictional force acts in backward direction in the front wheels and in forward direction in the rare wheels...why is it so??

    3. if a bicycle isnot being pedaled, then the frictional force acts in backward direction in both the wheels...doesnt it seem contrary to the fact given in (1)??
  2. jcsd
  3. Jan 6, 2012 #2


    Staff: Mentor

    wiki has an article on rolling friction:


    so it seems the mystery is item #1 it would seem the friction needs to point westward since as it rolls east the ball slows down unless the problem is related to the earth's rotation or something.

    for the bike it makes sense when you're coasting the rolling friction slows you down so its pushing you
    and hence points somewhat backward

    when you're pedaling the back wheel is under pedal power but the front isn't so rolling friction on the front wheel still points back. your pedaling exceeds the rolling friction of the back and in addition the tire would slip a bit which would change the direction of the frictional force I imagine.
  4. Jan 7, 2012 #3
    how the direction of friction changes as the tyre slips?
  5. Jan 7, 2012 #4

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    Consider something it may be easier to think about:
    A car is accelerating forwards.
    Therefore there must be a net, unbalanced, force acting forwards.
    The car is driven from the motor acting through the wheels in contact with the road surface ... so the force accelerating the car must be the contact force with the road ... which is friction. Therefore, the dominant friction-force on the accelerating car must be pointing forwards (in the direction of acceleration).

    Therefore - friction does not always oppose motion. What it does is sap energy ... which is why the car needs a motor.

    Now... bearing this in mind, instead of thinking "friction" think "net force", and do the problems:

    1. the net force on the ball must be pointing east if it is accelerating east ... which shows you that the question is under-specified: if it was accelerating and now moves at a constant speed, then there must be another net friction force opposing the motion.

    2. if the bike is being pedalled, it is being driven by the back wheel - accelerating the bike forward - so the net force from the back wheel must point forward. The front wheel will resist this motion, contributing to the drag, but not as much since the whole bike is accelerating. The context smooths out questions that cause trouble with #1.

    3. when you stop pedalling a bike, it slows down (accelerating in the opposite direction)... you've stopped driving the back wheel so its net friction changes direction.

    3 does not contradict 1 because the situation is different... made clear if we consider the case for 1 where the ball is accelerating east and not just at a constant speed. But still, it is not decelerating.

    I'd complain about #1 ... the nature of the motion is not specified and could be accelerating, decelerating, or uniform. The mere fact of motion in a particular direction is not enough to tell the direction of the net force. To properly answer these questions we need to use metadata - information not explicitly supplied in the question. IRL: this is almost always the case.
    Last edited: Jan 7, 2012
  6. Jan 7, 2012 #5
    so,in rolling friction,if a body is being accelerated,its frictional force always acts in the direction of acceleration(that may b -ve)/..but in uniform motion, the frictional force acts opposite to the motion of the rolling body..should i conclude like this?
  7. Jan 7, 2012 #6


    User Avatar
    Homework Helper

    This is mixing up the terms rolling resistance and static friction. A previous post already included a link to the wiki article on rolling resistance, which is a force that opposes rolling motion, due to hysteresis and deformation. Static friction is what keeps the surface of a rolling object from sliding on the surface the object is rolling on.

    So in the case of a bicycle being pedaled, the person generates a torque on the pedals that generate a torque on the rear tire, that resutls in the tire applying a backwards force onto the road, with the road applying an equal and opposing forwards force onto the tire. Technically it's the forward force of the road onto the tire that accelerates the bicycle forward (or maintains speed against other opposing forces such as aerodynamic drag). Static friction is what keeps the tire from spinning on the road, allowing it to apply a force to the road and the road to apply a force to the tire.

    Rolling resistance will oppose any rolling motion of the bicycle's tires, so if the bicycle is moving forwards, then rolling resistance results in a backwards force on the tire and a forwards force on the ground.
    Last edited: Jan 7, 2012
  8. Jan 7, 2012 #7

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    What rcgldr said: the questions were more about net force.
    The net force is always in the direction of the acceleration.

    The writer of the questions was expecting you to think about all the different friction forces and work out the sense that the question was asking about. It's testing your judgement, and, therefore, your understanding of the physics.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook