Free-body Diagrams for Bicycle and Person sitting on it

  1. A person is on a bicycle, there are several situations like the bike is at rest, accelerating,etc. The one I'm having most trouble with is when bike is in constant velocity.

    For the bike,I have gravity and force of person acting downward, normal force upward, force applied to the right and friction to the left.

    ON the person, gravity and force of bike on person is once again there. Horizontally, I know I need to have the friction of the bicycle, but what direction is the friction acting? And what is the force that balances out friction to achieve net force of zero?

    Thanks for you help.
  2. jcsd
  3. Doc Al

    Staff: Mentor

    You are assuming some friction force acting to slow down the bike? Like rolling friction, I presume?

    It depends on how you want to model it. On the person (assuming you ignore air resistance) there is no horizontal force. If you want to include rolling friction acting to slow the bike, then you also must have static friction opposing it to maintain constant speed.
  4. lets say the bike is moving forward. If so then the friction will be acting on the bike in the backwards direction, because friction is a form of resistance.

    The bike is traveling at a constant speed therefore, the person riding the bike is creating a force that is equal and opposite to the force of friction.
    This force is what balances out friction to achieve a net force of zero.

    hope that answers you question.
  5. Doc Al

    Staff: Mentor

    If that were true, then what force would balance out the force of the bike on the person?

    It's true that the net force on the bike must be zero, but the person doesn't exert a force on the bike (at least in a simple model that ignores the air resistance of the person).
  6. the person exerts a force on the bike by using their feet to pedal, this force is what allows the bike to travel. Since the question states that the bike is traveling at a constant speed, the air resistance in not affecting this FBD in this question.(yes it is in fact a simple model)
    however if it was not a simple model then the air resistance and the friction would have to total up to equal the force that the person is exerting forward in order to allow the bike to travel at a constant speed. If the air resistance and the friction did not add up an unbalanced force would arise(newtons first law), making the bike no longer travel at a constant speed but at a speed that is either decelerating or accelerating.
  7. Thanks for the replies guys. I made my original question a bit unclear. When I mentioned friction, I meant friction between the person and the bike seat, not the the rolling friction between bike and ground.

    From what I understand both of you make sense. Doc Al addressed the idea of there being no friction between person and bike seat, and that makes sense to me now. Suppose the bike wasn't in constant velocity though but was rather accelerating? I imagine that to cause the person's acceleration, there would have to be friction between person and bike seat. Even more, I think there might be applied force because when you hold on to the bike handles/brakes, you are hooking your fingers around the handles. You are putting a part of yourself in front the bike, so the bike will come from behind and apply a force onto you. I came to this idea thinking that if I left go of the handles, I will be seated on the bike as long as the acceleration of the bike is relatively low. If however the net force on the bike (and therefore its acceleration) becomes high enough, friction between person and seat won't be enough to cause the same acceleration as the bike, causing me to fall off. However, if I hold on to the handles, even if I am accelerating at a very quick rate, I won't fall off. It seems to me that there is another force giving me an acceleration (ie. force applied by bike on me in the horizontal direction)

    Let me know if that is in the correct line of thinking.
  8. Doc Al

    Staff: Mentor

    No, the purpose of pushing the pedals is to make the wheel turn thus creating a friction force between the ground and the tires. It's that friction force from the ground that drives the bike forward (if needed).

    The person does not directly push the bike forward. Don't forget Newton's 3rd law: If the person pushed the bike forward, then the bike would push the person back. But the person (and bike) are moving at constant velocity, so we know there's no net force between bike and person.

    For that matter, don't forget Newton's 1st law. If we pretend there are no external forces (like air resistance, ground friction, or gravity) slowing down the bike, then no force whatsover is required to maintain constant velocity of bike and rider.
    Yes, if the bike is accelerating then it must be exerting a net forward force on the person to accelerate him as well. That force will be a combination of friction from the seat and direct force from the handles, etc.
Know someone interested in this topic? Share this thead via email, Google+, Twitter, or Facebook

Have something to add?