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Question on forces/friction acting on wheel

  1. Jan 14, 2009 #1
    i have this burning question which i have thought about for a long time and yet i could not seem to figure it out. here it goes.

    when a wheel rotates to move a car or any vehicle, the friction would act on the opposite direction and so the friction (red arrow) would act in the same direction as the motion (black arrow), therefore propelling the car. However, given that this explanation is correct, how would the car travel at constant speed since theres no backward friction and there is always a resultant force (friction acting to the front) acting on the car? If you might say that the air resistance would oppose this force thereby allowing the car to travel at constant speed, i do not think that the air resistance is so huge to match the force, even if so, how then would the car be able to accelerate since the forces are always cancelled?
     

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  3. Jan 14, 2009 #2

    Doc Al

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    Let's ignore air resistance, rolling friction, and similar complications. To accelerate the car (assume no slipping), the ground must exert a forward static friction force on the tires. But when the car is moving at constant velocity, there's no need for a friction force from the ground.
     
  4. Jan 15, 2009 #3
    hmm... i dont get what u mean exactly... i mean u cant control the friction acting on the wheel can you? whether moving in constant velocity or not the wheel rotates in the same manner so wont the friction acting on it be the same?
     
  5. Jan 15, 2009 #4

    rcgldr

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    Friction just keeps the objects from sliding with respect to each other, there's no set force. If the car is going at a constant speed, then the net force is zero (otherwise the car would be changing speed).

    If there's a block resting on the ground, friction keeps the block from sliding, but if nothing is pushing the block the friction force is zero. If you push the block with 10 lbs of force and the block doesn't slide, the friction force is 10lbs. If you push with 20 lbs of force and the block doesn't slide the friction force is 20 lbs. At some point if you push the block hard enough it will start sliding, and that is the maximum force that friction can provide (for the given conditions).
     
  6. Jan 15, 2009 #5

    russ_watters

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    No. The static friction force between the wheel and the ground is determined by the output of the engine. The greater the engine output, the greater the torque on the wheels and the greater the force between the wheels and ground.
     
  7. Jan 16, 2009 #6
    hmmm ok... but all of you seem to be answering only one part of my question.. for the wheels right... the friction seems to be the one propelling the car forwards and so if like you stated that the greater the torque the greater the friction, wont both forces cancel out and the resultant force acting on the wheel would be 0 and therefore the car would be moving at constant speed. How then does the car accelerate? can someone please answer both of the question (how does it move in constant speed and how does it accelerate) at the same time?
     
  8. Jan 16, 2009 #7

    Doc Al

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    As I said earlier, if you ignore the complications due to air resistance and other dissipative forces, no tire friction is required for a car moving at constant speed. Of course, to overcome those dissipative forces some friction is required.

    To accelerate the car, there must be a friction force on the tires pushing forward. That is produced as a result of the engine creating torque on the wheel. No, the torque due to the engine and the torque due to tire friction do no cancel out--the tire must also accelerate and thus requires a net torque. More importantly, the tires exert an external force on the car, driving it forward.
     
  9. Jan 16, 2009 #8
    u say no friction is required but no matter what it will still be produced..
     
  10. Jan 16, 2009 #9

    Doc Al

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    Why do you think that?
     
  11. Jan 16, 2009 #10
    because when 2 surfaces meet there will definately be friction
     
  12. Jan 16, 2009 #11

    Doc Al

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    Really? I have a book lying on my desk. What's the force of friction on it?

    Realize that (in the absence of skidding or deformation) the friction between tire and road is static friction. Static friction will be whatever is required to prevent slipping, from zero up to the maximum value the surfaces can generate for a given normal force.

    Let's forget the car for a minute. Imagine just a wheel rolling without slipping along a perfectly horizontal surface at some speed. Assume the usual idealizations (no air resistance or deformation). In this case, there is no tendency to slip, thus the static friction is zero.
     
  13. Jan 16, 2009 #12
    yes the static friction may be zero but there would still be kinetic friction
     
  14. Jan 16, 2009 #13

    Doc Al

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    Nope. Kinetic friction would only exist if the tire slipped (skidded) against the ground. As long as it rolls without slipping, the contact patch of the tire is always at zero speed with respect to the ground. No relative motion of the surfaces means no kinetic friction.
     
  15. Jan 17, 2009 #14
    ok... if theres no static friction and theres no kinetic friction... how is the care even supposed to move??
     
  16. Jan 17, 2009 #15

    Doc Al

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    Movement (constant velocity) does not require a force. (Review Newton's first law.)

    Acceleration (change in velocity) requires a net force.
     
  17. Jan 17, 2009 #16
    i know... but from rest how does it even start moving?
     
  18. Jan 17, 2009 #17

    Doc Al

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    As stated already in this thread, to accelerate the car requires a friction force on the tires.
     
  19. Jan 17, 2009 #18
    ok man do u have msn or something? so u can explain better
     
  20. Jan 17, 2009 #19
    Your question can be turned around...but from motion how does it even stop moving?The point being made here is that if there is no resultant force the object will carry on in the same way..if it is at rest it will remain at rest,if it is moving at a constant velocity(straight line and steady speed)it will continue moving at a constant velocity.The object will only change its velocity(accelerate) if there is a resultant force.
     
  21. Jan 17, 2009 #20
    Your original Question;

    However, given that this explanation is correct, how would the car travel at constant speed since theres no backward friction and there is always a resultant force.


    With out some friction nothing will roll.
    Like a brick sitting on concrete, gravity is pushing down on the tire and the normal force is straight up and down and not backwards or forwards.
     
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