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Centripetal force- Car making a turn

  1. Jun 6, 2010 #1
    Hello everyone,

    I have got confused with this. u6l1c10.gif

    Normal the friction force acts to cancel the force. If I travel with 10 N force and friction is 5N, I travel at 5N. Why is that when you make a turn they don't create a net force, but act as separate forces?
     
  2. jcsd
  3. Jun 6, 2010 #2
    Friction (in reality it's not just friction in real tyres) is also the force that allows things to generate grip. Think about a car on ice, there is little grip force between the tyre and road, so the wheels spin. You can't turn or speedup/slow down.

    I'm not qiute sure what you mean about acting as seperate forces though, could you clarify?
     
  4. Jun 6, 2010 #3
    You're thinking of kinetic friction. That for something sliding. If you apply 10 N of force to the right, but it's opposed by 5 N of kinetic friction to the left, then there's a net force of 5 N to the right. However, this problem isn't about sliding. In this problem it's static friction, because the tires roll without sliding.

    As long as the car moves in a circular path with a constant speed, you know that the net force on the car must be pointed toward the center of the circle, and the friction happens to be is the net force on the car. The weight downward and the normal force upward add up to zero, so the net force, friction, is sideways, toward the center of the circle.
     
  5. Jun 6, 2010 #4

    Ranger Mike

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    see Race car suspension Class Sep9-09, 08:58 AM post
     
  6. Jun 6, 2010 #5
    Hey thanks for all the answers :smile: I think I have a major misunderstanding. When a tyre rolls on the floor, is the reason why it moves forward is due to friction, or the rolling action. So in a car to move forward, the tyres must roll and the road gives friction force to keep it moving forward? Is this right? All this time I was thinking rolling provides a forward force? So in ice how do the cars move forward, rolling of tyres is not going to make it move forward, do they just slide forward? Please tell me if I'm right? Thanks!!
     
  7. Jun 6, 2010 #6

    Ranger Mike

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    Newtons # First Law: Every body will persist in its state of rest or of uniform motion (constant velocity) in a straight line unless it is compelled to change that state by forces impressed on it. This means that in the absence of a non-zero net force, the center of mass of a body either is at rest or moves at a constant velocity.
    In my opinion what slows down the car is air drag , rolling resistance, drive line friction etc...and slip angle of the tires...
     
  8. Jun 6, 2010 #7
    You can't roll if you don't have friction. If there was no friction between the contact patch the tyre would just slide forward, not roll.

    On ice, there is very little friction, this is why the wheels just spin. You are applying a torque to the centre of the wheel and it's enough to overcome the friction so it just spins.
     
    Last edited: Jun 6, 2010
  9. Jun 6, 2010 #8
    The friction isn't "the reason why it moves", but: the reason why, instead of slipping off on a tangent line, which is a straight line that touches the circle at only one point, instead of doing that, the motion is continuously pulled inward into the circular path.
     
  10. Jun 6, 2010 #9
    It's confusing because the common usage of the word "velocity" in daily conversation is incorrect. Most people forget that a specific direction is part of the number that we call a velocity. For example, in Spanish-speaking countries, the speed limit on the road is called the velocidad maxima, translated as maximum velocity, where the law is only concerned about the speed and it doesn't care about the direction. A physicist would say that wording is incorrect, because it's not a velocity at all unless both the speed and the direction are indicated.

    Remembering that a specific direction has to be part of the value of the velocity, consider the car that is moving in a circle. Even if the speed is constant, the velocity is continuously changing, because there is a continuous change in the direction. That's why circular motion requires some force and acceleration, so that we can have that continuous readjustment of the direction.
     
  11. Jun 6, 2010 #10
    In a turn acceleration requires considerable friction between the tire and the road; in straight uniform motion very little friction is required as, say, when yo take your foot off the gas pedal and just coast in a straight line.

    the implication you state is untrue...if you add two forces together you obtain a net force...but that does not mean the separate forces have merged into one...onlt that you have decided to represent them as one force.
     
  12. Jun 7, 2010 #11

    rcgldr

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    Getting back to the OP ...
    The car is moving forwards but the front tires are turned inwards, and this causes the tires to generate an outwards force onto the pavement, which reacts with an equal and inwards force onto the tires, and the inwards force results in the inwards acceleration of the car allowing it to turn.

    The amount of friction determines the maximum force possible between tire and pavement.

    To continue, once the car starts to turn, it responds to the inwards acceleration with an outwards force, that is transmitted to the pavement at the tire contact patch, and even the rear tires will end up pointing a bit inwards of their actual path, along with an outwards force exerted to the pavement coexistant with the pavement's exerting an inwards force.
     
  13. Jun 8, 2010 #12
    Thanks everyone :smile: My knowledge on this is very poor. I'll take this step by step. So from what you guys are saying, if there is a single tyre, I push it, it rolls rather than slide due to friction. Is that right?

    Ok then in a car what creates the initial push? With the tyre I pushed it, in a car what gives this initial push to the car?

    I think may be the question I should ask is why does a tyre roll? What makes it roll in one direction.
     
  14. Jun 8, 2010 #13
    It's the static friction between the road and rubber that causes the force that enables rolling to occur.

    Driveshafts rotate and transfer torque to the centre of a wheel. This rotation at the centre acts as a 'push' at the contact patch.

    see: Torque and force relation.


    Do you mean why does it only roll forward? Or why does it only roll with 1 axis?
     
  15. Jun 8, 2010 #14
    Hey thanks for the reply :smile: I think I'm really confused on how the car is moving forward. Ok so after the intial push by the torque, the tyre hits the road, then road exerts a force and makes the tyre roll. Why does friction make the tyre roll forward, also why does rolling forward make something go forward, I mean if it rolling at the same place it won't move, how does friction make it roll forward? I think I'm confused with spinning and rolling forward, is spinning also cause by friction, or does it make it roll forward. So in no friction surface, do you slide? So if you press the accelerator can you slide faster?
     
  16. Jun 8, 2010 #15
    That's far too much to try and dissect and answer. I'm also not sure about how I can explain it differently, i'll have to let someone else have a crack at it.
     
  17. Jun 8, 2010 #16
    Ok simply on ice, if the tyres are not rolling forward and spinning, is the car sliding. So in this case is the car moving forward, due to force from the engine or it just sliding?
     
  18. Jun 8, 2010 #17
    It's just sliding as there is no friction (grip). The engine is just spinning the wheels, not making the car go forward.
     
  19. Jun 8, 2010 #18
    Hey thanks for the reply. So is the car stationary, or can it slide and move naturally not from the engine.
     
  20. Jun 8, 2010 #19
    Having no friction between the tyre and the road is the same effect as lifting the car up on a crane so the wheels dont touch the floor.

    You can physically push it and it will move. If you used the engine the wheels would spin but the car would go nowhere.
     
  21. Jun 8, 2010 #20
    Hey thanks :smile: So friction makes the car move forward. So for a given surface you can only reach a certain speed. Static friction is coefficient of surface and weight right. So is that the highest force you can get to move the car?
     
  22. Jun 8, 2010 #21
    Real tyres don't follow that exact relation but don't concern yourself with that for now, conceptually it's not important.

    But yes, thats correct the largest force that you can get to move the car is the static friction force. The friction between the tyre will not limit the speed, but the acceleration (which is determined by force a la newtons 2nd law).
     
  23. Jun 8, 2010 #22

    rcgldr

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    Without downforce, and with sufficient power, terminal velocity occurs when aerodynamic drag equals maximum friction force from tires. Normal cars do not face this issue. Only something like a Bonneville high speed vehicle has to deal with traction on salty surface versus aerodynamic drag at high speed issue.
     
  24. Jun 8, 2010 #23
    Drag will stop you reaching a speed long before the tyres will. Therefore for the puropses of this thread it's irrelevent. I don't want to confuse the OP's understanding by indroducing new variables such as downforce or the way tyres really grip.
     
  25. Jun 9, 2010 #24
    One can answer the question simply in this way. The patch of the wheel in contact with the ground tries to move back when its rotated by the engine. Static friction prevents it by the way of acting on the patch in the forward direction. Apply second law. Everything rigidly attached to the wheel (that is, the whole vehicle) should move forward with an acceleration a=F/m. But a clear understanding could be got by analysing how individual particles of the wheel behave when the friction acts.

    Rolling is a combination of translation and rotation. A wheel cannot roll at the same place. "A loud silence" is poetic but doesn't make sense.

    Spinning of the wheels is caused by the engine. remember that engine alone cannot make the vehicle move as its not an "external" force.
     
    Last edited: Jun 9, 2010
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