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Quick question on a car braking

  1. Jul 2, 2006 #1
    Just a quick question everyone. It's annoying me quite a lot as I used to know it, but I've not done physics for over a year and I'm getting quite rusty. My question concerns a car under braking, and how the its velocity is reduced. As I understand it, the frictional forces generated by the brakes producing a torque opposite to that of the rotation is unable to slow the car as it is an internal force, and only external forces can reduce its speed. Right?

    In which case how does the car actually slow down? I know it's something to do with the road pushing back on the car, but I'm so rusty I can't remember it.

    Thanks in advance.
  2. jcsd
  3. Jul 2, 2006 #2

    Doc Al

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    That's right. You need an external force--the friction of the ground against the tires--to slow the car. The brakes slow the wheels, but it's the friction of the ground on the tires that slows the car. If there were no friction--imagine braking on a sheet of wet ice--you could apply the brakes as much as you want, but the car won't slow down.
  4. Jul 11, 2006 #3
    I agree, and the tires only have so much static friction. Thus when you are going around a corner, you are more likely to loose control under braking or power from the engine. As the tire friction must also produce the acceleration to turn the car.
  5. Jul 11, 2006 #4


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    The above responses are all correct. All of a car's movements are ultimately due to friction between the tires and the ground. The largest accelerations that most high-performance car tires can withstand without breaking static friction are on the order of 0.9 g. This means that a car (any car) cannot accelerate faster than about 60 mph in 3 seconds, or corner a 1000-foot radius turn at more than 115 mph.

    This fundamental limit makes cars rather boring. I'd rather fly aircraft, which do not depend on friction and can accelerate and turn at any rate one wishes, given that you have the engine and the airframe strength.

    - Warren
  6. Jul 11, 2006 #5
    Not true with down-force onto those tires you can pull over 2 g at 100mph and somewhat less at lower speeds. Besides nothing ever happens accelerating off the line, its about going around corners. :wink:

    EDIT: Oh and can anyone sell me what the little warn thing is that shows up on the top of my posts?
    Last edited: Jul 11, 2006
  7. Jul 11, 2006 #6


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    That's just an indication of how many times you've pissed off the Powers That Be. If your count is above zero, you would have received a PM explaining your transgression.
    Last edited: Jul 11, 2006
  8. Jul 11, 2006 #7
    I figured it might be something like that, never before have I seen a forum with such a feature implemented.
  9. Jul 11, 2006 #8


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    Notice that I caught my spelling error and corrected it. :biggrin:
    I don't know what other sites are like, since this is the only one that I've visited. The moderation is lenient, but not overly flexible. That's why you can be assured that anything you learn here is reliable. It's the equivalent of a peer-reviewed journal; checks and balances are in effect to make sure that any faulty information is weeded out. I have been unashamedly guilty of giving bad advice (although I usually preface it with a caveat), and it's always caught by the experts. On a couple of occassions, that 'expert' has turned out to be a total idiot with an attitude, in which case the real experts eliminate him. You can believe that anyone with a Mentor or Science Advisor tag is the real thing. They'll sometimes disagree with each other, but in that case it's because the subject is up for interpretation. Unless you're a die-hard crackpot, you'll probably never receive a warning.
  10. Jul 12, 2006 #9
    It's somewhat ironic that this is here, as it is another thread I'm in where I'm getting a bit of an attitude from some people. But thats what happens when they don't think you understand and you present an alternative viewpoint to their established ideology.
  11. Jul 12, 2006 #10


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    It should not be surprising that people educated in a subject would get irritated that someone not educated in that subject would not trust them to educate them but would instead trust more in their uneducated "alternative viewpoint".
  12. Jul 12, 2006 #11
    Hmm see what I mean? You have no idea about my education, as I don't have any about yours. That said, I've never seen someone be so rude at any colloquium. I believe you are misinterpreting my "questions" on the other thread. Just because you ask something doesn't mean you don't already know the answer. Active listening.
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