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Maximum acceleration.

  1. Aug 3, 2005 #1
    Which is the maximum acceleration that a body (like a person or a car) can reach in the ground, and without using any extern help?
     
  2. jcsd
  3. Aug 3, 2005 #2
    The question is not clear at all.

    What do you mean by "external" (e.g. Gravity is external)
     
  4. Aug 3, 2005 #3
    Sorry, the question wasnt clear at all. I will be more specific: Is there a limit of the acceleration a car can reach using only its engine (that is, in a flat track for example)? My answer would be yes, the maximum acceleration that a car can reach in such conditions is g (gravitational acceleration) because the only force acting on it is the one due to the friction with the ground, and the latter depends on the normal force and the friction coefficient, but i´m not quite sure im right.
    Thanks.
     
  5. Aug 3, 2005 #4

    Stingray

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    Yeah, it's roughly g unless you hit something :smile:. It can be a lot more with special tires or aerodynamic help.
     
  6. Aug 3, 2005 #5

    pervect

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    Are rocket-cars considered to be "cheating"? They wouldn't have any limit on acceleration due to the coefficient of friction.
     
  7. Aug 4, 2005 #6

    Danger

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    You perplex me, Stingray. I thought that you were a professional racer. Don't try to tell me that a digger running 4 second quarters is accelerating at 1g max. My Roadrunner was a bloody joke on the street racing scene, and I pulled at least .7g going into 2nd gear. Off the line was probably less, but it was at least .5g, as can be attested to by my friend who got sacked by a can of juice that was on the dashboard when I lit up.
    Without aerodynamic aid your downforce is limited to 1g, but not your acceleration. That's purely a function of horsepower and traction. And as pervect pointed out, that only applies if you're relying upon engine-driven wheels as your motive influence. I've seen a car with a J-34 turbojet run the quarter in slightly less than 3 seconds from a dead-start. Try to tell me that it pulled less than 1.1g.
     
    Last edited: Aug 4, 2005
  8. Aug 4, 2005 #7

    Stingray

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    No, I'm not a professional racer. It's just a hobby. I don't have anywhere near the skill to be paid for it.

    Of course a jet or rocket-powered car can accelerate much faster than 1 g. I interpreted the original question to mean a car being accelerated only through its tires. Is that correct, mprm86?

    And you're right that dragsters do run much faster than 1 g. But they have the aforementioned special tires and wings. Street legal tires max out around 1 g, and that would only be for an AWD car. You say that acceleration depends on "power and traction," and that's basically true. I'm talking about available traction.
     
  9. Aug 4, 2005 #8

    Danger

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    As long as they don't offer to pay you to not do it, you're okay.

    A reasonable assumption.

    Ahhh... Well then, we're on the same boat. (But I still disagree about the brake thing. :tongue:)
     
  10. Aug 5, 2005 #9

    Mk

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    So the verdict is: No, there is no maximum speed.
     
  11. Aug 5, 2005 #10

    DaveC426913

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    "I've seen a car with a J-34 turbojet run the quarter in slightly less than 3 seconds from a dead-start."

    Holy cr*p.
     
  12. Aug 5, 2005 #11
    Yes, there is a maximum acceleration

    You all have forgotten your first physics class ever. Although friction plays a large role in acceleration, it plays virtually no role in maximum speed with a mechanical engine. Now, I say virtually because Special Relitivity states that the maximum speed you can attain is the speed of light or c. c is the maximum b/c of the resistance and/or friction that the threading of space-time itself plays. Therefore, with a mechanical engine, friction plays virtually no role in the maximum speed of your vehicle. This is b/c you can build your car thin enough and aerodynamic enough to overcome everything that places friction into the equation except space-time.
     
  13. Aug 5, 2005 #12

    pervect

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    Looks like flame-bait to me, or are you serious?

    Well, let's assume you are serious (and if it's flamebait, you wouldn't admit it, would you? I didn't think so).

    I've forgotten my first physics class - really. Not sure what it was, exactly. But I'm pretty sure it wasn't about relativity. Probably some of the younger posters still remember theirs, though, so it's highly unlikely we've all forgotten our first physics class.

    Special relativity does state that the maximum speed you can obtain is 'c'.

    This has nothing to do with resistance or friction of space-time (which does not exist), nor anything to do with "threading" of space-time (which doesn't even make sense - it's "not even wrong").

    It also has nothing whatsoever to do with maximum acceleration, and definitely nothing whatsoever to do with the original poster's question.
     
  14. Aug 6, 2005 #13

    Danger

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    My favourite from a perspective of design genius, even though it's a lot slower, is Les Shockley's "Shockwave". It's a (Kenny? Peterbilt?) semi tractor with 3 J-34's. That son of a ***** weighs 3 or 4 tonnes and blows through the traps in less than 14 seconds. Given the thrust-to-horsepower equivalence at sea level and under 500 kph, he's pushing about 30,000 hp. The 'burner pops' that he lights off while staging can deafen you at 100 meters. :biggrin:
     
  15. Aug 6, 2005 #14

    rcgldr

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    Coefficients of friction can be greater than 1. Top Fuel Dragsters pull about 4 g's at launch. Good table tennis rubber has a coefficient of friction around 7 or so, but is too soft to be used as a tire.

    Fastest land based acceleration is a rocket sled. 0 to 6400mph in about 6 seconds. In this case, a 4 stage rocket was used, the last stage pulling about 157g's. Here's a link, click on the picture to see the video.


    http://www.46tg.af.mil/world_record.htm

    Cool pic of a smaller rocket sled at 4800ft/sec.

    http://www.meggaflash.com/rocket%20photogr...0flashbulbs.htm
     
  16. Aug 6, 2005 #15

    Danger

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    Well... that was impressive! The bloody thing did it's whole schtick in about a tenth of the time that it took to load the file.

    That one just makes me think of the little dude in Mad Magazine's 'Spy vs. Spy' cartoons. :biggrin:
     
  17. Aug 7, 2005 #16
    Pervect, I really do not know why you would accuse me of flaming or baiting flaming, however, yes there is such a thing as threading of space and time. Although you may have heard it called something else, suck as the fabric of space. This is learned much when being taught about Einstein's Gravitational Theories. You must still be in Classical physics to not understand these theories.

    Secondly, yes, in my first physics class ever I did learn a small bit of relativity, however now I am a Senior at Florida State as a physics major and know quite a lot more than I did before.

    Third, to relate this back to the original question since you seem to have a hard time relating things of this nature back, friction on the tires, road, and even inside the engine all play a role in maximum speed and acceleration, however, with a MECHANICAL engine, mechanical being the key to this explanation, the only thing that limits max speed and acceleration are relativity. Einstein. Physics 101.

    Now, please do not flame me again.
     
  18. Aug 7, 2005 #17

    Stingray

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    Maybe I shouldn't speak for pervect, but he seems to know relativity quite well. And so do I. I've never heard of "threading of spacetime." "Resistance of spacetime" doesn't mean anything either, and in any case is irrelevant.

    This thread is about acceleration, not velocity. Even if it were about speed, bringing up relativity is just ridiculous. There are so many issues with any remotely conventional design that would have to be solved long before getting to c. The original question was clearly asking about a car, and nothing going anywhere near c would resemble such a thing in any way.
     
  19. Aug 7, 2005 #18

    Danger

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    Hey, now...! Let's not discount the RV from 'Spaceballs'. :grumpy:
     
  20. Aug 7, 2005 #19
    There is no maximum acceleration. Acceleration is a = dv/dt, and can be made as arbitrarily large as we want by decreasing dt. Any technical limitations are the engineer's fault.
     
  21. Aug 7, 2005 #20
    Thank you Icebreaker, you are absolutely correct. Stingray, you must not know a whole lot about relativity considering you dont even know about the fabric of space. When I refer to the threading of space I am referring to the fabric which if you know anything about relativity you cannot deny that Einstein defined the fabric of space with his Gravitational Theory. I think you are arguing for classical physics not modern physics.

    Also, the original question was not OBVIOUSLY for a normal car. The question was, is there a maximum speed or acceleration or does the friction of the road or air limit this. There are limits if you us differential equations but they will be located close to c. And Icebreaker is in fact correct for the actual equation, however if you were to take the limit, it should theoretically(as I have not done the calculation) be located close to c.
     
    Last edited: Aug 7, 2005
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