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Distance traveled given acceleration?

  1. Jul 23, 2015 #1
    I have what I'm assuming is a pretty simple question:

    I'm trying to compare the distance traveled by two vehicles given a time and acceleration. Specifically, I'm comparing a full-sized car (196" in length) that goes 0-60mph in roughly 3 seconds vs. a remote controlled car (27" in length) that features roughly the same acceleration? Do they both travel the same distance? Do I need to take into account an vehicle specifics, such as the length of the car or the circumference of the tires?

    Thanks.
     
  2. jcsd
  3. Jul 23, 2015 #2

    rcgldr

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    If the acceleration versus time is identical for both vehicles, then they travel the same distance. However, even if both of them accelerate from 0 to 60 mph in 3 seconds, if one of them accelerates quicker for 0 to 40 mph, then slower for 40 mph to 60mph, so it ends up 0 to 60 mph in 3 seconds, then the distance will be longer (more time spent above 40 mph than the other vehicle).
     
  4. Jul 23, 2015 #3

    sophiecentaur

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    Your problem here is that things don't scale as simply as you would like. This is particularly true if your model is electric and your full size is Internal Combustion. When a car goes from 0 to 60, the acceleration is far from constant (see the above post) because of engine characteristics and gear changes. If you could actually measure the acceleration over time (say with an IPhone App) for the car and the model, you might get some interesting graphical results. (Some good padding could be useful in the model car, in case of an accident.
    That would depend on what you actually want to find out. If you wanted to produce a 'true scale' model of speed over distance performance then you may find the easiest way could be to take your real car, see how far it travels when accelerating flat out and then do a number of runs of the model car over a scaled distance, using a range of power settings until it actually travels the required scale distance in your 3 seconds.
     
  5. Jul 23, 2015 #4
    To see how far they run, of course you should take their velocity into account. The same acceleration can imply few information of velocity, which just reveals their same changing rate to the time.
     
  6. Jul 23, 2015 #5
    Here's a little more context which might shed more light on the subject...

    I'm trying to compare a Tesla P85D, which can apparently do 0-60mph in 2.8 seconds vs. a 1/7 scale Traxxas XO-1 electric RC car, which can apparently do 0-60 in an astonishing 2.3 seconds. Since both are electric vehicles, can I assume, for these rough purposes, that acceleration is basically constant (or at least close enough)?

    sohiecentaur - I would definitely like to figure out a "true scale" model for the RC car as well. The RC car is 1/7 scale. Is there some sort of equation that will help me figure out the "if the car was full-sized, how fast is it actually going question?" My naive gut would love to believe that, if everything scaled accordingly (power, weight, etc.) the full-sized RC car would be able to do the 0-60 in a ridiculous 0.32 seconds. But my gut is also telling me that I'm probably wrong.

    Unfortunately, I don't actually have physical access to either of these vehicles, I'm merely basing my equations off of the specs listed on the respective websites.

    Some more info that will either confuse or clarify things furthur:
    The RC car has a 1650 kV electric motor. I found an online conversion calculator that put this at roughly 2212 hp. Is that correct/possible? If I scaled everything with my previously demonstrated naive assumptions, that would put the kV at 11,550 and the hp at 15,488. At which point the numbers are getting so large I feel I must be doing something (many things) wrong. Also, the RC car weighs in at a modest 10.3 lbs. I'm guessing it's not accurate to assume that a full-scale model will only weigh 72.1 lbs. (vs. the Tesla's 4,830 lbs.)

    Now that you all know the comparison I'm trying to make, what information do I need to make the comparison as accurate as possible?

    Thanks.
     
  7. Jul 24, 2015 #6

    rcgldr

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    The Kv rating is no load rpm versus voltage, in this case, 1650 rpm per volt. Wiki article:

    http://en.wikipedia.org/wiki/Motor_constants#Motor_velocity_constant.2C_back_EMF_constant

    A typical RC motor's torque is greatest at 0 (stall) rpm, and decreases linearly to zero as it reaches maximum rpm. This would mean the acceleration of the RC car is greatest at the start and decreases as speed increases, even ignoring other losses like internal friction, aerodynamic drag, ... .
     
  8. Jul 24, 2015 #7
    Ah. Awesome. That makes more sense.
     
  9. Jul 24, 2015 #8
    Okay, I found this website: http://webpages.charter.net/sinkwich/sdventure/html/sd_scale_speed.htm

    It has a scale speed converter. Given the time, 2.3 seconds, and the final velocity, 60mph, I was able to determine what I believe to be the distance traveled, roughly 2,429 inches. Using the converter, it gave me the following scaled speed of roughly 420mph.... which seems too easy an answer... since it's basically just the 60mph multiplied by 7 for the scaling. However, if that's accurate, I could live with it.

    Does anybody agree with this finding? Or dispute it?

    Thanks.
     
  10. Jul 24, 2015 #9

    Drakkith

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    Your gut is indeed correct. Different aspects of the car do not scale the same way. For example, the car is a 1/7th scale model, which means that all the dimensions are reduced to 1/7th of normal. However, the mass is scaled down MUCH more than 1/7th. The model is 4.67 kg with batteries.The Tesla P85D has a mass of right around 2268 kg. So the mass of the model is scaled down to 1/485th of normal. This has a drastic impact on multiple aspects of the car, especially acceleration. The motors scale similarly. They increase in mass MUCH faster than they increase in power. So if you try to scale up the model, you wind up with performance at least vaguely comparable to the Tesla P85D.
     
  11. Jul 24, 2015 #10
    I dispute it. Not sure how to work that calculator.
    I calculate 1214 inches assuming constant acceleration from 0 to 60.
     
  12. Jul 24, 2015 #11

    Drakkith

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    All that converter does is tell you that if a 1/7th scale model moves X inches over time, then the scaled up velocity is 7X/T. When you put in your velocity as 60 mph, the calculate automatically found the distance the model traveled in inches per 2.3 seconds. Then it just multiplied this by 7 and found the new scaled up velocity. It doesn't tell you anything about acceleration, as it assumes a constant velocity.
     
  13. Jul 27, 2015 #12
    Consider
    a = delta v/ delta t
    where delta's are finit then
    S = delta v * delta t =
    a * (delta t)^2
    So it seems for a moment.
     
  14. Jul 28, 2015 #13
    You don't need any scaling for this.
    IF they have the same acceleration, they both start from rest and they move for the same duration the distance traveled will be the same. It does not depend on the size, mass or shape of the objects.

    But this is a big "IF".
    Even if you know that both go form 0 to 60 in the same time, you can only say that they have the same average acceleration. The distance traveled cannot be found from just average acceleration. You have to know how the acceleration varies with time. So you cannot tell if they do or not travel the same distance from just the information given.
    Again, scale is irrelevant. Even if you have two cars with same mass and size, and they both go from 0 to 60 in the same time, it would be possible that they travel different distances.
     
  15. Jul 28, 2015 #14

    sophiecentaur

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    But I thought the whole thread was about scale models with correct scale speeds and accelerations???
    (I could be wrong, of course)
     
  16. Jul 28, 2015 #15
    It was deviated into this.
    The OP mentioned that he wants to compare the distances traveled by two cars with the same average acceleration.
    Because one of them is a model it was somehow assumed that scaling has something to do with it.
     
  17. Jul 28, 2015 #16

    sophiecentaur

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    That's a daft notion. By that reckoning, a big car would go faster and further than a small car.
     
  18. Jul 28, 2015 #17
    No, it wont. IF they have the same acceleration (at all times, not just average) they will go the same distance. And will have the same speed.
    Once we have the acceleration, it's all kinematics. The mass of the car does not matter.
    The question (originally) was not about dynamics, about which one will have more power etc.
    He explicitly said
    "that features roughly the same acceleration".
    Of course you will need more force to get the same acceleration on the real car and scaling will be important to figure out HOW to satisfy this condition.
    But once the same acceleration is assumed, it is all kinematics.
     
  19. Jul 28, 2015 #18

    sophiecentaur

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    Yet again a PF thread goes in circles. :biggrin:
     
  20. Jul 28, 2015 #19
    Or maybe just loops. :wink: :smile:
     
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