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Is it possible to reach speed of sound?

  1. Nov 1, 2011 #1
    This is probably simple but i just can't seems to work it out:

    assume no gravity or friction:

    car 1 traveling at a constant speed of 50m.s in one direction
    Car 2 traveling at a constant speed of 50m.s on top of car 1 in same direction
    car 3 traveling at a constant speed of 50m.s on top of car 3 in same direction
    etc etc.

    What is the speed of car 3 relative to the ground? 150m.s?
    What formula would you use to arrive at the answer?

    So if the above statement is correct,would it be correct in saying, if you had to pile more and more cars running at 50m.s, on top of each other, you should finally reach the speed of sound? Theoretically of cos.

    Thank you
  2. jcsd
  3. Nov 1, 2011 #2

    Doc Al

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    Vc/g = Va/g + Vb/a + Vc/b

    Sure, why not?

    The example might be better if you made it something like:
    - A flat bed train is moving at 50 m/s with respect to the ground.
    - On top of the train is a flat bed truck moving at 50 m/s with respect to the train.
    - On top of the flat bed truck is a car moving at 50 m/s with respect to the truck.
    - And so on, as much as you like
  4. Nov 1, 2011 #3
    Hi beddi. To add velocities like this that are in the same direction, you simply add them to each other. So car 3 is going 150 m/s (50 + 50 + 50) relative to the ground.

    And yes, with enough cars like that you could reach the speed of sound, although there's no reason why you couldn't just have one car going fast enough.
  5. Nov 1, 2011 #4
    Fantastic , my thoughts exactly, thanks guys

    Much appreciated..
  6. Nov 1, 2011 #5


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    Did you mean no drag? No friction implies the tires can't grip, so the cars couldn't move at all. I assume you mean reach the speed of sound, but without the high amount of aerodynamic drag that would occur as such speeds.
  7. Nov 1, 2011 #6
    Yip, drag would be the word... Anyway it was just a simple concept, obviously not possible in real life, as there are many factors which need to be taken into consideration.
  8. Nov 1, 2011 #7
    Yes, absolutely, since speed is very low, you can just add them up. But you have given that there is no friction, presummably, no air. Thus you don't have to worry about sound barrier or energy lost to friction, you can simply accelerate one car to speed of sound.
  9. Nov 1, 2011 #8


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    I'm not sure why you would pick on wind resistance here: it isn't the biggest factor affecting the realism of the scenario.
  10. Nov 1, 2011 #9
    Really? I think that when approaching the velocity of sound, the resistance is huge. And what about sonic boom is there is wind? It might destroy the car.
  11. Nov 1, 2011 #10


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    I can think of no realistic way of this scenario achieving anywhere close to the speed of sound, to start worrying about it.
  12. Nov 1, 2011 #11


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    Well, if there were no "drag" from the air, there would be no "speed of sound"!
  13. Nov 1, 2011 #12
    "Sound" seems to be a relatively low goal to set with this model.

    It would be much more interesting to ask about the speed of light. Or to give an arbitrary large velocity and ask how many cars would there have to be in order to reach that speed....

    This question kinda has potential and then lets you down.
  14. Nov 1, 2011 #13


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    Low? How do you even get the second car to achieve the OP's objective?
  15. Nov 1, 2011 #14
    Realistically, why not do it with one car?

    To do it with two, each car must be travelling at half the speed of sound(and I'm assuming we're talking about the speed of sound through air) with respect to the surface below it.

    For the purpose of a thought experiment, one could be more stimulated.
  16. Nov 1, 2011 #15


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    One way to get something to travel faster than the speed of sound is to put it on the end of a rapidly rotating rod. The rod could be made nicely aerodynamic and pivoted about its centre. Each element of the rod is going a bit faster than the one just inside its radius - the speed increases as you go outwards from the centre. This would be the same as your truck on a truck on a truck series.

    A bit of a fearsome beast, I admit (You would need quite a powerful motor to drive it all) and you'd really want not to be standing next to the experiment (get inside the bunker, I'd suggest) but you could get supersonic effects at the ends of the rod.
  17. Nov 2, 2011 #16
    The propeller blades of a Tupolev Tu-95 exceed the speed of sound.

    Also, the tips of whips can break the sound barrier. That's the crack that you hear...
  18. Nov 2, 2011 #17


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    I believe the crack of a bullwhip is actually the tip of the whip breaking the speed of sound...that's kind of like a modified version of this except with a flexible material rather than a rigid rod. But it's something you can do at home! (preferably, not inside your house...)
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