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How to measure speed without knowing the distance?

  1. Aug 19, 2013 #1
    Is there a cheap device or someone that I can easily build that will let me know the speed of a moving object? Its for one of my son's project...

    Thank you in advance.
  2. jcsd
  3. Aug 19, 2013 #2


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    If you can build someone easily, I would build a cop with a radar gun.

    What kind of project?
  4. Aug 19, 2013 #3
    I don't know what he plans to do he just said to me "With any tool I can measure speed without knowing distance?".
  5. Aug 19, 2013 #4
    Radar gun is all I can think of. I've some for about $50, but most are more than $100. I would try to figure out what the particular situation is though. There might be a tricky way to calculate the speed if there are other things that can be measured.
  6. Aug 19, 2013 #5
    And it is a motion on a curve and he said that he can't know the distance.
    Also, I have some electronic skill (not that much) if it is necessary.
  7. Aug 19, 2013 #6
    It sounds to me that he is thinking of radar guns and such like which measure a change in frequency ( of the radar gun) to determine speed
  8. Aug 19, 2013 #7


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    Does the device have wheels? If so, just use a rotary encoder type of marking + opto retroreflector setup to get some pulses from the rotation.

    Is the surface well-defined that the device will be moving over? Is there some texture to the surface, or some markings on the surface? If so, you may be able to image that surface and figure out the speed by how fast the surface is going by.

    More info would certainly help us give you better ideas, though. :smile:

    :rofl: You owe me a new keyboard!
  9. Aug 20, 2013 #8
    The thing that he wants to measure its speed is a cube. There is no texture in the surface, the cube's path is stable.
  10. Aug 20, 2013 #9


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    Just how does the cube move? Is it sliding down an inclined plane? Does the speed measurement device have to be on the cube, or can it be off to the side (like a video camera)?

    If you could just give us all the details, this would go a lot faster...
  11. Aug 20, 2013 #10
    The cube moves by curves, ya, an inclined plane. I can adhere to both proposition (camera or on the cube) if they don't use distance. Thank you. :shy:
  12. Aug 21, 2013 #11
    You could adapt the idea of a 'wind sock' used to indicate wind speed. A flat vertical vane pivoted at the top would deflect backwards when the 'car' is moving forward. Some calibration may be necessary.
  13. Aug 21, 2013 #12
    How big is the cube? Roughly what speed will it be travelling at? How far will it have to travel?
  14. Aug 21, 2013 #13


    Staff: Mentor

    You could do an optical motion capture approach. It would require some programming to analyze, but would probably be reasonably cheap and accurate. Alternatively you could mount some accelerometers on it. The processing is easier, but less accurate.
  15. Aug 21, 2013 #14
    how about an accelerometer? just put your android phone in the cube or whatever and write or download a dead reckoning app.

    \spoiler{ Protip: velocity is the integral of acceleration }
  16. Aug 21, 2013 #15


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    So, I hate to be pedantic here, but when I see something like this:

    "With any tool I can measure speed without knowing distance?"

    my initial inclination is to say that it didn't say without knowing ANY length!

    Distance is a length, as measure from one position to another. At least, that is the common use of the word. But there is another length here, and it is the length of the object that's moving. You don't call this the distance of the object, but rather, the length of the object. So is this length considered a "distance" that is also unknowable? If it isn't, then we have a very simple way of measuring speed without having to use a radar gun.

  17. Aug 21, 2013 #16
    I think you are being pedantic. Distance is a length, so length is a distance.
    He wants to measure speed without making a measurement of length, distance, displacement, etc, etc, I would say.
    I think it is an interesting question because it invites speculation about ways to determine speed.
    Better to consider what it says rather than what it does not say.
  18. Aug 23, 2013 #17
    Actually, he says he wants to measure the speed of a specific point in the cube, at the center. The cube is small, the size of a key. I don't know how fast it will be travelling but not that much.

    The optical motion method requires distance. I will search to see if the acceletometers are a good idea.

    That's maybe a good idea... Can you provide us with more informations?

  19. Aug 23, 2013 #18


    Staff: Mentor

    No it doesn't. If your fiducial marker has some detail then you can determine distance just from the image.
  20. Aug 24, 2013 #19
    I can't determine distance whatsoever.
  21. Aug 24, 2013 #20


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    Why not? You can take a picture or two, yes?
  22. Aug 24, 2013 #21

    if it's the size of a key to it you probably won't be ductaping a phone to it.

    I would use my basic stamp or raspberry pi and buy an accelerometer module for that which i would tack onto the gizmo and connect to the controller with some lightweight wires.

    but apparently there are serial and usb accelerometer dongles. maybe you could use one of these. just google around a little. but I suggest you don't buy anything until you're sure what you wanna do.
  23. Aug 24, 2013 #22
    if that's true, then we can't really help you.

    if you determine speed and time, you automatically determine distance. if you determine acceleration and time, you can determine velocity, and hence distance.

    perhaps you don't want to use a direct measure of distance, like a ruler, a camera, or a laser, but with whatever method you will use to determine speed, you will determine distance.
  24. Aug 24, 2013 #23
    Well Ok then. BTW the answer to your original question is no.

    You can’t measure speed without knowing distance. The formula for speed is…

    [itex]speed = distance / time [/itex]

    If you’re trying to calculate speed, and you don’t know distance, then you have one unknown equals another unknown divided by time. Is this some kind of riddle or something?

    I love the way you take the time to join and post on this forum, but you can’t be bothered to ask your son for details. Is he in a basement somewhere barking orders at you to get materials for his time machine project or teleportation device? Does he call you his minion?
    Last edited: Aug 24, 2013
  25. Aug 25, 2013 #24


    Staff: Mentor

    I agree with the previous two posters. This has become silly. Several different methods to measure velocity have been given. Please discuss the rest with your son.

    If you have future questions please post them as a new thread only after making sure that you have complete information about the actual constraints of the problem and the reasons behind those constraints.
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