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Difference Between Speed and Velocity ?

  1. Nov 8, 2012 #1
    Difference Between Speed and Velocity ??

    I want to as that what is the difference between speed and velocity.. these both seems to be same for me... but i'm in doubt... and similarly what is the difference between distance and displacement ???
     
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  3. Nov 8, 2012 #2
    Re: Difference Between Speed and Velocity ??

    Velocity and displacement are vectors meaning that the direction matters. If we face each other and walk in opposite directions with some speed, we will have the same speed but opposite velocities.If you walk in a straight line from your starting point to my starting point and I walk to your starting point we both travel the same distance but we have different displacements.
     
  4. Nov 8, 2012 #3

    Nugatory

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    Re: Difference Between Speed and Velocity ??

    Speed is a number: How fast is something moving? Velocity is a vector: How is the position of the object changing over time? Direction matters for velocity, but not speed.

    An easy example of the difference:
    If you tie a weight to a rope and then swing it in a circle around you.... The object will be travelling at a constant speed in its circular path. However, at one moment it will be moving to the east, and then a moment later to the north, and then a moment after that it will be moving to the west. So its speed is constant, but its velocity is continually changing.
     
  5. Nov 8, 2012 #4
    Re: Difference Between Speed and Velocity ??

    Here's something to consider: Einstein's special theory of relativity postulates that the speed of light is the same for all observers.

    Two observers moving relatively to one another will both see the same light go at the ~300,000 kilometers per second, relative to themselves. They may, however, see the light traveling in a different direction than what the other observer sees.

    So the speed of light is the same for all observers, but not the velocity of light.

    Sorry if that's too advanced for you at this point. If it doesn't make sense, but remember the rule so that when you hear a friend say "The velocity of light is the same for all observers". You reply "No, it's the speed of light that is the same for all observers."

    Another way of explaining the relationship between velocity and speed is to represent velocity as an arrow pointing in the direction of the motion. The size of the arrow is proportiional to the speed. So speed is called the magnitude of the velocity.
     
    Last edited: Nov 8, 2012
  6. Nov 8, 2012 #5
    Re: Difference Between Speed and Velocity ??

    Speed and velocity: that is very subtle. In many languages there is only one word, like there is in English only one word for acceleration. Speed can only mean the magnitude, while velocity can mean, like acceleration, either magnitude or vector (magnitude and direction). It has become customary in physics to use velocity exclusively for the vector quantity but this is not universal and was not common in the past. Moreover, translations from for example German tend to use "velocity" as the German word is non-specific. Thus old translations of some German papers by Einstein have "the constancy of the velocity of light", and the implied meaning (by the context) is in this case speed.

    Distance and displacement: that's similar but non-ambiguous: distance is only the magnitude, while displacement has direction.
     
    Last edited: Nov 8, 2012
  7. Nov 8, 2012 #6
    Re: Difference Between Speed and Velocity ??

    why speed can't have direction E.g 44 km/h towards East...??
    also why can't distance have direction... ???
     
  8. Nov 8, 2012 #7

    K^2

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    Re: Difference Between Speed and Velocity ??

    Because speed with direction is velocity. It's just how we name things.
     
  9. Nov 8, 2012 #8

    jtbell

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    Re: Difference Between Speed and Velocity ??

    By definition, speed has only magnitude, and velocity has both magnitude (speed) and direction.

    By definition, distance has only magnitude, and displacement has both magnitude (distance) and direction.

    Sometimes, we need only the magnitude, in which case we use speed and/or distance. Sometimes, we also need to take into account the direction, in which case we use velocity and/or displacement.

    [added] This is according to current usage in English-language textbooks. It was also this way forty years ago when I first studied physics. If the original poster is using a textbook that is a hundred years old or in a language other than English, he should clarify this.
     
    Last edited: Nov 8, 2012
  10. Nov 8, 2012 #9
    Re: Difference Between Speed and Velocity ??

    Latin vs. real English, I guess. When did Gibbs's vector concept really take hold?
     
  11. Nov 8, 2012 #10
    Re: Difference Between Speed and Velocity ??


    My previous post appears to have been deleted, silently, and without explanation. Strange that, eh?
     
  12. Nov 8, 2012 #11
    Re: Difference Between Speed and Velocity ??

    Well, speed could have a direction, but then it would be a vector quantity referred to as 'velocity.' The basic distinction I understand between the two is that velocity is a vector - it is a magnitude and a direction. Speed is the magnitude of that vector.

    Let's consider a particle that's moving along a plane: It is moving in the [itex]\hat{x}[/itex] direction at some km/h and in the [itex]\hat{y}[/itex] direction at some other km/hr. So, what you've got to do to get to a 'speed' is add these to vectors.

    Here's how I think about this, for better or worse: If you draw out the vectors on a sheet a paper, you can see that the velocities in each direction (the vector components) are perpendicular. Looks kinda like a right triangle, where the hypotenuse represents the vector magnitude of the velocity.

    So, how do we find the hypotenuse of a right triangle?
     
  13. Nov 8, 2012 #12

    jtbell

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    Re: Difference Between Speed and Velocity ??

    I still see it. No posts have been deleted from this thead as far as I can tell.
     
  14. Nov 8, 2012 #13
    Re: Difference Between Speed and Velocity ??

    I'm talking about the one before that. The nice thing about where I work is that NOTHING gets deleted.
     
  15. Nov 8, 2012 #14

    jtbell

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    Re: Difference Between Speed and Velocity ??

    I repeat, no posts have been deleted from this thread. As a mentor (moderator), I can see deleted posts. They are not actually physically deleted, merely "hidden" from ordinary members.
     
  16. Nov 9, 2012 #15
    Re: Difference Between Speed and Velocity ??

    In the speed-O-meter of Car, why we called it speed, instead of velocity, while car has direction ????
     
  17. Nov 9, 2012 #16

    Matterwave

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    Re: Difference Between Speed and Velocity ??

    The speedometer doesn't tell you the direction. Only the speed.
     
  18. Nov 9, 2012 #17

    phinds

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    Re: Difference Between Speed and Velocity ??

    Because the direction of the car is totally irrelevant to the reading on the speedometer. If you go in a curve, your speed can stay constant while your direction is constantly changing.
     
  19. Nov 9, 2012 #18
    Re: Difference Between Speed and Velocity ??

    yeh kiya baat hui ?????
    but Car runs in direction.. not in arbitrary point.. then why it has speed instead of velocity...
    velocity is used where, there is direction.. also car runs in direction.. then why car has speed instead of velocity ???? ///\\\
     
  20. Nov 9, 2012 #19

    A.T.

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    Re: Difference Between Speed and Velocity ??

    The car has both. The speedometer shows just speed.
     
  21. Nov 9, 2012 #20

    jtbell

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    Re: Difference Between Speed and Velocity ??

    If you mount a compass next to your speedometer, you now have a device that tells you the car's speed and direction... voilĂ , a velocityometer! :biggrin:
     
  22. Nov 9, 2012 #21
    Re: Difference Between Speed and Velocity ??

    Try " Wolfowitz, velociraptor" for a linguistic clue.
     
  23. Nov 10, 2012 #22
    Re: Difference Between Speed and Velocity ??

    One way to think about it:

    Distance is how far you've moved, ignoring direction. So if you go 5 km north, then 5 km east, then 5 km south, then 5 km east, you've traveled 20 km.

    Displacement is the distance of the shortest path from start to finish, or as the crow flies) including the direction. So in my example above, you'd have a displacement of 10 km east (5 km east + 5 km east, the north and south cancel out)

    Average Speed is distance/time
    Average velocity is displacement/time
     
  24. Nov 12, 2012 #23
    Re: Difference Between Speed and Velocity ??

    what is initial velocity and final velocity ??
     
  25. Nov 12, 2012 #24
    Re: Difference Between Speed and Velocity ??

    Initial means starting. If you start from rest, your initial velocity is 0.

    Final velocity is whatever your velocity is at the end of the time interval.

    For example, if I am going 10 km per hour east and I speed up to 50 km per hour east, my initial velocity is 10 km per hour east, my final velocity is 50 km per hour east. If it took me 10 seconds to speed up, then my acceleration is 40 km/h/10 s or 4 km/h/s east.
     
  26. Nov 18, 2012 #25
    Re: Difference Between Speed and Velocity ??

    Gibbs, the father of vector analysis in physics, seems to have established this tradition of using the words speed and velocity for the scalar and the vector, respectively.

    Gibbs's lecture notes on vector analysis and its use in physics circulated in a small circle since 1881. The publication of "Vector Analysis", the textbook based on his assembled notes (1901), had a huge influence. In the book, Gibbs recommended using the words speed and velocity as follows: Velocity is a vector quantity. Its direction is the direction of the tangent of the curve described by the particle. The term speed is used frequently to denote merely the scalar value of the velocity. This convention will be followed here.

    That recommendation was repeated by a popular physics textbook that was published a few years later: "A Textbook on Physics" by Duff (1909): For clearness such a phrase as 'twenty miles an hour' may be called the statement of a speed, which means the mere magnitude of a velocity without reference to the direction.

    Before Gibbs the distinction between the words speed and velocity was less clear. For example, Maxwell discussed speed, velocity, vectors and scalars in his book "Matter and Motion" (1877). He said: The rate or speed of the motion is called the velocity of the particle, and its magnitude is expressed by saying that it is such a distance in such a time, as, for instance, ten miles an hour, or one metre per second.

    * Vector analysis: a text-book for the use of students of mathematics and physics, founded upon the lectures of J. Willard Gibbs, by Edwin Bidwell Wilson, 1901.
    * Gibbs 1901 Duff 1909 Maxwell 1877
     
    Last edited: Nov 18, 2012
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