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Thread: Velocity vs. speed

  1. #1 Velocity vs. speed 
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    Hi everyone!

    I'm a translator from Spain and I am translating a text about pumps from Spanish into English and I have a big problem with the terms velocity and speed. I know they're not exactly the same thing, but I'd like to know if scientists use both terms as synonyms when talking about pumps or machines in general. I mean, does it make any difference to say, for example, "at high speeds" or "at high velocities"? After reading a lot I think that sometimes the difference is important: is not the same linear velocity than linear speed, but what do you say when you are speaking in general?

    I'd really appreciate it if someone would answer! :-D


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    It doesn't at all. Speed and velocity are synonyms. Velocity relates to mass and metrics though.


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    Despite what the last poster said, speed and velocity are not synonymous.

    Velocity is speed and direction . Thus while you can say that a car's speed is 70 kph, to give its velocity you also have to give the direction it is moving, such as 70 kph East. Thus 70 kph East is a different veloctiy than 70 kph North even though the speeds are the same.

    This becomes important when dealing with acceleration, because acceleration is a change in velocity. This can mean a change in speed, direction or both.

    An example would be an object traveling in a circular path at a constant speed. Even though its speed does not change, the direction it is moving is constantly changing. Thus it is in a constant state of acceleration.
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    He's right. But for your purposes velocity and speed are effectively the same. Remember velocity x mass=force and in that equation you don't say 70kph east. You would be correct in saying 75kph constant velocity or speed. Janus is right but he is splitting hairs basically.
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    Moderator Moderator Janus's Avatar
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    Quote Originally Posted by the man of science
    He's right. But for your purposes velocity and speed are effectively the same. Remember velocity x mass=acceleration and in that equation you don't say 70kph east. You would be correct in saying 75kph constant velocity or speed. Janus is right but he is splitting hairs basically.
    Velocity x mass = momentum, not acceleration, and momentum does have a direction.
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  7. #6  
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    Quote Originally Posted by the man of science
    He's right. But for your purposes velocity and speed are effectively the same. Remember velocity x mass=acceleration and in that equation you don't say 70kph east. You would be correct in saying 75kph constant velocity or speed. Janus is right but he is splitting hairs basically.
    So, have you ever heard the expression, 'c' is the velocity of light?
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    Quote Originally Posted by the man of science
    Remember velocity x mass=acceleration
    TMOS - Go away, troll. This person has come here with a serious question. We don't need any of your shenanigans.

    In response to the original question, velocity is a vector and speed is a scalar. However, the word velocity is often used when speed is meant. This would be quite common in a discussion about pump performance, where there would be no potential for confusion between vectors and scalars. The pump performance parameters are scalars.

    If you are translating, I think it would be safest to use "speed" if you know you are discussing a scalar quantity.
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  9. #8  
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    Thank you very much for all your answers!

    As Harold14370 says, the problem is to apply velocity and speed to pumps. In spanish we have just one word with two meanings so we have no problem. And as I'm not a sicentist it is really difficult for me to know if the author of the text about pumps is referring to a scalar quantity or not :?

    So, the best thing to do is translate it as speed, isn't it?
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    Wait did I seriously say acceleration I meant force. I feel like an idiot. Screw you Harold and thanks for affirming the point I was making.
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  11. #10  
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    The man of science, you don't seem to have any problem talking authoritively about things you know obviously little about and then insisting upon them even when you were quite obviously wrong. It is really annoying and you are coming across as a troll. It is not appreciated.

    lara:

    Yes, as harold said, speed would be the most appropriate word.
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    Quote Originally Posted by lara
    So, the best thing to do is translate it as speed, isn't it?
    Well... what I would say is that it would be technically corrrect, but might look funny depending on the context.

    Here is an example of some technical writing about pumps.
    http://www.tpub.com/content/doe/h101...h1012v3_70.htm
    Notice that they refer to flow "velocity" when they are actually discussing a scalar quantity. The word "speed" is used when discussing the rotational speed of the pump.
    The V graph assumes one particular speed (N) for the pump impeller. When the liquid being pumped enters the eye of a centrifugal pump, the pressure is significantly reduced. The greater the flow velocity through the pump the greater this pressure drop.
    Sorry for the wishy-washiness of my answer.
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  13. #12  
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    I found this link
    http://www.springerlink.com/content/v41kml5781003746/

    which is a translation on centrifugal pumps, from Russian - I think.

    The author uses the same principle Harold has suggested. Speed to refer to how fast the pump operates, i.e. number of strokes per minute. Velocity refers to the 'speed' of the fluid being pumped.
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