Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Speed Has Direction

  1. Nov 21, 2004 #1
    Many physics books imply that in classical physics speed has no direction component. This is usually implied first by not describing speed having a direction component. Secondly, velocity is described by stating that speed is the maginitude of velocity, plus a direction component. I think this is misleading.

    Speed is and has direction. Speed is magnitude in the x direction. Velocity is merely magnitude in the x, y, z direction.

    What do you think?
  2. jcsd
  3. Nov 21, 2004 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    Dearly Missed

    Speed is a non-negative scalar.
  4. Nov 21, 2004 #3

    Doc Al

    User Avatar

    Staff: Mentor

    Where did you get this idea? What if something was moving at 5 m/s in the y direction---by your idea it would have no speed?

    By definition speed differs from velocity. Speed is "how fast"; velocity is "how fast and in what direction".
  5. Nov 21, 2004 #4
    No, velocity is a vector be definition. Its magnitude is sqrt ( x^2 +y ^2 +z^2). Its velocity has components in the x,y,z direction, not magnitudes. Speed is by definition, sqrt ( x^2 +y ^2 +z^2), or, the magnitude of the velocity.

    Speed is not in the x direction. Speed is a combination of the direction of the three coordinates, x,y, and z. The only time speed is in the x direction is if thats the only direction of movement.
  6. Nov 21, 2004 #5
    By convention and definition, velocity is the vectory quantity (which obviously gives direction) and speed is a scalar quantity (the magnitude of the velocity vector).

    You are certainly free to come up with your own definitions. Just expect much confusion when it comes time to discussing things with other physicists. It will also likely get you very quickly dismissed by those scientists as someone who doesn't know what they're talking about.
  7. Nov 21, 2004 #6

    If you say in the y direction only, then it only moves in the y direction. That would be perfect vertical on the coordinate plane. Zero in the x direction and zero in the z direction. But of course, it would need to be exaxty over your head, because anyone stand around you would have relative x and z coordinates forced upon them.

    A magnitude needs a line. A line suggests direction in one dimension. The direction is along itself. One point, indefinite is zero dimension. Any point along the line is zero dimension. Because a point is indefinent, it cannot have direction. But, the line that is between two points is one dimension. Once the line comes to be, direction then manifests and I think is the line.

    When a tourist asks me about a place nearby that I can point to with my hand, I point in a linear direction. Directions couldn't be understood with a single dimensionless point, but a line between points. From my extended arm and point of tip of finger to the object the tourists seeks, I manifest the line. I gave the tourist direction(s). I gave them a line.
  8. Nov 21, 2004 #7
    A magnitude does not need a line. For example,,temperature can increase and decrease in magnitude but has no defined direction ,, it is a chacteristic at a given point.

    When you are traveling in a circle at a constant speed, there is no "line" involved as your direction is constantly changing (meaning your velocity is changing) but there is no change in the magnitude(speed) you are traveling. Does that make it any more clear?
    Speed is one component of velocity,,direction is the other component. Any change in either component is represented as acceleration.
  9. Nov 21, 2004 #8

    Doc Al

    User Avatar

    Staff: Mentor

    Sorry. I can't make any sense of that, or see the relevance to the distinction between speed and velocity.

    Are you able to see the difference between these two statements?
    (a) "The car is moving at 50 miles/hour"
    (b) "The car is moving at 50 miles/hour east"
    The first illustrates speed; the second, velocity.

    When giving directions to that tourist you are indicating a displacement, which is a vector quantity just like velocity. If, on the other hand, you just said "it's 1 mile away" you'd be giving the distance, a scalar quantity just like speed. Both displacement and distance are useful, but distinct, concepts.
  10. Nov 22, 2004 #9
    Temperature is commonly physically indicated by expansion or contraction of a substance in a cylindrical container. The cyclinder represent the an axis in which the magnitude moves along (one dimension). If the container where planer, then it would be a two dimensional magnitude. If the container were a sphere, such as a balloon, the magnitude would be in three dimensions.

    You don't have a change in speed or direction. A change of direction is always a change in magnitude or a change in magnitude is always a change in direction. Direction is magnitude along one of the xyz axis', otherwise you couldn't say xy or z direction.
    Last edited: Nov 22, 2004
  11. Nov 22, 2004 #10


    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    I think you have a very strange way of understanding what a "vector" is and what a "scalar" is. It appears that you a confusing the GRAPHICAL way to represent a vector (line length to represent the magnitude and it's orientation as a direction) with being an actual representation itself. You will have loads of problems, especially if you are going to be enrolling in more physics classes, if you continue to think this way.

    Is there a reason why you are redefining "speed"? I mean, if I make up a word and define it in a certain way, what makes you think you can adapt my word and redefine it anyway you like? Everyone who has responded to your claim has disputed your understanding of what "speed" is. It is DEFINED as the scalar of the velocity. Period! You and I are not free to negotiate its definition and make up our own. It is how we communicate with each other by understanding and agreeting to the "pre-ordained" definitions of a set of words and phrases.

    If you wish to adopt your definition, give it a different name. It certainly cannot use the word "speed", since that has a clear definition already (which everyone in this thread has adopted except you).

  12. Nov 22, 2004 #11

    Tom Mattson

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    This is nonsense.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook