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Speeds and vectors

  1. May 22, 2003 #1
    Not sure if this belongs here or in Maths. Just a question about convention and proper use of terms.

    It seems to me that if something is moving, it is moving in relation to something else. So any object at any speed automatically has a velocity, unless the speed is 0 maybe. Is there any situation when you would use the term Speed and it would have nothing to do with Velocity?

    This probably seems dumb, but I require an absolute definition that leaves no gaps.
  2. jcsd
  3. May 22, 2003 #2
    Vector = (magnitude, direction)

    Speed = Magnitude

    Velocity = (Speed, Direction)

    Meaningless content removed
    Last edited by a moderator: May 23, 2003
  4. May 22, 2003 #3
    No. Speed V by definition is simply a magnitude of velocity vector V
  5. May 22, 2003 #4
    Re: Re: Speeds and vectors

    just like you have a velocity vector....you find the Norm of the vector, then it is a speed
  6. May 23, 2003 #5
    you can pick whatever coordinate system you want, to get whatever velocity you want. The only time it matters is when the velocity is relative to some other coordinate system. You can have velocity magnitude without direction, but then it is not velocity in 3-d+t space, it's 1-d.
  7. May 23, 2003 #6
    A clear use of speed rather than velocity is to calculate the forces and power involved with a car travelling at a certain speed with respect to the earth. This is independent to velocity.
  8. May 23, 2003 #7
    Incorrect for forces (force is vector quantity) but correct for power.
  9. May 25, 2003 #8

    But if the frame of reference is the car, then within the other frame of reference, of the earth, if it is travelling at a straight line, then you can compute the forces within the car. The vectors wrt car frame of references are known.
  10. May 25, 2003 #9
    To ADAM: Speed Vs. Velocity

    Get this straight: Speed is just a magnitude with a unit of measurement but lacks direction in other words it's a scaler quanitity. If i say you are travelling @ 75 MPH all i know is you are travelling @ 75 miles per hour relative to the earth but i don't know which direction you are moving relative to the earth. In order for a scaler quantity to qualify for a vector quantity it must have a direction relative to earth or an object (usually frame of reference used is an object in Inertia since it's velocity is constant, that means it doesnot change direction). Earth is not in Inertia since it changes direction every singe moment because it rotates even though it's speed may be constant (also earth's speed is not constant, Kepler's law). Let's say the car is moving east relative to earth. Speed is 75 MPH and Velocity is 75 MPH East.

    When you say an object is moving it means object is moving compared to the relative of observer's body. Actually object can have a velocity of 0 compared to any body in universe: (Object in Inertia at rest) So in reality object can have a velocity of 0. Object at any speed greater than zero has a velocity relative to any object. Object at rest has no velocity because it's velocity is zero and it lacks direction. Velocity is very much preferred in physics because it gives a breif description of a moving body unlike speed. Most physics problems require directions to solve projectile problems so speed is not a good choice. It's highly unlikely a physicist would prefer the term "speed" over "velocity". In dialy life using velocity would be very odd and speed seems to be preferred.
    Last edited: May 25, 2003
  11. May 25, 2003 #10
    Work: W = (Fs)=Fscos([the])
    Power: P= dW/dt=(Fv)=Fvcos([the])
    Last edited by a moderator: May 25, 2003
  12. May 26, 2003 #11
    I would say something moves relative to another object rather than in "relation" to another object. Oh, and yes : speed = scalar, velocity = vector =)
  13. May 26, 2003 #12
    velocity of car from inside car =0
    acceleration of car from inside car =0
    acceleration of gravity from inside car > 0
    acceleration of gravity from outside car >0
    velocity of car wrt earth >0
    as you know, acceleration is absolute. velocity is relative.
  14. May 26, 2003 #13

    If x is a vector, you cannot say


    without giving some indication of the nature of the partial ordering.
  15. May 26, 2003 #14
    you can not say the car is travelling at a straight line
    you should say it travel a constant velocity
    and not constant speed too, because the it can be constant speed but not at the same direction
    and you must consider the gravity ....
  16. May 26, 2003 #15
    why the acceleration of car from inside car =0 !!
    and acceleration of gravity from inside car > 0 ??!!
  17. May 27, 2003 #16
    ok that was a little terse, what I was trying to get at is that inside a car moving at uniform velocity, the only acceleration measurable by accelerometer (any pendulum or spring) will be the acceleration of gravity. I was breaking it up into componants, but you certainly can add the vectors for the 3-d vector. Acceleration by changing velocity vector is equivalent to acceleration of gravity, and as a corrolary, acceleration is absolute, not relative.
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