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Homework Help: Average Speed vs. Average velocity

  1. Sep 24, 2006 #1
    Do you think that average speed and average velocities are usually the same for something in motion?
  2. jcsd
  3. Sep 24, 2006 #2


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    Generally, they are synonyms, but velocity is usually used in the context of physics, I guess.
  4. Sep 24, 2006 #3
    Ya I mean in Physics.

    Are they the same? what are the differences?
  5. Sep 24, 2006 #4


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    I guess there's no difference, except that you usually don't use the term 'speed' in physics.
  6. Sep 24, 2006 #5

    I have one more question, to find the instantaneous speed between lets say 0 and 3 seconds using a distance vs. time graph. I first draw the line fo best fit then I would find the slope right?
  7. Sep 24, 2006 #6
    It is like comparing apples and pears, they are completely different. One is a scalar the other is a vector quantity.
  8. Sep 24, 2006 #7
    Actually, the average speed is the [tex]\frac{total distance travelled}{total time taken}[/tex], while average velocity is [tex]\frac{total displacement}{total time taken}[/tex]. Remember, the displacement can be zero when the distance is not.
  9. Sep 24, 2006 #8
    ok so i'm confused now, when we're talking about something in MOTION is the average speed and the average velocity usually the same?
  10. Sep 24, 2006 #9


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    Hm, could you clarify what you meant by that?
  11. Sep 24, 2006 #10
    It is possible to have an average velocity of zero, for example.

    (You travel at 5 m/s for 2 sec and then you travel at -5 m/s (backwards) for 2 sec)

    ...and not so with speed (it is a scalar quantity).
    Last edited: Sep 24, 2006
  12. Sep 24, 2006 #11


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    I know, but I still don't understand the statement above. Nevermind.
  13. Sep 24, 2006 #12
    I wrote:

    "I do think that average speed and average velocity are usually the same for something in motion because it's still measuring time. The only diffference is that when calculating the velocity, you're calculating the rate at which the object changes it's postition."

    does that sound about right?
  14. Sep 24, 2006 #13
    Went offline for some time...What exactly did you not understand?
  15. Sep 24, 2006 #14
    Speed is a scalar quantity. If we designate forwards as positive movement and backwards as negative movement. I can run back and forth at 1m/s and arive where I started and my speed would still be 1m/s.

    Now if we are using velocity, it is a VECTOR quantity. This means that you need to indicate MAGNITUDE and DIRECTION.

    If you were to run 30,000 miles forward and then 30,000 miles backwards in 3 hours, your speed would be [tex]v=\frac{60000miles}{3hours}=\frac{20000miles}{hour} [/tex]

    BUT if you were to give velocity... [tex]\vec{v}=\frac{(30000miles)+(-30000miles)}{3hours}=\frac{0miles}{3hours}=\frac{0miles}{hr} [/tex]

    Get it?

    Notice the arrow above v to designate whether it is a scalar quantity of a vector quantity.. [tex] \vec{v}=velocity [/tex] [tex] v=speed [/tex]
    Last edited: Sep 24, 2006
  16. Sep 24, 2006 #15


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    If you are so 'aware' of the difference between vector and scalar quantities, then you should be more careful when writing equalities. :biggrin:
  17. Sep 24, 2006 #16
    Yes, Thanks I get it.

    But the question is really confusing, are they usually same or not?
  18. Sep 24, 2006 #17
    If you're moving with a constant velocity (speed is constant, direction is constant), then the magnitude of average velocity = average speed. In such a case, the dist-time graph will always be a straight line.
  19. Sep 24, 2006 #18
    In every day conversation, if someone were to ask you what velocity your ride can get you to school and back, and you're school is 200m away from your home, they are confusing speed with velocity. Lets say your car can get to school and back home in 10 minutes, well the velocity is [tex] \vec{v}=\frac{200m+(-200m)}{10min}=0m/min [/tex] This would be correct, but obviously they are asking for speed.

    While in school, I would say that velocity and speed are NEVER the same past grade 10. If a question asks for the velocity, you must take vectors into account as giving the speed will be wrong! Speed is NOT velocity, even though many people whom forget about physics class assume it is :P Get it?
    Last edited: Sep 24, 2006
  20. Sep 24, 2006 #19
    Even if you are travelling in a straight line in a forward direction at 10m/s, your velocity would be +10m/s while your speed is 10m/s... Small differance, but one indicates the direction, the other does not.
  21. Sep 24, 2006 #20
    lol ya thanks for trying.

    I'm in grade 10, so this is all new to me. We're learning how to calculate average speed/velocity and instantaneous speed/velocity using secent and tangent methods.
  22. Sep 24, 2006 #21
    I forget how to calculate the instantaneous velocity and speed using a graph.
  23. Sep 24, 2006 #22
    Hehe, you do that through calculus :)

    By "tangent methods" do you mean differentiation? Have you ever heard the term derivative? I am in grade 12 and am just learning about it now :P

    But to calculate the ALMOST instantaneous speed using a graph, simply draw a secant from one point to a point fairly close and estimate the slope. I think thats about as close as you can get without using calculus.

    Just remember these definitions, they are right out of my physics book.

    vector : A quantity, such as velocity, completely specified by a magnitude and a direction.

    scalar : A quantity, such as mass, length, or speed, that is completely specified by its magnitude and has no direction.

    If you take physics 20 next year, you will learn plenty about vectors and scalars, :).
  24. Sep 24, 2006 #23
    Thanks for the definitions, I wrote them down lol

    i got the instantaneous speed, how would calculate the instantaneous velocity? Which points do you use?

    The question asks "What is the instantaneous velocity between 0 and 3 seconds?"

    To get the speed I i used rise/run (line of best fit) got 2/3 = 0.7 km/min

    which points do i use to find the instantaneous velocity?
  25. Sep 24, 2006 #24


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    Last edited by a moderator: Apr 22, 2017
  26. Sep 24, 2006 #25
    I have not yet seen a vector where the orientation was declared. But anyways.
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