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Distance vs. Displacement?

  1. Jun 12, 2010 #1
    What the book says: Displacement is the shortest distance between two points, and can be positive or negative.

    Question: What about when you're working with two axes? To find the point you use the pythagorean theorem... making displacement always positive. Is this related to magnitude?


    Distance is...? I know that it measures every step between two points (not the shortest distance like displacement), but as for sign, I'm guessing that it is always positive because Speed = Distance/ Time and Speed is always positive.
     
  2. jcsd
  3. Jun 12, 2010 #2
    There are various definitions but one that relates to what you have said is that
    "displacement is distance travelled in a particular direction".
    Speed is defined in relation to distance, and velocity in relation to displacement.
    This means that displacement and velocity are vectors, and thus have direction, whereas speed and distance have only magnitude.

    If you walk half way around a circle you displacement would be 2r (in a particular direction) but the distance travelled would be πr.
    If you walk all the way round your distance travelled would be 2πr and your displacement zero. Your average speed and velocity would also be different for this reason.
     
  4. Jun 12, 2010 #3

    jtbell

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    Staff: Mentor

    This is a for a one-dimensional system, where the motion is back and forth along a line.

    In two or more dimensions, displacement is a vector quantity. It has components along each of the axes. (Actually, even in one dimension we can speak of displacement as being a vector that has only one component.)

    If your textbook eventually talks about vectors, it will probably talk about vector displacements.
     
  5. Jun 12, 2010 #4
    What I've gathered:

    Speed = Distance/ Time

    Velocity = Displacement/ Time

    Magnitude = length
    Direction = positive or negative

    Alright, so if a particle travels 10 meters to make a complete circle in 5 seconds, the speed is 2 m/s, while the velocity is 0 m/s?

    Is acceleration speed/ time or velocity/time?

    In addition: they represent speed with the letter V? Really? What do they represent Velocity with?
     
    Last edited: Jun 13, 2010
  6. Jun 12, 2010 #5

    jtbell

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    Staff: Mentor

    For one-dimensional motion, yes. For two-dimensional motion you need an angle, for example 35 degrees north of east.

    I would say "average speed" and "average velocity." The instantaneous speed may vary at different points on the circle. The instantaneous velocity must vary at different points on the circle, because the direction changes from one point to the next.
     
  7. Jun 13, 2010 #6
    My book tells me that acceleration is "how fast your speed changes". Is it speed or velocity?
     
  8. Jun 13, 2010 #7

    jtbell

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    Staff: Mentor

    Velocity. If you keep your speed constant but change your direction of motion, you accelerate. An example is circular motion at constant speed (uniform circular motion). You have an acceleration whose direction is always towards the center of the circle, and some force must produce that acceleration via F = ma.
     
  9. Jun 13, 2010 #8
    Velocity uses displacement/ time, Speed uses distance/ time, but when measuring instantaneous speed or velocity, they're basically equal aside from the fact that velocity has a direction.
     
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