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Velocity vs Acceleration

  1. Sep 7, 2008 #1
    What is the difference between average acceleration and average velocity?
  2. jcsd
  3. Sep 7, 2008 #2

    Doc Al

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    Staff: Mentor

    Velocity and acceleration are two different quantities. Velocity is the rate of change of displacement; acceleration is the rate of change of velocity.
  4. Sep 7, 2008 #3
    So can an object have the same average velocity and average acceleration?
  5. Sep 7, 2008 #4


    Staff: Mentor

    No, the units are different. You might have an object with an average velocity of 1 m/s and an average acceleration of 1 m/s², but those are not the same despite both having the number 1. Just like 1 orange is not the same as 1 apple.
  6. Sep 7, 2008 #5
    An athlete starts at point A and runs at a constant speed of 6.0m/s around a round track 100m in diameter. find the x and y components of this runners average velocity and acceleration between points a, b, c, d (equally apart on circle).

    So there is no acceleration because the speed is constant?
  7. Sep 7, 2008 #6


    Staff: Mentor

    Both velocity and acceleration are vector quantities. In other words they have a magnitude and a direction. If you change the direction of your velocity you have accelerated even if you have not changed the magnitude of your velocity (speed).
  8. Sep 7, 2008 #7
    ah....okay I see....thanks, i will start working on this problem!!
  9. Sep 7, 2008 #8
    So...tell can you please tell me if I am on the right track?

    A = (-50, 0) B = (0,50) C = (50, 0) D = (0, -50)

    Circumference = (pi * r^2) = 7854m

    time between points = 7854m / 6.0m/s = 1309 s

    X components:
    Vax, A = (0-50m)/1309s = -0.038 m/s

    Aax, A = (0-50m/s)/1309s = -.038m/s/s
  10. Sep 7, 2008 #9

    Doc Al

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    Staff: Mentor

    Careful: You found the area instead of the circumference.

    Correct this. Also: The points are spaced 1/4 circumference apart.

    To find the average velocity between points A and B, for example, first find the displacement between A and B. (It's a vector that points from A to B.)
  11. Sep 7, 2008 #10
    oops...okay so the circumference is 314m.

    Time between points is 314m / 6.0m/s / 4 13.0s between each section.

    so for A - b

    displacement Ax = (0-50m)/13s = -3.84 m/s
    displacement Ay, A = (50-0)/13s = 3.84 m/s

    VA = sq rt[(-3.84^2)+(3.84^2)] = 5.43 m/s

    Aax = 3.84m/s^2 Aay = -3.84m/s^2
  12. Sep 7, 2008 #11


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    Homework Helper

    Hi nwells1998,

    I don't believe this is right for the displacement, even though some of your numbers are right. Let's say point A is 50m to the west of the circle and point B is 50 m to the north of the circle. To find the displacement, you need to know how far B is from A, and also what direction B is from A. What would those answers be?

    Then the average velocity is found from:

    average velocity = displacement / time

    The average acceleration is (change in velocity)/(change in time). To answer that, you need to know what the velocities are at A and B. What are those?

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