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Homework Help: Average speed vs average velocity

  1. Mar 21, 2010 #1
    1. The problem statement, all variables and given/known data

    (a) A particle has a displacement r1 = (35.0i + 12.0j + 14.0k) m at 29.0 s. At 53.0 s the ball has a displacement of r2 = (35.0i + 6.0j + 11.0k) m.

    What is the average speed of the ball between 29.0 s and 53.0 s?


    3. The attempt at a solution

    Having checked the answers, I already know that the average velocity of the ball between 29.0 s and 53.0 s is:

    [tex]0i-0.25j-0.125k[/tex].

    The answers suggests that the average speed of the ball is 0.28 m/s. How do I obtain this answer?

    I got the average velocity using the formula [tex]v_{x, avg} = \frac{\Delta x}{\Delta t}[/tex] where x is the displacement. I think to find the average speed I must use [tex]v_{avg} = \frac{d}{\Delta t}[/tex] where d is the total distance traveled. But I don't understand why average velocity is in 3 components whereas the average speed is just a single number!
     
  2. jcsd
  3. Mar 21, 2010 #2
    Speed is a scalar quantity. Velocity is a vector, or course.

    What is the length of the vector [tex]0\hat{i}-0.25\hat{j}-0.125\hat{k}[/tex] ?
     
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