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Homework Help: Concept of avg speed

  1. Oct 4, 2004 #1
    ok i got back my test today and its some basic stuff, but i don't get the difference between AVG speed and AVG velocity.

    One of my problems was 'The length of the barrel of a blowgun is 1.2 meters. Upon leaving the barrel, a dart has a speed of 14 m/s. Assmuing that the dart was uniformly accelerated, how long does it take for the dart to travel the length of the barrel?' and for the answer I got .086s but the correct answer was .17 s

    Can anyone explain to me how the teacher got this?

    because all i did was 1.2/14 but to me it seems my teacher did 1.2/7 but why?

    And for the second part 'A skate boarder accerates from rest to 26 m/s in 10 seconds. His avg speed over this time interval is ?.

    The answer is 13 m/s but i have not a clue how he got this

    Can someone explain to me?

    Thanks in advance
  2. jcsd
  3. Oct 4, 2004 #2

    Doc Al

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    Staff: Mentor

    You used the final speed of the dart as it left the barrel. But the speed while it's in the barrel goes from zero (it starts from rest) to the final speed of 14 m/s. For uniformly accelerated motion, the average speed is [itex]v_{ave} = (v_i + v_f)/2[/itex]. So the average speed is only 7 m/s.

    Same idea.
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