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Homework Help: Average speed, etc.

  1. Sep 15, 2007 #1
    1. The problem statement, all variables and given/known data
    A bullet leaves the muzzle of a gun at a speed of 400 m/s. The length of the gun barrel is 0.50 m. Assuming the bullet was uniformly accelerated, (a) what was it's average speed inside the barrel? (b) how long was the bullet in the gun after it was fired?

    2. Relevant equations
    (b) d= 1/2 at^2 or t^2 = 2d/a

    3. The attempt at a solution

    d= 0.50 m ........ vo=400 m/s ....... v= ????

    I don't know how to find the velocity!

    I can't figure out how to solve the problem if I don't have the velocity, for (b) and No clue how to find the average speed (distance/time) if I don't havea time variable. Please help me!
    Last edited: Sep 15, 2007
  2. jcsd
  3. Sep 15, 2007 #2
    Try working with Vf^2 - Vi^2 = 2ad
  4. Sep 15, 2007 #3
    Want to tell me what those variables stand for? Thanks.

    (V, I don't have the velocity, f? wtf does that mean? i? wtf? I can't even get the acceleration b/c i don't know the velocity.
  5. Sep 16, 2007 #4
    I guess I'll tell you what those variables mean, even though you should already know them if you've so much as opened your physics textbook.

    Vf = final velocity
    Vi = initial velocity
    d = displacement

    Now you have Vf, Vi, and d, so you now have the acceleration. Because you have the acceleration, you can then find the time. Because you have the time, you can then get the average velocity.

    Is that simple enough for you?

  6. Sep 16, 2007 #5
    Those variables arn't the same in my textbook. Unfortunately, my book was copyrighted in 1992, I use v as the final velocity, Vo as the final d, dunno what you mean.


    I can't even do VECTORS.
  7. Sep 16, 2007 #6
    Don't give up. It will all really make sense once you find a way to relate it to what you already know. You intuitively know that a = (Vf-Vi)/t, etc...

    Just don't let the variables overwhelm you; they are really not something that someone just decided to make up one day. They really make sense, if you take the time to figure out why and how they relate.

  8. Sep 16, 2007 #7
    Can someone relate the numbers that go with the variables? That way it'll make the problem easier. I still haven't been able to figure this out, and it's killing me.

    My uncle told me...

    d= Vot, which algebraically would be distance/velocity

    so i got .5/200=.0025 but for some reason I don't think that's right

    for a) i got 200 m/s, because the starting speed is 0 m/s, the final is 400 m/s, my uncle, who is a science major and is a science coordinator for k-12, told me to divide it by 2, so i got 200 m/s. is that correct?
    Last edited: Sep 16, 2007
  9. Sep 16, 2007 #8
    For your info, I'm not asking for the answer, just numbers to go with the variables. Then I can answer it myself.

  10. Sep 16, 2007 #9


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    Staff Emeritus
    Science Advisor

    Unfortunately, different texts/people use different symbols, but CaptainZappo is on the right track.

    This is a helpful reference - http://hyperphysics.phy-astr.gsu.edu/hbase/mot.html#motcon
    Look at the plate "Description of Motion in One Dimension"

    For constant acceleration, a, and initial velocity v0, the velocity at distance d is given by

    v2 = v02 + 2 * a * d

    The average velocity is simply the algebraic average (v + v0)/2.

    I think one can find time to leave or travel down the barrel know the distance and the average velocity (or speed).
  11. Sep 17, 2007 #10
    Ehh. My head hurts.

    I'll ask my teacher, maybe he'll be able to explain it to me.
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