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Please help Easy motion question

  1. Sep 23, 2009 #1
    Please help!!! Easy motion question!!!

    1. The problem statement, all variables and given/known data
    Please explain these steps to me. My physics teacher went too fast :(

    v=a*t
    d=v*t
    d=(a*t)*t
    d=at2
    Vaverage=Vf/2
    =1/2Vf
    d=1/2Vf*t
    d=1/2at2

    Thank you for your help

    2. Relevant equations
    v=a*t -> d=1/2at2


    3. The attempt at a solution
    N/A
     
  2. jcsd
  3. Sep 23, 2009 #2
    Re: Please help!!! Easy motion question!!!

    Hi kevinli , welcome to PF ^^

    first equation (v=a*t) is for Velocity = acceleration * time

    in units = (m/s²)*(s) = (m/s) which is the SI unit for Velocity ^^


    the second one (d=v*t) is for distance = velocity * time

    in units = (m/s) * (s) = (m) which is also the SI unit for Distance


    In this equation (d=(a*t)*t) he just substitued v with (a*t)

    and he got this one (d=at2)



    Here in this equation (Vaverage=Vf/2) i think he assumed Vi = 0
    the initial velocity = 0 so the average Vf-Vi=Vf and divide by 2 ^^


    He used the average velocity = (1/2)Vf in this equation d=v*t Instead of v

    So he will get d= (1/2)Vf * t

    and from first equation v=a*t

    Or we can say Vf=a*t
    we get

    d=(1/2)at²

    And finally i want to tell U that these equations will help U In free fall or projectiles
    I hope i have explained what U want , & hope that u get it well now if it is what u want to know ^^
     
  4. Sep 24, 2009 #3

    Redbelly98

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    Re: Please help!!! Easy motion question!!!

    Welcome to PF kevinli :smile:

    What is v here? We talk about average, initial, and final velocity. Which velocity is it?

    That should read
    d = Vaverage*t​

    The above 2 equations are wrong.

    Look at these last two equations. If they are true (which they are), that implies that
    Vf = ____?​
     
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