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Homework Help: Calculating constant velocity, deceleration, and total displacement

  1. Jun 24, 2012 #1
    1. The problem statement, all variables and given/known data

    An object starts at rest and accelerates at 2.5 m/s^2 for 12 seconds. It then moves at a constant velocity for 5.0 seconds and then slows down uniformly at 1.5 m/s^2 until it stops. Calculate:
    a) the constant velocity
    b) the time taken for deceleration
    c) the total displacement

    2. Relevant equations

    a=v/t or any of the five equations for uniform acceleration

    3. The attempt at a solution
    I know how to do part c) but i need the first two answers which i'm not sure how to use.
    For a) i don't know which method to use: a=v/t or some kinematic equation, or which acceleration to use or which time (the 12 seconds while its accelerating, or the 5 seconds for constant velocity.
    For b) i don't know how to do either because it i based on part a)
  2. jcsd
  3. Jun 24, 2012 #2
    Hi pamelajanas !!
    Welcome to PF !!

    Hints :
    Employ the first equation of 1-D (uniform acceleration) motion to solve for constant velocity, part (a). Also note that a = Δv/Δt and not v/t.
    Consequently you will know which kinematic equation to apply to solve for (b) and (c).

    Get on it !! :smile:
  4. Jun 24, 2012 #3
    okay so using the first equation for part a) I would have to rearrange it in order to find v2?
    v1 = 0 m/s (which makes sense)
    Δt= 12s or 5s (12s, which is the time for the acceleration or 5s, which is the time for the constant velocity)
    Δd= .. i am not even sure

    and i DID mean the a=Δv/Δt, which i do or do not have to use?
  5. Jun 24, 2012 #4
    i'm still not sure how to do this.. :(
    could you please look just above this, which are the reasons why i do not understand this
    Last edited: Jun 24, 2012
  6. Jun 25, 2012 #5
    First equation of 1-D motion is :


    You are to find v and you are given u and t. (The object starts from rest.)
    To reach the velocity v via acceleration you're given time as 12 seconds.
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