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Piecewise Linear Question

  1. Sep 11, 2008 #1
    1. The problem statement, all variables and given/known data
    An ice sled powered by a rocket engine starts from rest on a large frozen lake and accelerates at +11.0 m/s2. At t1 the rocket engine is shut down and the sled moves with constant velocity v until t2. The total distance traveled by the sled is 5.30x10^3 m and the total time is 81.0 s.

    Find t1, t2, and v.


    a= +11.0 m/s^2
    deltat= 81.0 s
    deltax or d= 5300 m
    t1= ?
    t2= ?
    v= ?

    2. Relevant equations
    well, i tried to use:

    1. d= Vi*t + .5at^2
    2. d= Vi + at^2
    ^ I'm not sure if those are right to use.

    3. The attempt at a solution
    Okay, well what I did first was this:

    I used the first equation listed and put in everything I knew, which ended up with this:
    d= 5.5t^2

    Then, I used the second equation to also solve for d.
    d= 11t

    I know the entire distance is 5300m, so I then did:

    5300= 5.5t^2 + 11t

    however, i'm not exactly sure as to where i can use the deltat of 81.0s. I know that I'm going to have to use the quadratic forumla from here, but I don't know what to do. I was thinking of doing something like this:

    5300= 5.5t^2 + 11t (81-t)

    But, I'm not sure that is accurate.

    I'm stuck right now as of what to do. Help would be muchh appreciated. Thanks!
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Sep 12, 2008 #2


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    Homework Helper

    Hi avabby,

    This second equation is not true.

    The first equation is right, but there are several more kinematic equations that would be helpful. What are they?

    Your approach is not working because it looks like are trying to describe the motion all at once. However, here the acceleration is changing: first the sled has an acceleration, and later it moves with zero acceleration. The kinematic equations you are using are for constant acceleration only, so you will need to split up the motion into two parts and treat them separately. (For example, find an expression for the distance the sled travels while it is speeding up, and find a separate expression for the distance travelled while it is moving at constant speed.) What do you get?
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