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Homework Help: Acellertaion and Velocity Help

  1. Oct 9, 2008 #1
    Acellertaion and Velocity Help!!

    Setting a world record in a 100 m race, Maggie and Judy cross the finish line in a dead heat, both taking 10.0 s. Accelerating uniformly, Maggie took 2.00 s and Judy 2.80 s to attain maximum speed, which they maintained for the rest of the race.
    (a) What was the acceleration of each sprinter?

    (b) What were their respective maximum speeds?

    (c) Which sprinter was ahead at the 6.00 s mark?

    (d) By how much was that sprinter ahead?

    ok..so i've talked to like 10 million people..and nobody seems to know how to do it..

    i have no idea where to start...i have to find the acceleration first...im just not sure if their initial velocities would be 0 or what?
  2. jcsd
  3. Oct 9, 2008 #2
    Re: Acellertaion and Velocity Help!!

    Well , since the acceleration is uniform , meaning constant, you would apply the kinematic equations for position, velocity and acceleration.
  4. Oct 9, 2008 #3
    Re: Acellertaion and Velocity Help!!

    yea thats exactly what i thought!

    but thats the thing..i dont know which equation to start with

    i thought i would use the position equation
    X final = Xinitial + Vinitial(t) + 1/2(a)t^2

    and solve for acceleration

    but i dont know what the initial velocity is..or how to find it
  5. Oct 10, 2008 #4
    Re: Acellertaion and Velocity Help!!

    The way I approached this problem is to first consider one runner's graph of velocity in relation to time.

    Since there's a uniform acceleration to a final velocity, the graph will have two phases. The first phase will be the acceleration phase which is a constant slope up to Vf (final velocity). The second phase will be a horizontal slope at V=Vf all the way until t=10.

    The area under that graph is the distance covered and thus must be = 100.

    For Maggie, you know the area of the rectangle in the second phase is Vf*8 (the number of seconds she spent at Vf). The first phase's area will be the area of a triangle because of the acceleration. That area will be (1/2)*Vf*(2).

    Thus, (1/2)*Vf*2 + Vf*8 = 100
    This gives you the final velocity. You can find accceleration from there, and so on.
  6. Oct 10, 2008 #5
    Re: Acellertaion and Velocity Help!!

    Common sense,, the intial velocity is 0 saying as they havent started running yet.
  7. Oct 10, 2008 #6
    Re: Acellertaion and Velocity Help!!

    A slightly simpler solution:
    Take one of the runners, for example the first one, and write the equation for the distance traveled (total):

    d=1/2 a1 *t1^2 + vmax*(t-t1)

    Then you have
    Solve for a1;

    Repeat for the other runner.
  8. Oct 10, 2008 #7
    Re: Acellertaion and Velocity Help!!

    o thanks a lot

    which sprinter was ahead at 6.00 s?

    i think i use the position function...but i dont know what to plug in
  9. Oct 11, 2008 #8
    Re: Acellertaion and Velocity Help!!

    Plug in the values for accelerations. Put t=6 seconds. Calculate the distance for each one.
  10. Oct 11, 2008 #9


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    Homework Helper

    Re: Acellertaion and Velocity Help!!

    You need to first determine the distance each sprinter covered for each of their acceleration phases.

    x = 1/2*a*t2

    Then you figure the additional distance covered at their top speed with the remaining time budget to 6 seconds.
    As before you know their top speed was a*t of the acceleration phase.
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