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Kinematics question about a runner

  1. Sep 26, 2015 #1
    1. The problem statement, all variables and given/known data
    A sprinter can accelerate with constant acceleration for 3.00 s before reaching top speed. He can run the 100-meter dash in 10 s.

    What is his speed as he crosses the finish line?

    2. Relevant equations

    3. The attempt at a solution
    So I broken up the question into two parts, one where he has constant acceleration, the other where he has constant velocity.

    idk where to go from there, can someone point me in the right direction?
  2. jcsd
  3. Sep 26, 2015 #2
    You're approach to breaking the problem up into two parts is a good approach. You know that the first part of the spring takes three seconds, while the second portion at the top speed takes seven seconds. One approach would be to set the total distance to be the sum of the distance traveled under the constant acceleration part, and the distance traveled under the constant velocity part. This leaves you with determining the acceleration. This can be done by using the average velocity formula, (since the r.h.s. involves the quantities you either know or want), and substitute that in to the formula that relates the squares of the velocities, distance, and acceleration. Sub in the distance, solve for a, and then plug into your distance formula. Then rearrange for vf and that should do it.
    Last edited: Sep 26, 2015
  4. Sep 26, 2015 #3
    Technically in both parts the acceleration is constant.

    Can you try and apply one of those kinematic equations you listed above?
  5. Sep 27, 2015 #4
    Thank you both for taking the time to reply! Your instructions led me in the right direction. I took the liberty of scanning my work so anyone else having trouble with this type of question can see the steps. scan__1443369936_68.147.204.233.jpg
  6. Sep 27, 2015 #5
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