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    ciciccz's Avatar
    ciciccz Posts: 4, Reputation: 1
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    #1

    Oct 5, 2011, 06:12 PM
    2D kinematic problem
    A sprinter can accelerate with constant acceleration for 3.70 before reaching top speed. He can run the 100-meter dash in 10 s.
    What's he's velocity when crossing the finish line?
    FadedMaster's Avatar
    FadedMaster Posts: 1,510, Reputation: 148
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    #2

    Oct 5, 2011, 06:17 PM
    Please note that we do not give answers on here, but we do help you with your homework. Have you made any attempts on your problem? If so, please post them. If not, please let us know where you are stuck.

    3.70 what? What are your units?
    ciciccz's Avatar
    ciciccz Posts: 4, Reputation: 1
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    #3

    Oct 5, 2011, 06:29 PM
    since he travels 100m in 10s, which means the average velocity is 10m/s but I don't find that useful at all.
    since acceleration is not given, the first part covered distance is 0.5a*3.7^2, then the distance remaining will be 100-0.5a*3.7^2. therefore the average velocity for the second half will be (100-0.5a*3.7^2)/6.3. To be honest, I don't even know if I am doing this right, but I couldn't find anyway to solve this problem... please helppppp
    ciciccz's Avatar
    ciciccz Posts: 4, Reputation: 1
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    #4

    Oct 5, 2011, 06:33 PM
    Sorry, it is 3.7 seconds.
    FadedMaster's Avatar
    FadedMaster Posts: 1,510, Reputation: 148
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    #5

    Oct 5, 2011, 08:17 PM
    Okay, for me it always helps to just chart out what my knowns and unknowns are.

    We can also look at this in two parts, the first part where the runner accelerates, and the second part where the runner is no longer accelerating and has reached his maximum velocity.

    For first part:














    For the second part:














    Now that it is mapped out a little bit, are you able to see what you need to do next? If you have formulas written out in front of you it should be a little bit easier to see what to do.

    Also, is this calculus based physics you are taking, or just an intro course?
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    ciciccz Posts: 4, Reputation: 1
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    #6

    Oct 5, 2011, 08:46 PM
    ... I think I just made this question harder for myself. I thought he will decelerate after accelerating. Sigh... took me more than an hour figuring this out
    Unknown008's Avatar
    Unknown008 Posts: 8,076, Reputation: 723
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    #7

    Oct 6, 2011, 02:23 AM
    Could you post the answer that you got?
    FadedMaster's Avatar
    FadedMaster Posts: 1,510, Reputation: 148
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    #8

    Oct 6, 2011, 05:30 AM
    Quote Originally Posted by ciciccz View Post
    .... i think i just made this question harder for myself. I thought he will decelerate after accelerating. sigh... took me more than an hour figuring this out
    Believe me, it happens to the best of us. I always over think problems. Please post what you found so we can take a look.

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