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Homework Help: Velocity and Acceleration Equational Problem (need help making sense)

  1. Sep 9, 2014 #1
    1. The problem statement, all variables and given/known data

    A man is running at speed c (much less than the speed of light) to catch a bus already at a stop. At t=0, when he is a distance b from the door to the bus, the bus starts moving with the positive acceleration a. Use a coordinate system with x=0 at the door of the stopped bus.

    Part A:
    What is xman(t), the position of the man as a function of time?
    Answer symbolically in terms of the variables b, c, and t.

    2. Relevant equations

    v(t) = V0+at

    3. The attempt at a solution

    I know that I need to use this equation:

    v(t) = V0+at

    But I am confused because the answer is: xman(t) = -b+ct. This doesn't make sense to me because in order to use this equation, c would need to be the acceleration, not the velocity. I tried reviewing the book and it really doesn't help. I know that the mans speed is constant, but with an acceleration c, wouldn't his velocity increase in time?

    Sorry if this is a simple question, I just am trying to understand the foundations so that I can better apply myself later in the course. Thank you for your time.
  2. jcsd
  3. Sep 9, 2014 #2
    Welcome to PF!

    If you are asked to find displacement , why should you be using equation for finding velocity .

    You need to use the equation x(t) = x0 + ut+(1/2)at2 .

    Here x0 = -b , u=c , a=0 .

    Does that make sense ?
  4. Sep 9, 2014 #3
    Oh wow.... thanks!

    I was looking through the equations that I know, and I couldn't arrive at the answer with anything that I had (so I thought). I immediately looked for an equation that looked similar to the answer, which was what I gave above. Thanks for making it clear, I didn't really ever think to set accel. to 0, which was what was made me stuck. I appreciate you helping me! :)
  5. Sep 9, 2014 #4
    You are welcome :)
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