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Homework Help: Motion Equation Problem

  1. Sep 14, 2013 #1
    1. The problem statement, all variables and given/known data
    Sam is running at 3.8 m/s and is 75m behind John who is running at a constant velocity of 4.2m/s. If Sam accelerates at 0.15m/s^2, how long will it take him to catch John?

    2. Relevant equations

    d = (vf+vi/2)t
    vf = vi + at
    d = vit + 1/2at^2
    vf^2 = vi^2 + 2ad
    d = vft - 1/2at^2

    3. The attempt at a solution

    I honestly do not know how to solve this problem...
    I think I should probably find Δd of John first? His acceleration = 0, his initial and final velocity is 4.2m/s... correct? and for Sam, his initial velocity = 3.8m/s, his acceleration = 0.15m/s^s and would his Δd = -75m ....

    I really don't understand this question or what I should begin to solve first and how...
    Any help would be appreciated!
  2. jcsd
  3. Sep 14, 2013 #2
    For Sam to catch John both of them should have the same point in space
  4. Sep 14, 2013 #3
    Start by defining a distance d from the initial position of Sam to the point they will meet. Then look at the formulas.
  5. Sep 14, 2013 #4
    So initial position or Δd of Sam = -75m + 3.8m/s... no?
    and point they will meet is 4.2m/s + 75m? since they're both in constant motion?
    I'm still really confused.
  6. Sep 14, 2013 #5
    Always look at your units to see if the answer makes sense!!!

    The initial position of Sam can be taken as 0 and the point where they will eventually meet is a distance d from it.Now you use what you know about Sam and John plus the formulas to find two equations with the time it takes them to reach the point d .Solve them to find t.
    Try drawing a diagram if you still have problems understanding.
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