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Time Interval in Motion

  1. Feb 6, 2013 #1
    1. The problem statement, all variables and given/known data
    From t=0 to t=5.00 min, a man stands still, and from
    t=5.00 min to t=10.0 min, he walks briskly in a straight line at a
    constant speed of 2.20 m/s. What are (a) his average velocity vavg (v average)
    and (b) his average acceleration aavg in the time interval 2.00 min to
    8.00 min? What are (c) vavg and (d) aavg in the time interval 3.00 min
    to 9.00 min?

    I know the solution, but my question is: why is the entire time interval for part a is equal to delta t= 8–2= 6 min? Where did the 8 and 2 come from? Part b and c, I also am confused by the time intervals. I have the solution attached and confusions marked in red. Can anyone clarify this?

    2. Relevant equations
    Average velocity: Delta x/delta t

    Average acceleration: Delta v/delta t


    3. The attempt at a solution
    (The solution is attached).
     

    Attached Files:

    Last edited: Feb 6, 2013
  2. jcsd
  3. Feb 6, 2013 #2

    haruspex

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    The time intervals are all arbitrary values selected by the problem setter. They have no special significance.
     
  4. Feb 6, 2013 #3
    I still don't understand :frown: They do matter because I would need to use those values to find the average velocity. Wouldn't I subtract 10 - 5 minutes instead of 8 - 2 minutes since it says: "From t=0 to t=5.00 min... and from t=5.00 min to t=10.0 min..."?
     
  5. Feb 6, 2013 #4

    haruspex

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    Yes, of course they matter. Those are the values you are told to use:
    But you asked where they came from. The answer is that they came out of the problem setter's head.
     
  6. Feb 7, 2013 #5
    check with Bashar
     
  7. Feb 8, 2013 #6
    There something must be wrong with my eyes, because I "misread" the problem. It's sad though because I was trying to figure out where the times came from for a couple of days, and I finally saw "his average acceleration aavg in the time interval 2.00 min to 8.00 min?" It like miraculously appeared to me.

    Thank you for all those that helped. Seriously think something wrong with my eyes.
     
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