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Homework Help: Acceleration of a runner

  1. Sep 10, 2006 #1
    I been on these two question back and forth trying to figure out where to start at. Each time I think I get it I end up going way off what im suppose to do and it seem like I have trouble with doing conversion. :cry: Please help in any way to make this problem easier to solve.

    In order to pass a physical education class at a university, a student must run 1.0 mi in 12.0 min. After running for 10.0 min (assume constant speed), she still has 610 yd to go. What constant acceleration does she need over the final 610 yd in order to make it?


    An indestructible bullet 2.00 cm long is fired straight through a board that is 10.0 cm thick. The bullet strikes the board with a speed of 400 m/s and emerges with a speed of 275 m/s. (To simplify, assume that the bullet accelerates only while the front tip is in contact with the wood.)

    (a) What is the average acceleration of the bullet through the board?

    (b) What is the total time that the bullet is in contact with the board?

    (c) What thickness of board (calculated to 0.1 cm) would it take to stop the bullet, assuming that the acceleration through all boards is the same?
  2. jcsd
  3. Sep 10, 2006 #2
    do one problem first, then the other, don't go back and forth. Convert the different units into the same one, either mile into yd or yd into mile. Use the formula V=D/T to find the student's initial velocity and then use the formula D=ViT+1/2AT^2 to find the acceleration. If you still can't figure it out, i can't help you.
  4. Sep 10, 2006 #3
    okay I found out what I was doing wrong which was finding the right velocity but i finally got the answer.

    what trouble me the most is problem #2: I think i need to use
    V^2 = V_o^2 + 2AdeltaX which inputs to 275^2 = 400^2 + 2A(.05)
  5. Sep 10, 2006 #4
    the deltaX or distance is 10.0cm(the thickness of the board) minus 2.00cm(the length of the bullet) because you assume that the bullet only accelerates when it's head is in contact; you put .05 instead of 8 when you plugged it in.
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