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One-Dimensional Motion with Constant Acceleration

  1. Jun 17, 2015 #1
    1. The problem statement, all variables and given/known data
    A rock is released from rest from the top of a very high cliff, and accelerates downward at g.
    Approximately how far does the rock travel in the first 7 seconds of its free-fall? (Assume no air friction.)

    2. Relevant equations
    X=Xo+VoT+1/2AT^2

    3. The attempt at a solution
    I understand how to work out the problem, but I do not know why it works. You simply substitute the numbers in to get the distance. But for some reason, you are not given (nor do you use) initial position like the equation calls for. Can someone explain why it's simply thrown away? The initial position can't be 0, as it's being dropped from a mountain.
     
  2. jcsd
  3. Jun 17, 2015 #2

    Dick

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    The question is asking how far the rock travels. That's the difference between the final position and the initial position. ##x_0## cancels when you take the difference.
     
  4. Jun 17, 2015 #3

    HallsofIvy

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    It sounds like you are memorizing formulas without really understanding them. That's a bad idea. You are free to take the "0" point any where you want. I suggest taking x= 0 at the top of the cliff. Further, the fact that rock is "released from rest" tells you that the initial velocity is 0. Finally, you can take the acceleration due to gravity to be either negative or positive- as long as you interpret the results in the same way- you are free to choose "positive" or "negative" either up or down.
     
  5. Jun 17, 2015 #4
    You can ignore the mountain altogether : you have a ( constant ) acceleration rate, a starting velocity (u = 0) and a time ( 7 seconds ), so use newtons rules of motion.
     
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