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Hip Fracture fall

  1. Jul 30, 2012 #1
    1. The problem statement, all variables and given/known data
    Falls result in hip fractures are a major cause of injury and even death to the elderly. Typically, the hip's speed at impact is about 2.0 m/s. If this can be reduced to 1.3 m/s or less, the hip will usually not fracture. One way to do this is by wearing elastic hip pads. If the typical pad is 5.0 cm thick and compresses by 2.0 cm during the impact of a fall, what constant acceleration (in m/s^2 and in g's) does the hip undergo to reduce its speed from 2.0 m/s to 1.3 m/s?

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    2. Relevant equations

    V^(2)=Vo^(2) + 2A(X-Xo)


    3. The attempt at a solution
    (1.3m/s)^(2)=(2.0m/s)^(2) + 2A(0.05m-0.02m)

    A= -38.6m/s^(2) [EDIT] sorry!

    Is this right? Thanks!
     
    Last edited: Jul 30, 2012
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  3. Jul 30, 2012 #2

    TSny

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    Hi Toranc3. The quantity (X-Xo) represents the displacement during the deceleration. I don't interpret the problem as saying that this displacement is 3.0 cm. Can you convince me that you should take the difference between the 5.0 cm and the 2.0 cm?

    Also, I don't think you did the math correctly. [EDIT, your math is fine!]

    Finally, what should be the units for the acceleration?
     
    Last edited: Jul 30, 2012
  4. Jul 30, 2012 #3

    PeterO

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    Note the problem says it was compressed by 2 cm, not compressed to 2cm thickness.
     
  5. Jul 30, 2012 #4
    I think I see it now. So because it decreased by 2 cm that is the only distance in which the velocity decreased(decelerated). So nothing happened with the rest of the 3 cm thickness. Therefore I use the distance from 0 to 2 cm. Would it be correct to say all of that?
     
  6. Jul 30, 2012 #5

    TSny

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    Yes, I think that's right.
     
  7. Jul 30, 2012 #6
    Thank you TSny and PeterO! :)
     
  8. Jul 31, 2012 #7

    PeterO

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    Correct. No padding squashes to zero thickness, this particular padding is apparently designed to crush from 5cm to 3 cm under the load.
     
  9. Jul 31, 2012 #8
    My mother broke her hip by turning in an akward manner, then she fell. Old bones break easily. Her doctor says this is common in the elderly. They break bones first, doing what for younger people is ordinary activity, and then they fall. Padding would not help.
     
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