Hip Fracture Falls: Reduce Speed with Elastic Hip Pads

[Toranc3]

Homework Statement

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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Homework Equations

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

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!

 

[TSny]

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?

 

[PeterO]

Homework Statement

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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Homework Equations

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

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!

Note the problem says it was compressed by 2 cm, not compressed to 2cm thickness.

 

[Toranc3]

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?

 

[TSny]

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?

Yes, I think that's right.

 

[Toranc3]

Thank you TSny and PeterO! :)

 

[PeterO]

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?

Correct. No padding squashes to zero thickness, this particular padding is apparently designed to crush from 5cm to 3 cm under the load.

 

[Pkruse]

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.