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# Re: Was embedded timber post (80 Tons trailer impact on Structure)

• To: seaint(--nospam--at)seaint.org
• Subject: Re: Was embedded timber post (80 Tons trailer impact on Structure)
• From: "William Haynes" <gtg740p(--nospam--at)gmail.com>
• Date: Fri, 23 May 2008 15:15:12 -0400

```"The quantity Casey is figuring is an energy not a force, because the
relationship he's using is the definition of kinetic energy."

agree

On Fri, May 23, 2008 at 2:53 PM, Christopher Wright <chrisw(--nospam--at)skypoint.com> wrote:
>
> On May 23, 2008, at 12:41 PM, Casey K. Hemmatyar wrote:
>
>> Just to refresh your memory, it  has a 28,340 k-ft of Force but not
>> Energy.
>
> Not to nag, but you're both wrong.
>
> David figured the energy correctly but misinterpreted the work as a moment.
> If you're looking at the  and came up with the average force that would
> decelerate the truck in 30 inches. If you ran the truck into a post the post
> would have to deform 30 inches under the influence of that force--actually
> the total of the deformation of the post and the deformation of the truck
> would be 30 inches. What would actually happen is that the initial force
> would be much greater than the average; the post would snap off and the
> truck would be damaged but probably keep moving.
>
> Casey also calculated the energy, only in metric units and then put it equal
> to a force, which it isn't. I'm not sure how the calculation got carried
> out, but the mass in kg of an object that weighs 80 tons = (80)(2000)/2.2046
> = 72575 kg and 70 mph = (70)(88/60) = 102.667 ft/sec or (0.3048)(102.667) =
> 31.293 m/sec That makes the kinetic energy (72575)(31.293)^2 =71069 kJ, not
> 39779 kJ.
>
> The quantity Casey is figuring is an energy not a force, because the
> relationship he's using is the definition of kinetic energy.
>
>
> But the actual bottom line is that running a truck into an embedded timber
> post, will snap the post and not do much to slow the truck, except if the
> truck jackknifes and rolls after the impact. If you've ever seen a collision
> of a truck and a telephone pole, it's pretty clear.
>
> Christopher Wright P.E. |"They couldn't hit an elephant at
> chrisw(--nospam--at)skypoint.com   | this distance" (last words of Gen.
> .......................................| John Sedgwick, Spotsylvania 1864)
> http://www.skypoint.com/~chrisw/
>
>
>
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--
William

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