William Couture wrote:

> On 4/10/07, David VanHorn <dvanhorn@microbrix.com> wrote:
>> Does anyone know where to find the acceleration of a baseball when it is
>> hit? I'm looking for high end numbers like in a major league home run.
>> Gee and Jerk if I can get it.
> 
> Order of magnitude calculation:
> 
>    Incoming baseball velocity:  -90mph  (good fastball) (note negative sign)
>    Homerun distance:  350 feet

[...]

>    V ~= 106 ft/second
> 
>    Assume impact takes place for a duration of 1/10th of a second:
> 
>    a = (V1 - V0) / t
> 
>    a = 106 - (-90) / 0.10

Why is it so difficult to convince people that SI units make sense? It's so
sad to see a good thought go down the drain because of silly unit errors :)

Using proper units with numbers in equations would help, too -- but when
not using SI units, this gets inconvenient rather quickly, so it's not that
common.

The last equation above would look like this, with the proper units:

a = (106 ft/s - (-90 miles/hr)) / (0.10 s)

Adding/subtracting ft/s to/from mph without conversion is not a good
thing...

The SI alternative would look like this:

a = (32.3 m/s - (-40.2 m/s)) / (0.1 s)

To me, this just looks like fewer conversion errors... :)

>    a = 1960ft/second^2
> 
>    a ~= 61.25 g

The result would then be 

a = 725 m/s^2 = 74 g

The number is not that different in the end (and in the same order of
magnitude), but that's just by chance. The factor from mph to ft/s is ~1.5,
but it could have been 3.8 just as well -- or instead of adding the
absolute speed values, they could have been subtracted and the result would
have been way off :-/

Gerhard

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