Basic Algebra Question

[k4dm0nky]

So for this equation above.. for the length do you times the length by 12" to get it in inches then multiply it to the 4th power? Or do you do the 4th power for the length and then multiply by 12?

I've been tricked with this type 2 times, where different sources do it different ways?!

[acadia]

I try to convert all of my dimensions to the appropriate units before I do any calculations, but in this case I'm not 100% certain. Anybody else? Which sources did you look at?

[alleycatbabe]

Typically you want to convert first.

[k4dm0nky]

Kaplan seems to mutliples first, Ballast multiples after. Hmm... wonder what Mike would say.

[AKK_VT]

Not sure why, but I have never been able to make this formula work with wL4 (L to the 4th)… I always convert it to WL3 (L to the 3rd), where W = wL

[Mike-SE]

I'm back.
Units, Units, Units. Remember these three things.
w has units of lbs/ft (or kips per foot)
L normally has units of feet.
If you convert the L from feet to inches you must also convert the w to lbs/inch. A bunch of work and prone to errors.
The solution (the way I DO IT in the office
Use w in units of lbs/feet
Use L in units of feet, so L^4.
THEN multiply by 1728 (12^3)
so the equation becomes:
5*w*(L^4)*1728/384/29000/I

See the attached for how to see the "units" in the equation.

[NYCPM]

Mike, thanks for the explanation. So if I understand correctly, you're suggesting that the equation should be expressed as (5w(L^4)1728)/(384EI) ?

Also, are we likely to see questions like this on the exam? Having tried a few of these practice problems, the factors can go up to the billions!

[Mike-SE]

yes, your interpretation is correct. Yes, you should be able to use this equation on the exam. The actual equation is included in the reference material so you do not need to memorize it, just know how to use it.

[k4dm0nky]

So is the way Kaplan does it is incorrectly? For L they took 14' and multiplied it by 12... and took 168 and took that to the 4th power. ?

[Coach]

I recommend forgetting about the 1728 (because it's unique to this equation) and always writing down and converting ALL units first.

[k4dm0nky]

So you recommend how Kaplan suggests then Coach?... convert units first as this: (14' x 12")^4 ?

[Coach]

So you recommend how Kaplan suggests then Coach?... convert units first as this: (14' x 12")^4 ?

The best way to avoid errors is to write it all out. Of course, you will need to know the units for E and I.

[k4dm0nky]

Is that a yes or a no? lol

[Mike-SE]

IF coach agrees with Kaplan I will cry!

[k4dm0nky]

okay I got this to work... this is how I did it.

I took small 'w' and multiplied it by the length to get it into big 'W' lbs. That removes one 'l' power so its l^3 now.

(length' x 12")^3

That gets the right answer. It's all about getting the little w and turning it into big W. The units get screwy if you don't do that I am finding. Thanks guys!

[acadia]

The little w still had inches in it before you multiplied it by the length to get just lbs or kips. You can keep L^4, but you would have to convert little w to inches too.