Question

Water flowing through a 2.8 cm-diameter pipe can fill a 200 L bathtub in 5.8 min.

Flow rate Q = volume/time = speed * cross section area
Q=200/5.8=speed*.028....i get 1231.52
or
Q=(200/348)=speed*.028...i get 20.52

both are wrong so what do i do?
then i did
speed=200/(pi*.25^2) ...i got 3.97 which is wrong

then i did v=Q/A=.2m^3/348s
divided by pi*(.014)^2 so i got 3.585559637e^-8...this is also wrong...plz look into it

Answer 1

YOu are not using units, nor labeling your work with units.
YOu are not using area, you are using diamter. Area= PI(diameter/2)^2
That would correct the top part, but you need to change liters to the same unit as area (cm, dm, or whatever).
I recommend diameter as .28dm, volume as 200 dm^3, that would give speed in dm/min. most would change that to dm/seconds.

You are relying on your calculator to think for you. There are multiple errors on each line.

Looking at how lost you are, I will do this for you <b>one time only</b>

flow rate: 200dm^3/(5.8*60) sec=.548dm^3/sec

speed= flow rate/area=.548dm^3/sec *1/(PI*(.28dm/2)^2)
= .548dm^3/sec * 1/.0616dm^2
=8.90dm/sec

Now. check that. Understand it. Finally, does the idiot computer expect answers in dm/sec? If not, think for it and convert.

Notice how I put units in each quantity.

Put your units in the calculations, do it with pen/pencil. Make the calculations check your work (you have to end with units that support the quantity).

Answer 2

I feel I'm being helped in Asia ...sorry to be pain in the neck. just a question if u don't mind...how do u convert from dm/s to m/s

Answer 3

dont worry i figured it out...i didn't have to convert...it was a problem of putting wrong brackets in my calculator