Help solving equation

[94c4lt1]

Hi Guys,

I am new and I was hoping I could get some help with an equation.

time= (.15 w-s/mm^2)/(720 uW/cm^2)

w-s is watt seconds and uW is microwatts.

mm^2 is millimeters squared and cm^2 is centimeters squared.

I have it simplified pretty well, only I am having difficulty with the w-s and w. I am not sure how to simplify that more. I can convert w-s to joules, but I can't get the energy to convert to power (watts).

Can anyone help solve this and tell me how to do it?

Thanks.

[Tormod]

Welcome!

Hope Bo or Tim gets a chance to help you out here. They are our math experts.

[94c4lt1]

Originally Posted by Tormod

Welcome!

Hope Bo or Tim gets a chance to help you out here. They are our math experts.

Thanks for the welcome. Great site you have here.

Thanks.

[Tim_Lou]

well, you just have to convert the units to standard units and simplify it.
(.15 w-s/mm^2)/(720 uW/cm^2)=
(1.5*10^5 ws/m^2) / (7.2 w/m^2)
time comes out as 2.1*10^ 4 seconds.

[IrishEyes]

Tim, you are just so cute sometimes! I have NO DOUBT that you will become famous in some way

I mean, really... you make it sound so simple... "well, you just have to convert the units to standard units and simplify it." ... like it's something every single one of us does all day long, right? I have a hard time remembering to 'carry the one' sometimes, and you act as if this were no more than adding 2+2... amazing.

[Tormod]

No, in the other thread he couldn't comment on infamous' calculations (I mean, any kid could). So Tim's not a wizard, he's just faking it.

[94c4lt1]

Originally Posted by Tim_Lou

well, you just have to convert the units to standard units and simplify it.
(.15 w-s/mm^2)/(720 uW/cm^2)=
(1.5*10^5 ws/m^2) / (7.2 w/m^2)
time comes out as 2.1*10^ 4 seconds.

Thanks for you help.

Doing it your way gives me 2.1*10^3 though....

Can someone double check this for me?

When I simplify, I get:

(150 w-s/m^2)/(.072w/m^2)=2083.3

Am I doing something wrong?

[94c4lt1]

Originally Posted by Tim_Lou

well, you just have to convert the units to standard units and simplify it.
(.15 w-s/mm^2)/(720 uW/cm^2)=
(1.5*10^5 ws/m^2) / (7.2 w/m^2)
time comes out as 2.1*10^ 4 seconds.

My other question is the units. w-s means watts per second = w/s. From the P=W/t (power equals energy divided by time). Here is the unit equation:

((w/s)/m^2)/(w/m^2) = (w/sm^2)/(w/m^2) = 1/s

Or am I wrong somewhere?

[Tim_Lou]

((w/s)/m^2)/(w/m^2) = (w/sm^2)/(w/m^2) = 1/s
yes, it is true, but it the ws means w/s in your equation:
time= (.15 w-s/mm^2)/(720 uW/cm^2)
it cannot be true, it would be:
1/time= (.15 w-s/mm^2)/(720 uW/cm^2)

anyway, yeah im not a math wizard, im just a science and math nerd.

[Tim_Lou]

time= (.15 w-s/mm^2)/(720 uW/cm^2)
(.15ws/mm^2) * (10^6 mm^2/ 1 m^2) = 1.5*10^5 ws/m^2
(720 uW/cm^2) * (1w/10^6 uW) * (10^4 cm^2 / 1 m^2) = 7.2 w/m^2

hmm, maybe you forgot the mm^2 and cm^2?
1 m^2 = 1000*1000 mm^2 = 10^6 mm^2
1 m^2 = 100*100 cm^2 = 10^4 cm^2