Help solving equation

[Tormod]

"not a math wiz". Lol.

[maddog]

Originally Posted by 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.

I will take a stab...

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

where uW = 10^6 W, cm = 10 mm, so

time = (15 Ws/cm^2)/ 720 * 10^-6 W/cm^2
= (1/48) * 10^6 (Ws/cm^2)/(W/cm^2)
= (1/2)(1/4)(1/6) * 10^6 Sec
= (.125)(.167)*10^6 sec
~= 2*10^4 sec

Yeah your big goof is mixing mm with cm, a common problem.
There are 10 mm to a cm. The units you mention is in mks system (in meters) yet
the equation is to be put in one or the other {mm, cm, m -- take your pick}

Hope this helps. I am a bit rusty, though I will stand by my work.

Maddog

[ImZ]

I think you are doing this all wrong guys. But check my solutsion. Maybe I`m wrong because I have slept only 3 hours and I often make mistakes when I`m tired. I really hope I didn`t make mistakes this time. Check it.

[ImZ]

Here it is

[hashes]

time= (.15 w-s/mm^2)/(720 uW/cm^2)
time= (.0015w-s/cm2)/(720uW/cm2)
time=(.0015w-s/cm2)/.000720W/cm2)
time=2.08333...3s

[94c4lt1]

Originally Posted by hashes

time= (.15 w-s/mm^2)/(720 uW/cm^2)
time= (.0015w-s/cm2)/(720uW/cm2)
time=(.0015w-s/cm2)/.000720W/cm2)
time=2.08333...3s

After redoing the equation and checking a few things, I think Tim was right.

I got confused when converting cm^2 to mm^2. I was just using 1cm=10mm when it is cm^2 to mm^2. So 1cm^2=100mm^2. Therefore the answer is 20833.33 s.

Thanks for all the help guys.

Wow there sure are alot of answers.

[ImZ]

As you wish but.....
If you convert cm to mm then.....
0,15 w-s/mm2: 0,072 w/mm2=2,083 s

It`s your own choice but if Tim was here I belive he would admit he`s mistake.
I`m absolutely sure that this solution is the right one. This equation is really simple and I really don`t think I was wrong

[maddog]

From ImZ's work, I see how I was off by a factor of 10. At least from an Astronomer's point
of view this puts me in the ball park (w/i factor of 10).

Maddog

ps: 94c4lt1: ImZ's calculation is the correct one. You do need to account for .15 => 1.5e-1 in
addition to cm => 10 mm and then squaring. That would give 2083 sec.

[Tim_Lou]

sorry, i almost forgot this topic...

ok, lets do the calculation all over again:
.15 (ws/mm^2)
notice its 1/mm^2, not mm^2
so, (.15 ws/mm^2) * (1 mm^2/ 10^ -6 m^2) = .15 * 10^6 ws/m^2 = 150000
720 uW/cm^2 (*its 1 over cm^2) * (1cm^2 / 10^ -4 m^2) = 720 *10^4 uw/cm^2
(720 *10^4 uw/m^2) * (1 W/ 10^6 uW) = 720 * 10^ -2 = 7.2
150000/7.2 = 20833.333~