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Old Jul 18th 2017, 05:37 AM   #1
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Question Incidence angle - tilted plate

Would you please be able to assist on the following question:

A vertical plate is subjected to rays from a light source located 35 degrees above the horizon. The plate itself is tilted 12.5 degrees around its vertical axis reducing the exposure to the light source.

Find the incidence angle between the light source and the front face of the plate.

Thank you in advance!
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Old Jul 18th 2017, 11:34 AM   #2
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Slopes to find incidence angle

I hope someone has a better solution for you, but I believe the 35 degree angle makes a slope of about 0.700208, and the 12.5 degree angle makes another slope of 0.221695 that is perpendicular to the first slope. The hypotenuse of those slopes is 0.734465, which seems to correspond to an angle of approximately 36.296 degrees, but that does not seem quite right maybe. There could be a mistake in my math. Here is my solution in JavaScript:

with(Math)atan2(sqrt(pow(sin(35*PI/180)/cos(35*PI/180),2)+pow(sin(12.5*PI/180)/cos(12.5*PI/180),2)),1)/PI*180

Last edited by RonSpain; Jul 18th 2017 at 11:36 AM.
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Old Jul 18th 2017, 12:02 PM   #3
Pmb
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I Ron and Yanislav,

I see that you're both new here and as such you may not be aware of the forum rules

Must read before posting a problem : please show your attempt!
The Helpers and the staff of the forum have agreed that they won't give a full answer to a problem posted with no attempt (except maybe in some cases). The Helpers are here to help you to understand your weaknesses by going through the asked questions with you and not here to solve your physics problems.
It is imperative to show an attempt to a problem. If you don't know how to start, say it. Helpers will give you a push in the right direction.

Welcome to the world of physics!
This particular forum is not for physics but to introduce yourself.
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Old Jul 26th 2017, 10:20 AM   #4
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I was not aware of those specific rules. I joined a few months ago, but I have been offering help for questions for years on other sites. I usually do not fully answer a question that sounds like homework because I do not intend to do random strangers' schoolwork (and many questions do seem to be schoolwork), but this question sounded like it could perhaps be a real-world problem, and I felt my answer had a good chance of being wrong (though I have not yet found an error). And the last time I tried to reply to someone with hints and clues on a forum like this, the moderator thought it was a separate topic and moved it. As always, I did try to explain my process here instead of simply providing an answer, so the asker can use his or her own judgment and learn. While many may appreciate giving and receiving help with schoolwork, I think I would prefer forums for discussing physics and math, and not for doing the job that visitor's teachers are being paid to do. Are there any other unusual rules that I need to know here? Let me know.

Regardless, I hope my answer helped Yanislav to learn how to solve problems like this. That JavaScript formula is not as complicated as it may look. I simply used the sine divided by the cosine to find the slope for each angle, then you know how to calculate the hypotenuse of the slopes: find the square root of the squares of the slopes. That gives us our new slope, right? Then I use the atan2 function to find what angle fits that new slope.
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Old Jul 26th 2017, 10:20 AM   #5
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Join Date: Apr 2017
Location: North Carolina
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I was not aware of those specific rules. I joined a few months ago, but I have been offering help for questions for years on other sites. I usually do not fully answer a question that sounds like homework because I do not intend to do random strangers' schoolwork (and many questions do seem to be schoolwork), but this question sounded like it could perhaps be a real-world problem, and I felt my answer had a good chance of being wrong (though I have not yet found an error). And the last time I tried to reply to someone with hints and clues on a forum like this, the moderator thought it was a separate topic and moved it. As always, I did try to explain my process here instead of simply providing an answer, so the asker can use his or her own judgment and learn. While many may appreciate giving and receiving help with schoolwork, I think I would prefer forums for discussing physics and math, and not for doing the job that visitor's teachers are being paid to do. Are there any other unusual rules that I need to know here? Let me know.

Regardless, I hope my answer helped Yanislav to learn how to solve problems like this. That JavaScript formula is not as complicated as it may look. I simply used the sine divided by the cosine to find the slope for each angle, then you know how to calculate the hypotenuse of the slopes: find the square root of the squares of the slopes. That gives us our new slope, right? Then I use the atan2 function to find what angle fits that new slope.
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