Help with Isaac physics optics question

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#1
https://isaacphysics.org/questions/light_circ_mirror
Part D has been confusing me for a while now... I think it's more mathematical in comparison to physics/equations etc.
The SA of the cap is 2πRh, so surely SA of shell is 2πR(x+h), so then one would need to figure out R next, in order to find values for x and h, but how do you find R?
*(the above method is just how I went about trying to solve the question, I have no idea whether it's right or wrong, although there's most likely a better one somewhere as I haven't gotten anywhere with it)
Last edited by user342; 1 month ago
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1 month ago
#2
(Original post by user342)
https://isaacphysics.org/questions/light_circ_mirror
Part D has been confusing me for a while now... I think it's more mathematical in comparison to physics/equations etc.
The SA of the cap is 2πRh, so surely SA of shell is 2πR(x+h), so then one would need to figure out R next, in order to find values for x and h, but how do you find R?
*(the above method is just how I went about trying to solve the question, I have no idea whether it's right or wrong, although there's most likely a better one somewhere as I haven't gotten anywhere with it)
https://www.thestudentroom.co.uk/sho...php?p=88232022
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#3
Thanks, although I think I'm a bit more confused now... could you check whether my above method and formulae are correct?
If so, should I change R to (x^2+15^2)^1/2 then open up brackets?
Last edited by user342; 1 month ago
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1 month ago
#4
(Original post by user342)
https://isaacphysics.org/questions/light_circ_mirror
Part D has been confusing me for a while now... I think it's more mathematical in comparison to physics/equations etc. ...
Agree. Perhaps is just geometry.

(Original post by user342)
... The SA of the cap is 2πRh, so surely SA of shell is 2πR(x+h),...
I don't really see the logic. Isn't x+h = R? What do you understand by SA of shell or SA of spherical shell?

(Original post by user342)
...although I think I'm a bit more confused now...
This kind of writing or comment is rather inferior. Be specific of what are you confused of.

(Original post by user342)
...If so, should I change R to (x^2+15^2)^1/2 then open up brackets?
How do you arrive (x^2+15^2)^1/2?

It would better that you write your thinking logically or your working with some explanation for others to understand you better instead just "spitting out" formula or equation.
Last edited by Eimmanuel; 1 month ago
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#5
(Original post by Eimmanuel)
Agree. Perhaps is just geometry.

I don't really see the logic. Isn't x+h = R? What do you understand by SA of shell or SA of spherical shell?

I would say the reverse. R is already given in the question.
Good point, I didn't recognise that R=x+h. How would you find SA of shell then? And what do you mean when you say R is already given in the question?
i got to this (x^2+15^2)^1/2 as x R and 1/2 of d (d is 30) are right angled triangle, so wanted to use pythagoras to try to get R out of the way
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1 month ago
#6
(Original post by user342)
Good point, I didn't recognise that R=x+h. How would you find SA of shell then? And what do you mean when you say R is already given in the question?
i got to this (x^2+15^2)^1/2 as x R and 1/2 of d (d is 30) are right angled triangle, so wanted to use pythagoras to try to get R out of the way

(Original post by user342)
….How would you find SA of shell then? ….
What do you understand by SA of shell or SA of “spherical shell”?

(Original post by user342)
And what do you mean when you say R is already given in the question? …
The R is not for this question. I was reading an email while replying to your post.
Yes, R is not given for this question. I would edit the post.

(Original post by user342)
…i got to this (x^2+15^2)^1/2 as x R and 1/2 of d (d is 30) are right angled triangle, so wanted to use pythagoras to try to get R out of the way
Correct.
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