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Application of Calculus to the Physical World

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Jefferson :
1992 Q4 (c) ii. (see attachment)

Could someone explain to me why d is a constant?

Why does it not change with length l or radius r? (i.e. a variable)

Thank you.

blyatman:
d is a function of l and r, both of which are constants. Hence, d is a constant.

Jefferson :
Hi,
Thanks for replying.

I think 'l' and 'r' are variables, otherwise differentiating will just return nothing (i.e. 0).

I've just read the question again and it does say

"for the given d",

So it is indeed a constant. 

It was my mistake.

 

blyatman:
Sorry I didn't read the question properly. Now that I have, I don't fully understand what they're doing. They differentiated V w.r.t l, so l is a variable. But d is a function of l, so I get what you're saying now about d being a variable. Bit confusing there, but if you've figured it out then all is well.

DrDusk:

--- Quote from: blyatman on July 18, 2019, 03:51:35 pm ---Sorry I didn't read the question properly. Now that I have, I don't fully understand what they're doing. They differentiated V w.r.t l, so l is a variable. But d is a function of l, so I get what you're saying now about d being a variable. Bit confusing there, but if you've figured it out then all is well.

--- End quote ---

I guess technically then they've taken the Partial derivative

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