Why Does the Shell Theorem Lemma Allow Multiplication by \cos \varphi?

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SUMMARY

The discussion centers on the application of the Shell Theorem and the justification for multiplying by \(\cos \varphi\) in the context of vector forces. The lemma allows for the simplification of force contributions by projecting the differential force \(dF\) along the radius vector \(r\). This projection is valid due to the properties of dot products in vector calculus, specifically when dealing with radial components in spherical coordinates.

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320px-Shell-diag-1.png


I am concerned regarding a lemma of the shell theorem. Specifically, I am concerned with the idea that due to the vector nature of the forces, that one can simplify this:

271c7a94be4496a99e2534e2ceae7751.png


into this:

63898081c163a2fc2c132f68be1bf017.png


Could somebody precisely explain why we're allowed to multiply in the [itex]\cos \varphi[/itex] in the second equation?
 
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It's a dot product with the radius unit vector. You're projecting the contribution of dF (which I'm inferring is along s) along the radius vector r
 

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