Hi everyone,(adsbygoogle = window.adsbygoogle || []).push({});

I'm trying to understand the integral on http://www.phys.lsu.edu/~jarrell/COURSES/ELECTRODYNAMICS/Chap14/chap14.pdf (page 14)

I get all the steps except the how to get from eq. 48 to eq 49. I'm not really sure how to compute all the dot products. He lets the angle between n and β be θ

and angle between β and [itex]\dot{β}[/itex] be θ(0)

Attempt at solution:

[itex]n\cdotβ=βcos(θ)[/itex]

[itex]β\cdot\dot{β}=β\dot{β}cos(θ(0))[/itex]

[itex]n\cdot\dot{β}=\dot{β}cos(θ-θ(0))[/itex]?

If this is correct, do I proceed by applying the difference identity of cosine in the last dot product and then square the whole thing? There are going to be a lot of terms, so before wasting more time on expanding, let's first see if what i'm doing is in fact the right way to do this integral.

Many thanks!

**Physics Forums | Science Articles, Homework Help, Discussion**

Dismiss Notice

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Integral with dot products

**Physics Forums | Science Articles, Homework Help, Discussion**