Lecture Notes for 120 - UCLA Department of Mathematics
Lecture Notes for 120 - UCLA Department of Mathematics
Lecture Notes for 120 - UCLA Department of Mathematics
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3.1. THE FUNDAMENTAL EQUATIONS 57<br />
shows that<br />
dB<br />
dt<br />
=<br />
✓ dB<br />
dt · N ◆<br />
N<br />
However, we also have<br />
This implies<br />
Finally the equation<br />
0= dB<br />
dt · N + B · dN dt = dB<br />
dt · N + ds<br />
dt ⌧<br />
dN<br />
dt =<br />
dB<br />
dt =<br />
⌧ ds<br />
dt N<br />
appleds dt T + ⌧ ds<br />
dt B<br />
is a direct consequence <strong>of</strong> the other two equations.<br />
The <strong>for</strong>mula <strong>for</strong> the curvature follows from observing that<br />
!<br />
dT<br />
ds · N = a v d |v|<br />
|v| |v| 2 · N<br />
ds<br />
= a<br />
|v| ·<br />
=<br />
a (a · T) T<br />
|a (a · T) T|<br />
a · a (a · T) 2<br />
|v||a (a · T) T|<br />
= |a|2 |v| 2 (a · v) 2<br />
|v| 3 |a (a · T) T|<br />
|a (a · T) T|<br />
=<br />
|v| 2<br />
q<br />
where |v||a (a · T) T| = |a| 2 |v| 2 (a · v) 2 .<br />
The <strong>for</strong>mula <strong>for</strong> the binormal B now follows directly from the calculation<br />
T ⇥ N = 1 ✓ ◆<br />
a (a · T) T<br />
|v| v ⇥ |a (a · T) T|<br />
= 1 ✓<br />
◆<br />
|v| v ⇥ a<br />
|a (a · T) T|<br />
=<br />
v ⇥ a<br />
|v||a (a · T) T|<br />
= v ⇥ a<br />
|v ⇥ a|<br />
In the last equality recall that the denominators are the areas <strong>of</strong> the same parallelogram<br />
spanned by v and a.<br />
To establish the general <strong>for</strong>mula <strong>for</strong> ⌧ we note (with more explanations to<br />
follow)