The Normal Component $a _ { \mathrm { N } } ( t )$

Multiple Choice

The normal component $a _ { \mathrm { N } } ( t )$ of the acceleration of a particle moving along the curve $\mathbf { r } ( t ) = \sin ( 2 t ) \mathbf { i } - \cos ( 2 t ) \mathbf { j } + 2 t \mathbf { k }$ is

A) 4
B) $\sqrt { 8 t ^ { 2 } + 1 }$
C) $\sqrt { 56 }$
D) $\sqrt { 65 }$
E) $\frac { 8 t } { \sqrt { 8 t ^ { 2 } + 1 } }$