Find the magnitude of a particle's acceleration vector at t = 0 for a particle moving with position vector r of t equals components the cosine of t, t cubed

Question

Find the magnitude of a particle's acceleration vector at t = 0 for a particle moving with position vector 

A	1
B	0
C	2
D	3

Answer 1

r(t) = < cos(t) , t³ >

acceleration vector at t=0 --> v(t) = t/dt (r(t))

So, v(t) = < -sin(t) , 3t² >

And, A(t) = < -cos(t), 6t >

at t=0

A(0) = < -cos(0) , 6(0) >

A(0) = < -1, 0 >

So, (A(0)) = sqrt ( -1² + 0 ² )

= sqrt 1

=1

Therefor, (A(0)) = 1

The correct answer is A.