- #1
Pi Face
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Homework Statement
A 1 kg mass is attached to a spring with constant k = 16 N/m. Find the motion x(t) in amplitude-phase form (2.37) if x(0) = 1 and x′(0) = −1.
Ignore damping forces.
Homework Equations
combined with 3
The Attempt at a Solution
So I know m=1, k=16, c=0, x0=1, and v0=-1
w=sqrt(k/m)=sqrt(16)=4
md^2x/dt^2+kx=0
characteristic equation
mr^2+k=0
r=+/-iw=+/-4i
x(t)=c1*cos(4t)+c2*sin(4t) (e^0t = 1 and is excluded)
initially x0=1 so
1=c1
v=x'=-4c1sin(4t)+4c2cos(4t)
use intial value of -1 for v
-1=4c2cos(0)=4c2
c2=-1/4
A=sqrt(c1^2+c2^2)=sqrt(17)/4
tan(phi)=c2/c1=-.245, add pi =2.90
so now we have
x(t)=Acos(wt-phi)=sqrt(17)/2*cos(4t-2.90)
the book says the answer should be sqrt(17)/16*cos(4t-6.038)
so I all have right is the angular frequency
where did I mess up?