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The position of a 39.8 g oscillating mass is given by the equation x(t)= (2.00cm)cos(5t- 4.00[tex]\pi[/tex]) where t is in seconds. Determine Velocity at t= 0.250s

M= 39.8 g

T= 1.2575 rad/s

K= 9.95e-1 N/m

initial postion= 2 cm

## Homework Equations

V[tex]_{x}[/tex]= -[tex]\omega[/tex]Asin([tex]\omega[/tex]t + [tex]\phi[/tex])

## The Attempt at a Solution

the attempt i just substitued

V[tex]_{x}[/tex]= -5 rad/s*2sin( 5.00(0.250)-4[tex]\pi[/tex]).... but this didn't work and i dont understand why???