How to determine phase constant?

[Bigworldjust]

Homework Statement

A 225g mass attached to a horizontal spring oscillates at a frequency of 4.00 Hz. At t=0s, the mass is at x = 5.00 cm and has v_x = -37.0cm/s . Determine the phase constant.

Homework Equations

velocity = v(t)
v(t) = Aω·cos( ωt + ϕ )

The Attempt at a Solution

-37 cm/s = ( 5.2122*10^-2 m )( 25.133 rad/sec )cos( 0 + ϕ )
-0.282 = cos( ϕ )
ϕ = cos⁻¹( -0.282 )
ϕ = 106.40º = 1.857 radians ◄---

However that answer is wrong, and I am not sure why. Can anyone help?

 

[ehild]

The result is correct, but you could have used both x(0) and v(0) to calculate the tangent of the phase constant, and decide about the quadrant by examining the sign of the sign and cosine.
It is possible that the SHM was defined as x(t)=cos(ωt + ϕ). Try it. ehild

 

[Bigworldjust]

So instead of using -37 use 5 instead? I tried that and got 1.5324 radians and it's still wrong, lmao. Hmm..

 

[ehild]

How did you get 1.5324 radians?

If x=Acos(ωt+φ), v=-Aωsin(ωt+φ), X(0)=5=Acosφ, v(0)=-37=-Aωsinφ:

tanφ=7.4/(8π)...

Both sine and cosine are positive so the angle is in the first quadrant.

ehild

 

[Bigworldjust]

Ah, thanks I see where I made the error. It was .2863rad. Thanks a lot!

 

[Bigworldjust]

Actually now I am having trouble with the last part, lol. The question is to determine the position at t = 5.00s .

I did this:

Position = s(t)
= A·sin[ ωt + ϕ ]
= ( .052122 m )sin[ (25.133 s⁻¹)( 5.0 s ) + .2893 ]

And got .01478 m, but that is wrong. Any help?

 

[ehild]

But we have figured out that the position was x=Acos( ωt + ϕ) with ϕ=0.2893...


ehild

 

[Bigworldjust]

But we have figured out that the position was x=Acos( ωt + ϕ) with ϕ=0.2893...


ehild

Wow, I didn't even realize, hah. Thanks again for all the help!