# Easy standing wave question

i_hate_math

## Homework Statement

A rope, under a tension of 209 N and fixed at both ends, oscillates in a second-harmonic standing wave pattern. The displacement of the rope is given by
y=0.49sin(πx/3)sin(12πt)
What are (a) the length of the rope, (b) the speed of the waves on the rope, and (c)the mass of the rope? (d) If the rope oscillates in a third-harmonic standing wave pattern, what will be the period of oscillation?

T=2π/w

## The Attempt at a Solution

I have solved part a,b,c but I got part d wrong. I simply used T=2π/w
==> T=2π/(12π)=0.1666666...=0.167s
And this is not the correct answer.
Please point out where I did wrong

Homework Helper
Gold Member

## Homework Statement

A rope, under a tension of 209 N and fixed at both ends, oscillates in a second-harmonic standing wave pattern. The displacement of the rope is given by
y=0.49sin(πx/3)sin(12πt)
What are (a) the length of the rope, (b) the speed of the waves on the rope, and (c)the mass of the rope? (d) If the rope oscillates in a third-harmonic standing wave pattern, what will be the period of oscillation?

T=2π/w

## The Attempt at a Solution

I have solved part a,b,c but I got part d wrong. I simply used T=2π/w
==> T=2π/(12π)=0.1666666...=0.167s
And this is not the correct answer.
Please point out where I did wrong
The given 12π is for the second harmonic. Part d asks about the third harmonic.

i_hate_math
The given 12π is for the second harmonic. Part d asks about the third harmonic.
I see, but is it okay to assume that the length of the rope remains the same?

Homework Helper
Gold Member
I see, but is it okay to assume that the length of the rope remains the same?
Yes.

• i_hate_math
i_hate_math
Yes.
With the same L, I got speed v=sqrt(T•L/m) where T is tension, so v remains the same too. the wavelength is changed since its now in third harmonic pattern, L=6=(3/2)λ.
==> λ=4
==> λ•f=v and from part b, v=36 is the correct answer
==> v=1/9=0.111...

Homework Helper
Gold Member
With the same L, I got speed v=sqrt(T•L/m) where T is tension, so v remains the same too. the wavelength is changed since its now in third harmonic pattern, L=6=(3/2)λ.
==> λ=4
==> λ•f=v and from part b, v=36 is the correct answer
==> v=1/9=0.111...
Did you mean T=0.111?

i_hate_math
Did you mean T=0.111?
• 