Homework Help: Waves (String with Weight)

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1. Jan 14, 2017

Vanessa Avila

1. The problem statement, all variables and given/known data
A 1.60-m string of weight 1.30 N is tied to the ceiling at its upper end, and the lower end supports a weight W. When you pluck the string slightly, the waves traveling up the string obey the equation y(x,t)=(8.50mm)cos(172rad⋅m−1(x)−2730rad⋅s−1(t))

a) How much time does it take a pulse to travel the full length of the string?
b) What is the weight W?
c) How many wavelengths are on the string at any instant of time?

2. Relevant equations
Acos2π(x/λ- t/T)
k = 2π/λ
w = 2π/T

v =√(F/μ)

3. The attempt at a solution
I found the mass of the string by using F = mg and got 0.133 kg. With this mass, I was able to find the density (m/L) which is 0.0831m/kg

for the time, I thought i should do
T = 2π/2730 rad/s

for how many wavelengths, I thought of doing:
λ = 2π/172 rad/m, but that was wrong.

2. Jan 14, 2017

TSny

OK, except for the units.

You calculated a time here. But is it the time that is asked for in part (b)? What does the symbol T stand for in your calculation?

I think you calculated λ correctly. But part (c) is asking for "how many wavelengths are on the string".

3. Jan 14, 2017

Vanessa Avila

Oh T stands for the period. How do we use the period to find small t? and how do we find how many wavelengths are on the string? and my bad! I mixed up the units for density. It should be kg/m. Thanks for that!

4. Jan 14, 2017

TSny

Can you find the speed of a wave pulse using the information given? Hint: the speed of a wave pulse is the same as the wave speed of the wave y(x,t) that is given in the problem.

I think "how many wavelengths are on the string" means how many wavelengths fit into the length of the string.

5. Jan 14, 2017

Vanessa Avila

So it's v = λ/T ?

T = 2.302E-3
λ = 0.0365 m

v = (0.0365/2.302E-3) = 15.9 m/s

6. Jan 14, 2017

TSny

7. Jan 14, 2017

Vanessa Avila

Would I then solve for time by doing v = distance/time by using the length of the string as distance?
15.9 m/s = 1.60m/t
t = 1.60 /15.9 = 0.101s

8. Jan 14, 2017

TSny

OK

9. Jan 14, 2017

Vanessa Avila

:) My answer for t was correct for that one. yay!

And now for the weight, I'm not sure where to start.

10. Jan 14, 2017

TSny

The weight W probably has something to do with the tension in the string.

11. Jan 14, 2017

Vanessa Avila

would it be 1.30 N as well since it's pulling the weight up?

12. Jan 14, 2017

TSny

Consider a free body diagram for the object hanging on the end of the string.

13. Jan 14, 2017

Vanessa Avila

Oh woop. It has Tension up and mg down. I don't have T, but i can solve that by v = √(Ft/μ)

So my Ft is 21 N which means W is also 21 N. That answr was correct! :)

Now for finding how many wavelengths, do I use the λ I solved?

14. Jan 14, 2017

TSny

Great!

Sure.

15. Jan 14, 2017

Vanessa Avila

But what equation do I use to solve for the n numbers of wavelenghts?

16. Jan 14, 2017

TSny

Suppose the wave y(x, t) that is given in the problem exists on the string. If you took a picture of the string at a particular instant of time, how many wavelengths would you count along the string?

17. Jan 14, 2017

TSny

The problem is a little confusing. If you "pluck" the string to get a "pulse", it would not have the form y(x, t) that is given in the problem (which represents a "sinusoidal wave" spread all along the string).

18. Jan 14, 2017

Vanessa Avila

So if it's just a pulse then it's just 1?

19. Jan 14, 2017

Vanessa Avila

oh wait. So i have a string 1.60 m long, and each wavelength is 0.0365
should it then be
1.60m/0.0365 to find how many there are?

20. Jan 14, 2017

TSny

But, the function y(x, t) given in the problem is not really a pulse. It is a sinusoidal wave spread along the string. So, there are a certain number of wavelengths that would occupy the string at a given instant of time.

21. Jan 14, 2017

TSny

Yes!

22. Jan 14, 2017

Vanessa Avila

alright! Awesome! Thank you so much :)

23. Jan 14, 2017

TSny

You are welcome. Good work.