Isaac Physics question - standing waves

Original post

by Halleline

I need some help on an Isaac physics question. It’s super simple but I can’t seem to get it correct, so if anyone could point out my mistakes I would be super grateful!

The answer is in 3sf btw

Your second equation does not make sense.
It is your algebraic manipulation that causes the problem.

f₃ = 3f₁
I use f₁ as the fundamental frequency.
$f_1 = \frac{1}{2L} \sqrt{\frac{T}{\mu}}$

When you move 2L from right to left, they should become numerator.

Maybe you want to practise your algebra... :smile:

Reply 2

Original post

by Eimmanuel

Your second equation does not make sense.
It is your algebraic manipulation that causes the problem.
f₃ = 3f₁
I use f₁ as the fundamental frequency.
$f_1 = \frac{1}{2L} \sqrt{\frac{T}{\mu}}$
When you move 2L from right to left, they should become numerator.
Maybe you want to practise your algebra... :smile:

It does become the numerator as you take the reciprocal of 1/2L so I’m not sure what equation you’re referring to ?

Reply 3

Isaac Physics question - standing waves image_edit.jpg

Original post

by Halleline

It does become the numerator as you take the reciprocal of 1/2L so I’m not sure what equation you’re referring to ?

Reply 4

Original post

by Eimmanuel

Yeah that is what I did but when I further rearrange for mu , it says it’s incorrect . But I also used the equation my professor gave me and that’s coming out wrong too ? So I either can’t do significant figures anymore, I can’t convert ( only length from cm -> m though ) or my professor can’t do algebra either.

So I am even more confused then before 😀

image.jpg180.0KB

Reply 5

Original post

by Halleline

When you make $\mu$ the subject for
$f_n = \frac{n}{2L} \sqrt{\frac{T}{\mu}}$ ,

what do you get?

The $\mu$ equation that your prof gets is incorrect. You can do a "unit analysis" to verify it.

The SI unit for $\mu$ is kg/m.

Can you post the link for the question?

Reply 6

You can use this widget to check the rearrangement.
https://www.wolframalpha.com/widgets/view.jsp?id=4be4308d0f9d17d1da68eea39de9b2ce

Reply 7

Original post

by Eimmanuel

When you make $\mu$ the subject for
$f_n = \frac{n}{2L} \sqrt{\frac{T}{\mu}}$ ,
what do you get?
The $\mu$ equation that your prof gets is incorrect. You can do a "unit analysis" to verify it.
The SI unit for $\mu$ is kg/m.
Can you post the link for the question?

that’s the link:
https://isaacphysics.org/questions/phys_linking_15_q3?boardhttps://isaacphysics.org/questions/phys_linking_15_q3?board=bfl22_standing_waves_1=bfl22_standing_waves_1

(edited 11 months ago)

Reply 8

Original post

by Eimmanuel

When you make $\mu$ the subject for
$f_n = \frac{n}{2L} \sqrt{\frac{T}{\mu}}$ ,
what do you get?
The $\mu$ equation that your prof gets is incorrect. You can do a "unit analysis" to verify it.
The SI unit for $\mu$ is kg/m.
Can you post the link for the question?

This is what I get rearranging

Reply 9

Original post

by Halleline

This is what I get rearranging

Used that equation but it’s incorrect too

Reply 10

Original post

by Halleline

This is what I get rearranging

Isaac Physics question - standing waves image_01_edit.jpg

Not sure why you bring L^2 to the numerator.
Or else the second last equation is good.

Reply 11

Original post

by Halleline

Used that equation but it’s incorrect too

You need to practise your algebra manipulation skills.

You need to use SI in using the equation or else you will have "units mismatch" (common mistakes in A level), do the conversion from kg/m to g/m as the final step.

Reply 12

https://www.wolframalpha.com/input?i=solve+f%3D%283%2F%282*L%29%29*%28T%2Fu%29%5E%280.5%29+for+u

Original post

by Halleline

This is what I get rearranging

Reply 13