Solving Physics Homework: Frequency Change

[Scholar1]

Homework Statement

A cord vibrates with a frequency of 3.0 Hz when a mass of 0.60kg is hung up from it. What is its frequency if only 0.38kg hangs from it?

Homework Equations

f= 1/2pi sqrt k/m

The Attempt at a Solution

3.0= 1/2pi sqrt ( k/0.6)
k=213.18 N/m

f= 1/2pi sqrt (213.18/ 0.38)
f= 37.2 Hz[/B]

 

[PietKuip]

The ratio between the masses is 60:38, so the ratio of frequencies is the square root of that.

But what is this about: an oscillating mass on an elastic string or a vibrating cord under tension?

 

[Scholar1]

@PietKuip it just says an elastic cord vibrates when a mass is hung from it. Also, what is wrong in my calculations is it that you are supposed to use grams for the mass?

 

[Scholar1]

Actually it can't be because I used kg that is what you are supposed to use.

 

[PietKuip]

Units don't matter, just use the ratio.

It is quite a difference if the mass is oscillating up and down on a string or whether the string is in a transverse oscillation without the mass moving. Different kinds of motion, different formulas apply.

 

[Scholar1]

@PietKuip I understand that the ratio method will work. But I am a bit concerned why what I did above did not work as my textbook specifically said to use that formula.

 

[PietKuip]

Weird textbook. How can one learn any physics that way? The wording suggests a different mode of motion than for the case where this formula applies.