# Standing Waves

1. Apr 6, 2004

### new324

Once, again I find myself in need of a little help. Any hints/tips would be well appreciated.
In the arrangement shown in (attachment), a mass m can be hund from a string (with a linear mass density of .002 kg/m) that passes over an ideal pulley. A string with a length of L=2 meters is connected to a vibrator of constant frequency, Find the mass m of the object when the standing waves with a fundamental frequency of 20 Hz are observed in the string.

Update: Alright I substitued a few equations and have determined the Tension. My assumption is that the mg is going to be half of the tension (half is mg the other half is the wave). Is this assumption correct? If not, any hints?

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Last edited: Apr 6, 2004
2. Apr 6, 2004

### Staff: Mentor

Think about it: The tension in the string must support the weight of the mass. Right?

3. Apr 6, 2004

### kuengb

No, it's tension=m*g. Look at the situation without a wave: At the end of the string you'll have to pull with a force of m*g to prevent the mass from falling; hence the tension in the whole spring is m*g. If you run the oscillator now (with an arbitrarily small amplitude), this won't change, force can't be "splitted up" into two halfs in the same string.

Bruno

4. Apr 6, 2004

### new324

Awesome. You guys are such a huge help to me. I always try and make things tougher than they are. Thank You!