# Vibrating Objects

1. Aug 7, 2007

### danago

Hi.

Im having some problems with conceptually understanding vibration. I understand that every object has its natural frequency at which it tends to vibrate when disturbed. What im not fully understanding is how an object can have multiple natural frequencies. Why would an object vibrate at one frequency over the other?

Thanks,
Dan.

2. Aug 7, 2007

### chaoseverlasting

When an object is at a temperature above absolute zero, its particles oscillate due to the thermal energy they possess. Due to these vibrations, the object has a 'natural frequency'.

Now, if, for example, a certain frequency peaks when the objects natural frequency peaks and falls when the natural frequency falls, it increases the amplitude of vibration of the object. This frequency is usually an integral multiple of the objects natural frequency.

Also, the natural frequency will change with temperature as the thermal energy of the molecules of the object changes.

Hope that helps.

3. Aug 7, 2007

### Oerg

Hmmm how would you explain microwave ovens? They use microwaves to 'agitate' the food molecules at their natural frequency. But as the food heats up, wouldnt the frequency the microwave emit have to change over time? Or is it not necessary as the object would hear up in a period much faster than the time the microwave must change its frequency in order that the food heats up proeprly??

4. Aug 8, 2007

### andrevdh

The propagation speed of a disturbance along the length of the object is constant and does not depend on the impinging disturbance. Due to the fixed length of the object it can support only periodic disturbances that will form (standing) wavelengths that "fits" the length of the object. So if you disturb it at twice its fundamental frequency it would support only half the wavelength ... Disturbances with "other" frequencies will not form standing waves in the object and it would not resonate (respond to the frequencies).