# Natural frequency

1. Feb 9, 2014

### Manraj singh

I have googled it but couldn't find a simple, easy to understand answer answer. Why does a body have a particular natural frequency. Can it be changed by any way?

2. Feb 9, 2014

### ZapperZ

Staff Emeritus
Go back a bit. Where did you get the information that a body has a natural frequency in the first place? This is where you have to cite your source, and explain why you could not use it to get more information from that source.

Zz.

3. Feb 9, 2014

### jagdishdash

natural frequency of a body depends on its density which is an intensive property of that body
as density varies body to body of different elements
natural frequency for that particular body made of that particular element remains the same
we can't change natural frequency easily if density some changes then natural frequency will change

natural frequency =1/√density
that's the relation between natural frequency and density

hope u understood

4. Feb 9, 2014

### nasu

What do you mean by "natural frequency of a body" when you write all this?

5. Feb 9, 2014

### sophiecentaur

I agree. What sort of natural frequency are we taking about? If you don't know then can you say where you read about it?

6. Feb 10, 2014

### Khashishi

Sound waves will travel through a number of different materials. Depending on the shape of an object, you can have standing sound waves in the the object at various resonant frequencies. Generally speaking, the resonant frequencies will be lower if the object is bigger (and the material is kept the same), since it takes sound waves longer to bounce back and forth. So, that's why a xylophone has different sized blocks.

7. Feb 10, 2014

### sophiecentaur

Animal tissue is very 'lossy' for mechanical vibrations. Hence, I would think it very unlikely that you could stimulate a mechanical resonance at any frequency.
otoh, it would not surprise me to find that a human body could have a detectable EM resonance at around 100MHz, (for a 1.5m tall person)

But I think the OP involves more non-Science than Science. We'd need a proper reference before taking it more seriously.

8. Feb 10, 2014

### olivermsun

Fish swim bladders are actually really good for this. But I'm not sure that's the kind of "body" the OP was thinking of...

9. Feb 11, 2014

### Staff: Mentor

I reckon there's sure to be a frequency of back-and-forth shaking where the brain shows greatest tendency/propensity to slosh around in the cranium. Though I'm not volunteering to be the experimental subject.

This may or may not be the 'body' that the OP had in mind.

10. Feb 11, 2014

### AlephZero

Actually, the experimental results aren't necessarily what you expect. (This has been studied seriously - e.g. the effect of vibration on helicopter pilots) Up to a certain level, performance actually improves with increased vibration - presumably because you are trying harder to overcome it.

But having once had a "demo" of the effect of up to about 1.5 g vibration levels over a range of frequencies (strapped in with a safety harness to stop you bouncing out of your seat), I don't recommend it - and don't try it with a full stomach, unless you are wearing protective clothing.

11. Feb 11, 2014

He posted this here,General Physics so he's not talking about Biology.
or "Resonance".

12. Feb 11, 2014

### sophiecentaur

I never thought of that example when I made my reply but I was thinking about human 'bodies'.
The swim bladder is a good example of resonance and the loss mechanism is weak. There is a resonant mass of air, surrounded by a dense medium (tissue). The boundary is a big mis-match and so the loss is small, giving a high Q. I can't think of any equivalent High Q structure in humans, except, of course,the frequency selective sensors in the Cochlea..

The example of bodily resonance under high g conditions is interesting - fairly low Q but double or triple the displacement of an organ when subjected to violent disturbance could seriously affect the damage done. Loading limbs with extra mass or wearing massive / 'stiff' clothing could change the resonant frequency under those circumstances

It is always better when the context of an enquiry is made clear in OPs. It saves the responses going up blind alleys. But I do sympathise; when the whole story is told and a question asked, the response is often "That's the wrong question; this is the question that needs to be answered".

We need more input from the OP, I think.

13. Feb 11, 2014

Staff Emeritus
For heaven's sake - we've spent 11 messages trying to guess what the OP had in mind. Does it not make more sense to let him or her explain it?

14. Feb 11, 2014

### sophiecentaur

Yep; that seems to be the way on PF. It is interesting to see the way the ripples spread from one small pebble.

15. Feb 11, 2014

### voko

Any material body or a system of material bodies have a range of frequencies they vibrate at if excited and then left alone. Those are their natural frequencies. They depend on the shape and the material of the bodies.

A body that is not left alone can vibrate at a different frequency. Whether this is called a "natural" frequency is a matter of convention. For example, a string to which some constant tension is applied is usually said to have natural frequencies that depend on the magnitude of tension. If, however, a periodic tension is applied, then we will most likely say the string undergoes forced oscillations, whose frequency may be different for the string's natural frequency.

16. Feb 11, 2014

Isn't the natural frequency which makes the glasses break in Opera's?

17. Feb 11, 2014

### Staff: Mentor

Unlikely - a "body" is just an object. It just isn't in common use for native English speakers. The OP is almost certainly asking about this:
http://en.wikipedia.org/wiki/Natural_frequency

18. Feb 11, 2014

Staff Emeritus
And now it's up to 16 messages. I don't want to close this and wait for the OP to ask to have this reopened, but we're not going to make any progress trying to guess what he means. That will only sow more confusion.

19. Feb 11, 2014

### ZapperZ

Staff Emeritus
I completely agree with Vanadium. You folks are tripping over yourselves in not only guessing what the OP meant, but also in offering your answers without even bothering to see if this is what the OP was asking for.

News flash: when you help someone, it isn't about YOU. It is about the person you are trying to help!

I am closing this thread. If the OP comes back and still interested in this, he/she can contact me to have this reopened AND to clarify further what had been asked.

Zz.