Hailey P.

asked • 03/20/24

Solid objects do not actually vibrate, correct?

The solid object as a whole does not vibrate because the molecules and atoms that vibrate are fixed in their position in the object, correct?

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Valentin K. answered • 03/20/24

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All solid objects in the real world vibrate because their molecules are not perfectly fixed in position. Guitar strings vibrate, metal pipes vibrate, crystals vibrate etc.


The so called "rigid body" in Physics is just a simplification model - it is completely inelastic and all its molecules are at fixed distances from each other. There is no such thing in the real world.

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Hailey P.

So, are you saying solid objects do vibrate then? Like a table or a night stand? Without any outside force applied to the object to cause the object to vibrate?
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03/21/24

Valentin K.

tutor
Any solid object can vibrate when a force creates a vibration (elastic wave) through it. If there were no frictional forces, the vibration would continue forever, with no need of the external force. If there is no friction, a wave on a surface of a lake or will continue moving forever without a force that keeps it going. Similar elastic waves can propagate through a metal rode. They die off only because there are frictional forces that dissipate the energy into heat.
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03/21/24

Valentin K.

tutor
Atoms and molecules of a solid object are not completely fixed in place. They do vibrate but in a disorganized way and with very small amplitudes, similar to the Brownian motion the molecules of a liquid. This is not considered a macroscopic elastic wave of vibration, which is an organized vibration of many molecules in the same direction, like the wave that arises when you hit a solid object and dies off due to frictional forces that dissipate the wave energy into heat.
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03/21/24

Hailey P.

So are you saying the disorganized forces of molecule & atoms vibrating cancel each other out and that is why the solid object itself doesn’t vibrate?
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03/21/24

Valentin K.

tutor
Yes. A vibrating object like a drum or a guitar string has elastic waves propagating through it. To start such a wave, an external force is needed to create an initial displacement, which will be a synchronized motion of molecules at some part of the body - you hit the drum or you pluck the guitar string. The random motion of molecules, which is present in everything at temperature above the absolute zero Kelvins, is not synchronized and is not considered a vibration in the sense of an elastic wave propagating through the body.
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03/21/24

Hailey P.

So the solid vibrate is what you are saying? Sorry, just trying to make sure I understand you!
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03/21/24

Valentin K.

tutor
Yes every solid can vibrate if you create an elastic wave through it. A metal rod will vibrate if you hit it with a hammer, which will create elastic waves propagating through it. These waves are organized motion of the molecules, not disorganized microscopic thermal motion that is always present. If you want to understand that further, you will have to read a physics textbook.
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03/21/24

Hailey P.

I understand if a force is applied to it, but what about a solid, like a table that is not touched, that would not vibrate correct? Even though the molecules and atoms do bc their movement cancels each other out, making the solid object unable to vibrate, correct?
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03/21/24

Dong-ryul Jeon J.

You can think of a solid object as a container of gas molecules. When gas molecules move around at random directions and speeds, the container does not move. To make the container move, the gas molecules must have a net direction. To make the gas molecules move in one direction, an external action (a force) is needed.
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03/22/24

Hailey P.

Gotcha! So they cancel each other out, and the solid object does not move as a whole macroscopically, correct? Thank you for replying and for your response!
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03/22/24

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