Cantilever Beam First Mode of Vibration

[Oscar6330]

I developed a COMSOL model and calculated the first 6 modes of vibration of cantilever beam. Later out of curiosity, I looked up some videos on youtube to see actual cantilever. I found this video which shows the first mode.



How do you know that its first mode of vibration? More specifically, how do you differentiate between normal vibrating beam and first mode (they look quite the same)?

 

[Oscar6330]

any help please!

 

[nasu]

How do you know that its first mode of vibration? More specifically, how do you differentiate between normal vibrating beam and first mode (they look quite the same)?

What do you mean by "normal vibrating beam"?

 

[Oscar6330]

What do you mean by "normal vibrating beam"?

From Normal vibrating beam/ cantilever, I mean a beam which is vibrating at a freq other than its mode freg. For e.g if first mode a beam is at 100 hz and its vibrating at 40Hz.

 

[nasu]

Now I understand. One way to go is the following:
If you vibrate the cantilever and change the excitation frequency continuously, you can tell the normal modes by an increased amplitude. For low damping this may be very sharp.
If you measure amplitude versus frequency, you will see some peaks corresponding to the normal modes (or resonant frequencies).
For the setup in the clip I guess you can tell visually when it is near the normal mode, from the increased amplitude.