How to make an object to resonate

[TheBIGuy]

Hello,

For my physics school experiment I need to:
1. Strike a copper ball, get it to vibrate
2. And then measure its resonating frequency.

How do I do that? I guess it should be simple, but I don't know much about physics.

I know there are programs I can use to record the sound and extract the frequency of that sound, but the first part is what bothers me the most - how do I get a copper ball to vibrate?

 

[phinds]

Is this a hollow ball or a solid ball of copper? Is the ball hanging from a thread/string/whatever or is it sitting on the ground/table/whatever. We are not mind readers here.

 

[TheBIGuy]

Oh sorry, I forgot to mention. The ball is sitting on the table and it is hollow.

 

[phinds]

Oh sorry, I forgot to mention. The ball is sitting on the table and it is hollow.

Sitting on a table, I think it would be very hard to get it to resonate, but I could be wrong. Try tapping it with a piece of metal, like the handle of a dinner knife. If it's going to resonate, it will resonate, but I suspect not for long if at all.

 

[TheBIGuy]

What's the difference if it's sitting on the table or on the ground? Any ideas how do I make it to resonate longer? Do you know how can I "strike" the ball?

And also, because it is round, it should produce the same sound on all the surface, right? Because, if I do that with some object that isn't round, it produces a different sound on different part of that object.

 

[berkeman]

What's the difference if it's sitting on the table or on the ground? Any ideas how do I make it to resonate longer? Do you know how can I "strike" the ball?

And also, because it is round, it should produce the same sound on all the surface, right? Because, if I do that with some object that isn't round, it produces a different sound on different part of that object.

Because sitting on the table, the table will quench the oscillation fairly fast. Energy is given up to the table. As opposed to if it is hanging from a thread, very little of the vibrational energy is absorbed by the thread.

If you do some Google searching, you can see what some of the vibrational modes of a hollow sphere are. There are a number of ways it can vibrate. Do they identify what the fundamental mode is?

I can think of another way to measure the resonant frequency instead of striking it. What other way can you think of to excite resonant oscillations in an object?

 

[NascentOxygen]

Oh sorry, I forgot to mention. The ball is sitting on the table and it is hollow.

What is the ball's diameter? How much does it weigh?

 

[TheBIGuy]

4" hollow copper ball. I don't know for the weight.

 

[berkeman]

What is the speed of sound in copper? You can make your initial estimates for the resonant frequency from that...

 

[DEvens]

Ask yourself, what does it mean for an object to resonate? Once you know that, how will you measure such a thing?

What does it mean when the problem states "strike a copper ball, get it to vibrate"? How could you get it to vibrate? Does striking the ball mean only smacking it with some kind of hammer? Is there any easy way you could encourage the copper ball to vibrate longer so you can more easily hear the frequency it is vibrating at? (That is part of why the previous poster was talking about how the ball is suspended.)

Hint: You are going to need some way to determine frequency, presumably of sound. There are many ways of doing that. For example, you could start with some standard frequency sources (tuning forks for example) and see which sounds the most like the vibration of the object you are testing.

It is unlikely to be the speed of sound in copper that determines the resonant frequency. The speed of sound in copper is about 5 km/s, so would indicate something like a 50 kHz primary frequency in a 4 inch object. That is not much like ringing you will hear from smacking such an object. I mean, you can hear the ringing, and probably can't hear 50 kHz.