# Resonance question

1. Apr 22, 2015

### Scheuerf

What causes resonance? Why does a vibrating object only cause another object to vibrate of the two objects have the same natural frequency?

2. Apr 22, 2015

### brainpushups

Resonance is caused by some type of periodic driving force.

That is not true. You can make something oscillate without hitting the natural frequency. The amplitude of the oscillations is greatly increased when you are near the natural frequency however.

3. Apr 23, 2015

### Staff: Mentor

It doesn't take much energy at the appropriate frequency to cause vibrations to build up in a low-loss resonant system.

4. Apr 23, 2015

### tech99

A resonant device, such as a swing, acts as an energy store. You can keep giving it a small push at the right moment of every cycle, and the swing gradually builds up in amplitude. It is summing and storing the energy from all the small pushes. If there is another resonator nearby, tuned to the same frequency, only the slightest coupling between the two will, in the same way, cause the gradual transfer of energy from one to the other.

5. Apr 27, 2015

### Philip Wood

Just wondered if you'd actually DONE what tech99 was describing (that is given a swing or pendulum "a small push at the right moment of every cycle"), and, by contrast, at WRONG moments each cycle. Doing this gives you a useful "feel" for what happens.

A word of warning: A small push every other cycle, every third cycle etc., would ALSO cause amplitude build-up and so the resonant curve would have multiple peaks – not what's shown in the textbooks. The textbook response curve is for a SINUSOIDAL driving force, not for a driving force consisting of a sequence of pulses.

.

6. Apr 27, 2015

### rumborak

As another side comment, the driving force does not have to be periodic at all. That's how violins work: The bow scratching across the string creates essentially a noise input signal to the string, but the resonance frequency of the string gets selectively amplified.

7. Apr 27, 2015

### stedwards

Many nice YouTube videos if you search on coupled oscillators.

8. Apr 28, 2015

### jerromyjon

I learned long ago from "Mr. Wizard" that vibration caused by friction is due to a "slip and stick" alternation.

9. Apr 28, 2015

### tech99

Your description of the violin sounds good, but I have found that relaxation oscillators (slip and stick etc) are reluctant to be locked to a resonant circuit. Maybe this is just the experimental set up or it may be something fundamental.

10. Apr 28, 2015

### Scheuerf

If an object is vibrating, and is next to an object with a different natural frequency. That object could also start vibrating a small amount if I'm understanding what everybody is saying. Does that object vibrate at its own natural frequency, or the natural frequency of the object causing it to vibrate. Also is there a difference between natural frequency and ressonace frequency?

11. Apr 28, 2015

### rumborak

"Resonance" always refers to the natural frequency(ies) of the receiving object.
(With maybe the exception of antennas, where they transmit best at their own resonance frequency. But even then, the actual antenna is the receiving object from the driving circuit)

12. Apr 28, 2015

### Scheuerf

I always hear people saying natural frequencies or a set of natural frequencies. How does an object have more than one natural frequency?

13. Apr 28, 2015

### jerromyjon

Simply put a structure could be resonant at varying frequencies and any of the frequencies could lead to disastrous effects of the same or different failure points.

14. Apr 28, 2015

### rumborak

Imagine a box whose width, height and depth are different from each other. In this case resonance comes from bouncing waves from one wall to the opposite, which means there will be three different frequencies the box will resonate at.

15. Apr 28, 2015

### brainpushups

Consider two masses connected by three springs which are fastened to rigid walls and consider longitudinal displacements only. There are two natural frequencies; one where the masses are in phase and one where the masses are out of phase. Extend this idea to a continuous string consisting of atoms connected by forces that behave like springs and you have as many natural frequencies as there are atoms (in this model). Check out any 'coupled oscillators' chapter of a classical mechanics text for more detail.

16. Apr 29, 2015

### Philip Wood

At the driving frequency, in the steady state.

If the driver is suddenly switched on the object's oscillations will be the sum of damped oscillations at the 'object's' natural frequency and oscillations at the frequency of the driver. The damped oscillations will die down, leaving just those at the frequency of the driver.

I've put 'object' in quotes because 'system' or 'oscillatory system' would be a better term. A mass attached a spring (of negligible mass) with the other end of the spring anchored would be an example of such a system. The mass must be treatable as a rigid body, all points on it moving together. That way we have a system with a single natural frequency. [If, for example, the mass were distributed along the spring, rather than being in one lump, we'd have a system with multiple natural frequencies (if the ends of the spring were constrained.)]

Last edited: Apr 29, 2015
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook