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This article states that

If an object is being forced to vibrate at its natural frequency, resonance will occur

I can understand how this definition applies to a playground swing: if person A pushes person B, who is on the swing, in a certain way, the swing will swing higher.

If I'm not mistaken, resonance can also occur when an object is caused to vibrate at a multiple of its natural frequency.

However, I'm having trouble understanding resonance with this experiment: enter image description here

If I were to flick the tallest mass on the left side, the corresponding mass on the right side will vibrate with the same frequency. This is an example of forced vibration. However, no matter how much "frequency" I give the mass on the left side, the mass on the right side will still resonate. How does this make sense if resonance can only occur with multiples of an object's natural frequency?

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If I were to flick the tallest mass on the left side, the corresponding mass on the right side will vibrate with the same frequency.

It is because it is a symmetric system. If the right side had a different height , it would have a different natural frequency than the left and the right side would respond always with its natural frequency, different from the the left.

This is an example of forced vibration. However, no matter how much "frequency" I give the mass on the left side, the mass on the right side will still resonate.

at its own frequency, not the forced frequency you impose on the left. Even the left will decay at its own frequency, once the disturbance stops.

How does this make sense if resonance can only occur with multiples of an object's natural frequency?

From the available frequencies each undisturbed object will pick those it resonates to: An airplane passes close overhead, the window panes resonate at their natural frequency, although the sound waves of the airplane contain a lot of frequencies.

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  • $\begingroup$ Why would the other mass vibrate "at its own frequency"? $\endgroup$
    – Xcoder
    Feb 27, 2020 at 1:03
  • $\begingroup$ I guess because it is not directly forced and the set up of the tongs is not good to transfer random forces to them through the axis. $\endgroup$
    – anna v
    Feb 27, 2020 at 5:12
  • $\begingroup$ regarding @annav 's last comment - the resonant system, structure, etc. is like a bandpass filter. It admits energy only at its natural frequency and attenuates outside of that frequency. Energy that's generated by a bob on the left enters the support rod at its natural frequency and is received on the right only by the bob that has the same natural frequency. 'Sympathetic' vibrations. It's the same basic principle used in radio communications between transmitters and receivers. $\endgroup$
    – docscience
    Feb 29, 2020 at 0:47

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