A lost nail and hanging balls

I was teaching yesterday. Physics. Tenth class physics. I was clearing some doubts for the two kids. Their board examination is approaching fast.

One of them had written a test some time back. The test paper had been prepared by his mother. He hadn't answered some questions. He didn't know the answers to them. One of the unanswered questions needed him to describe an experimental method for demonstrating the phenomenon of resonance.

'There are so many mentioned in your book. You should have remembered one of them', I remarked.

He tried to explain. 'Well, I remember real life examples like that of a motorcycle which at times starts vibrating vigorously at some speed when the forced vibration matches it's natural frequency. But I am not sure about an experimental method as such.'

'Of course there are. Let's look into your book'. I opened the chapter on sound, found some experiment, and without bothering to read it, told him, 'Here look, you can quote this experiment to show resonance.' I was pointing to a diagram present in his Physics text-book which was very similar to one that's present below. There were four pendulums, the alternate ones having equal lengths.

I continued, 'so you see, you can use these hanging balls to show resonance. Can't you?'.

Wait a second. What did I just say? Hanging balls? Yep, that was right. I knew it from the way the kids looked at each other and couldn't control their smile. It was embarrassing. And I totally forgot how else to refer to the balls. Hanging balls they were. And hanging balls I called them. I had totally forgotten the existence of this word called 'pendulum'. We were smiling together, somehow holding our laughs.

The doubt followed. It was not too clear to them how resonance was demonstrated here, in this hanging balls experiment.

'You can bring a vibrating tuning fork near one of the balls, and when the frequency of vibration matches the natural frequency of the ball, the ball will start oscillating with increased amplitude. At the same time, the alternate ball which is hanging through a thread of equal length will also start oscillating because for everything else remaining constant, frequency of a ball hanging from a thread depends upon its length. For the same reason, the other two balls won't oscillate with increased amplitude. Their natural frequency is in mismatch with the externally applied frequency. Is it clear to you guys now?'

It was fun! I am happy I had the balls to deliver the explaination in a serious tone, without letting my embarassement show off. :)

Well, this was the bit on hanging balls. The funny bit. The 'lost nail' story however isn't that funny. It is rather a true and painful story of a nail that I lost. Today. It belonged to my right foot hallux which now lies nail-less and bandaged. Two guys accidently jumped over my right foot. And it happened. In fractions of second. No pain. Twisted nail. A cool sensation. But for everything that goes, another pops up to take place. So all I have to do is wait for a new nail to grow up. And it sure will. Lost nail over hanging balls, anytime ;)


  1. dhruv said...
    oh this happens a lot. when you are "in the zone" you dont realize these "mistakes". You're too engrossed in getting your point across clearly and efficientyl.. I've experienced it myself and to come to think of it, its very strange and hilarious when you remember it later on! =P

    Felt sorry for your toe. Take care of it! Must've hurt a lot! :(
    ankit said...
    accha hai !!
    Raveena said...

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