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← Investigators SciencePitch and vibration25 minutesStd 6–8

Bottle Xylophone Puzzle

✨ Tapping the fullest bottle gives the lowest note, but blowing across the very same bottle gives the highest.

Two pairs of bottles: when tapped, the fuller bottle vibrates as glass plus water and gives the lower note, but when blown across the top, the same full bottle has a short air column and gives the higher note.
How it works

🧰 You need

  • six identical glass bottles or glasses
  • water
  • a measuring cup
  • a metal spoon
  • a ruler

⚠️ Tap gently so the glass does not chip, and clear up any breakage with an adult.

👀 What you will see

When tapped, the bottle with the most water gives the lowest note. When blown across, that same full bottle gives the highest note. The two orderings are exact opposites.

🧪 Do it

  1. Line the bottles up and fill them with different measured amounts of water, from empty to almost full.
  2. Measure and write down the water height in each bottle.
  3. Tap each bottle gently on the side with the spoon and listen.
  4. Put the bottles in order from lowest to highest note and record the order.
  5. Now blow across the top of each bottle instead of tapping, and order them again.
  6. Compare the two orders and write down what you notice.

💡 Why it happens

Pitch depends on frequency, the number of vibrations per second, measured in hertz. When you tap, the glass and the water vibrate together, so more water means more mass to move and a slower, lower-pitched vibration. When you blow, it is the column of air above the water that vibrates, and a short air column vibrates faster, giving a higher pitch. That is why the two methods give opposite orders from the same six bottles.

🔁 Now try this

Measure the air column length in centimetres for each bottle and plot your blown pitch order against air column length.

🤔 Think about it

Why does a double bass need such long thick strings compared with a violin?

🧘 Ask the Acharya

“Why does a double bass need such long thick strings compared with a violin?” — ask him and he will explain it from your own chapters.

Ask Guru →

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