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Why a Wet Finger Makes a Glass Sing

8th grade nonfiction passage about friction and resonance: 256 words, six questions, an answer key and vocabulary.

8th grade nonfiction friction and resonance 256 words

Why a Wet Finger Makes a Glass Sing

Run a wet fingertip slowly around the rim of a thin glass, and the glass may begin to hum with a clear, ringing note. The sound seems to appear from nowhere, because your finger is making no noise at all. It is a small demonstration of two ideas from physics: friction that grabs and lets go, and the natural vibration of objects.

A finger moving across a rim does not glide smoothly. Instead, it sticks to the glass for an instant, drags the rim a tiny distance sideways, and then slips free. As soon as it slips, it sticks again, and the pattern repeats hundreds of times each second. Every stick and slip gives the glass a small push.

Every object has certain rates at which it vibrates most easily, and these are called its natural frequencies. A glass is shaped something like a bell, and when it is pushed at just the right rhythm, it vibrates more and more strongly, an effect known as resonance. The sticking and slipping of the finger settles into that rhythm, so the glass rings at one steady pitch.

The pitch depends on the glass itself. A smaller, thinner or stiffer glass rings higher, while a larger one rings lower. Pouring water into the glass lowers the pitch, because the water adds weight that the vibration must move. The water on your finger matters as well, since a clean, wet fingertip grips and slips in just the right way. A dry or oily finger does not work nearly as well.

Questions

  1. What does the wet fingertip do as it moves around the rim? in the text

  2. How often does the stick and slip pattern repeat? in the text

  3. What is resonance? in the text

  4. Why does the glass ring at one steady pitch? think about it

  5. Why does pouring water into the glass lower the pitch? in the text

  6. Why might a dry finger fail to make the glass sing? think about it

Words to know

friction
the rubbing force between two surfaces that touch
rim
the top edge of a cup or glass
frequencies
the number of times something vibrates each second
resonance
strong vibration caused by pushes at just the right rhythm
demonstration
a way of showing how something works
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One passage, three levels

The same topic and facts written three ways, so a mixed class can read and talk about one text. Each group’s sheet shows only a shape; the key says which is which.

● Easier · 238 words · Flesch-Kincaid grade 2.8

Run a wet finger slowly around the rim of a thin glass. The glass may begin to hum with a clear, ringing note. The sound seems to come from nowhere, because your finger makes no noise. It shows two ideas from physics. One is friction that grabs and lets go. The other is the natural shaking of objects.

A finger on a rim does not glide smoothly. It sticks to the glass for a moment. It drags the rim a tiny bit sideways. Then it slips free. As soon as it slips, it sticks again. This happens hundreds of times each second. Each stick and slip gives the glass a small push.

Every object has certain rates at which it shakes most easily. These are its natural frequencies. A glass is shaped like a bell. When it is pushed at just the right rhythm, it shakes more and more strongly. This is called resonance. The sticking and slipping of the finger settles into that rhythm. So the glass rings at one steady pitch.

The pitch depends on the glass. A smaller, thinner or stiffer glass rings higher. A larger glass rings lower. Water in the glass makes the pitch lower, because the water adds weight that the shaking must move. The water on your finger matters too. A clean, wet finger grips and slips in just the right way. A dry or oily finger does not work well.

▲ This passage · 256 words · Flesch-Kincaid grade 6.9

Run a wet fingertip slowly around the rim of a thin glass, and the glass may begin to hum with a clear, ringing note. The sound seems to appear from nowhere, because your finger is making no noise at all. It is a small demonstration of two ideas from physics: friction that grabs and lets go, and the natural vibration of objects.

A finger moving across a rim does not glide smoothly. Instead, it sticks to the glass for an instant, drags the rim a tiny distance sideways, and then slips free. As soon as it slips, it sticks again, and the pattern repeats hundreds of times each second. Every stick and slip gives the glass a small push.

Every object has certain rates at which it vibrates most easily, and these are called its natural frequencies. A glass is shaped something like a bell, and when it is pushed at just the right rhythm, it vibrates more and more strongly, an effect known as resonance. The sticking and slipping of the finger settles into that rhythm, so the glass rings at one steady pitch.

The pitch depends on the glass itself. A smaller, thinner or stiffer glass rings higher, while a larger one rings lower. Pouring water into the glass lowers the pitch, because the water adds weight that the vibration must move. The water on your finger matters as well, since a clean, wet fingertip grips and slips in just the right way. A dry or oily finger does not work nearly as well.

■ Harder · 258 words · Flesch-Kincaid grade 10.1

Draw a wet fingertip slowly around the rim of a thin glass, and the glass may begin to hum with a clear, ringing note. The sound appears to emerge from nowhere, because your finger produces no noise whatsoever. This small demonstration illustrates two ideas from physics: friction that alternately grabs and releases, and the natural vibration of objects.

A finger traveling across a rim does not glide smoothly. Instead, it clings to the glass for an instant, drags the rim a tiny distance sideways, and then slips free, and as soon as it slips, it clings again, so that the pattern repeats hundreds of times every second. Each cycle of sticking and slipping delivers a small push to the glass.

Every object has particular rates at which it vibrates most readily, and these are called its natural frequencies. A glass is shaped somewhat like a bell, and when it is pushed at precisely the right rhythm, it vibrates more and more strongly, an effect known as resonance. The finger's sticking and slipping gradually settles into that rhythm, so the glass rings at one steady pitch.

The pitch depends on the glass itself, since a smaller, thinner or stiffer glass rings higher, while a larger one rings lower. Pouring water into the glass lowers the pitch, because the water adds weight that the vibration must move. The water on your finger matters as well, because a clean, wet fingertip grips and slips in exactly the right manner, whereas a dry or oily finger does not perform nearly as well.

Download as one PDF

How to use it

  1. Press Read it big (or B) to put the text on the projector, or Full screen (F) to fill the whole screen.
  2. Ask the questions one at a time and use Show answer when the class has decided. A shows or hides every answer.
  3. Print student copy for the class, or Print with answers for yourself.

Ideas for class

  • Before reading, put friction, rim, frequencies, resonance and demonstration on the board with their meanings from the Words to know section and have students predict what the text will be about.
  • Read it aloud once while students follow along, then have them reread it alone or with a partner. At a calm 100 words a minute it takes about 3 minutes to read aloud.
  • Use the Read it big button (or B) to put the text on the projector, and Show answer on one question at a time so the class commits to an answer first.
  • Question 4 asks the reader to infer. Ask students to point to the words in the passage that led them to their answer.
  • Ask students to write one question the passage did not answer, then look for the answer in a book or on a classroom screen.
  • Print the student copy for independent work and keep the answer-key copy for yourself.

Related

Questions

What reading level is "Why a Wet Finger Makes a Glass Sing"?

It was written for 8th grade. It has 256 words and works out to a Flesch-Kincaid grade level of about 6.9. That is a formula's estimate from sentence length and syllables, and it is not a Lexile or a guided reading level, so check it against your students.

What do the questions ask?

There are six. 4 ask for something the text says directly and 2 ask the reader to work something out from clues. Each has an answer in the key, and the answers are written as a teacher would accept them, not as the only possible wording.

How long does it take to read?

About 3 minutes read aloud at 100 words a minute. For a one-minute fluency check, paste it into the fluency timer and use the word count.

Can I print it?

Yes. Print student copy gives the passage, the questions with lines to write on, and the word meanings. Print with answers gives the same page with the answers filled in. Both are made in your browser.

Is this passage from a published book?

No. It was written for Peabody Teach, so there is no book to buy and no login to read it.

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