Waves: Light and Sound — Answer Key
Part A: Fix the Sentence
Each sentence has an error. Rewrite it correctly on the line.
1. Fix the sentence:
Sound is made when objects stay perfectly still and quiet.
Corrected: Sound is made when objects vibrate and create waves in the air.
In Grade 4 science, sound is produced by vibrations. When something vibrates, it pushes air particles back and forth, creating sound waves that travel to our ears.
2. Fix the sentence:
Faster vibrations always make a sound quieter and lower.
Corrected: Faster vibrations make a sound have a higher pitch.
Grade 4 students learn that frequency (how fast something vibrates) controls pitch. A bird chirp vibrates fast and sounds high; a drum vibrates slow and sounds low.
3. Fix the sentence:
Sound can only travel through empty space and never through water.
Corrected: Sound can travel through air, water, and solid materials.
In Grade 4 science, sound needs particles to carry the vibration. Air, water, and solids all have particles, so sound moves through each of them, but not through empty space.
Part B: Fill in the Blank
Write the missing word or number on each line.
1. When an object vibrates, it makes the air around it move and creates a sound wave.
Grade 4 students learn that vibration is the source of every sound. The shaking object pushes air particles to make a sound wave.
2. A sound with a very high pitch comes from vibrations that happen many times each second.
In Grade 4 science, frequency is the number of vibrations per second. Higher frequency means higher pitch, like a whistle or a tiny bell.
3. When you bang a drum hard, the bigger vibration makes the sound louder.
Grade 4 learners connect amplitude to volume. A larger vibration means a taller wave, which our ears hear as a louder sound.
4. Sound cannot travel in outer space because there are no particles to vibrate.
In Grade 4 science, sound waves need a medium of particles to pass the vibration along. Space is mostly empty, so sound has nothing to push through.
Part C: Short Answer
Answer each question in one or two complete sentences.
1. Explain how a guitar string makes a sound that you can hear.
Sample answer: When a Grade 4 student plucks a guitar string, the string vibrates back and forth very fast. Those vibrations push the air around the string, creating sound waves. The waves travel through the air to our ears, and we hear the note.
This Grade 4 answer shows that sound starts from vibration, becomes a wave in the air (the medium), and reaches the listener through that wave.
2. Why can two friends talking underwater still hear each other a little?
Sample answer: Sound waves can travel through water because water is made of tiny particles that pass the vibration along. When a Grade 4 friend talks underwater, the vibrations of their voice move through the water particles to the other friend's ears.
Grade 4 students learn that sound moves through any medium with particles. Water particles are packed close together, so they can carry sound waves.