One common misconception is the idea that noes and antinodes are the same as the crest and trough of a wave. They’re not. A node is a place on the wave that is permanently at rest. An antinode is where the wave is at its maximum (it will travel through a large up and a large down displacement).
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When a wave travels from one medium to another, like sound waves traveling in the air and then through a glass window pane, it crosses a boundary. Whether the wave continues to the new medium (and even how it goes through), or whether it bounces and reflects back, or a bit of both depends on the boundary.
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Straight waves are what happens when something moves back and forth in a medium like water. These are interesting when they hit a diagonal plane barrier, because when the incident wave reaches the barrier, the waves always reflect at the same angle that they approached the barrier with (called the Law of Reflection).
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Waves bend when they go from one medium to another when the speed changes. It’s a really important topic in light (not so much with sound), because it’s how lenses, eyes, cameras, and telescopes work. The bending of sound waves happens naturally in the air above the earth when it’s warmer than the surface of the earth. The sound waves that travel through the warmer air are faster and the ones that travel through cooler air are slower. When the sound waves go from warmer to cooler air (less dense to more dense air), they become bent back down toward the surface.
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When waves pass around small (we’re talking small compared to the wavelength of the wave) objects, they diffract. People in the audience of a concert can hear really well if they are sitting right behind a pillar because the sound waves are large enough to bend around it (which is actually because of both diffraction and reflection effects). Diffraction helps sound bend around obstacles. You can sometimes hear conversations around corners because of diffraction.
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Often two (or more) waves travel through the same spot. If you’ve ever listened to an orchestra, you’re hearing the sounds from many different instruments all playing at the same time. If more than two boats are on the lake, their wakes churn up the water together. Here’s how we handle this in physics…
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What if two waves of the same wavelength and amplitude travel in the same direction along a stretched string? What will the string look like? We know about the idea of superposition adding the waves together, but what does the string actually look like?
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In previous lessons we’ve learned that energy is the ability to do work, and that work is moving something a distance against a force. The concept of energy is fairly easy to see as far as lifting things or pushing things go. We are exerting energy to lift a box against the force of gravity. We are exerting energy to pedal our bike up a hill. But how does this energy stuff relate to light, electricity, or sound? What’s moving against a force there?
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Damping is when a spring, swing, or other vibrating object loses its energy over time. It means that without adding energy into the system, like pumping on a swing or hitting a drum head, the object will eventually come to its non-vibrating (equilibrium) position.
Imagine the kid on the swing again. Why does the kid move past the equilibrium point without stopping?
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The concept of frequency is very important to understanding energy. When it comes to electromagnetic waves it is frequency that determines whether the wave is radio, light, heat, microwave or more. It’s all the same type of energy, it’s the frequency that determines what that energy actually does. With sound energy the frequency determines the pitch of the sound.
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Imagine a police car on the side of the road with lights and sirens on full blast. You’re also parked and you hear the same frequency (say 1,000 Hertz) of the siren. However, if you’re driving at 75 mph toward the police car. you’re going to hear a higher frequency (1096 Hz), and if you’re driving away at 75 mph, you’re going to hear a lower frequency of 904 Hz. Why is that?
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The restoring force slows down the object as it moves from its resting but speeds it up when it heads back to the resting position, and that’s what creates the vibration. We’re going to take a look at the forces in a pendulum from the point of view of Newton’s Laws.
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You’ll need a pendulum for this experiment. A pendulum is really nothing more than a weight at the end of something that can swing back and forth. The easiest way to make one is to get a string and tape it to the edge of a table. (The string should be long enough so that it swings fairly close to the ground.) Tie a weight to the bottom of your string and you’ve got a pendulum.
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Since we can’t see soundwaves as they move through the air, we’re going to simulate one with rope and a friend. This will let you see how a vibration can create a wave. You’ll need at least 10 feet of rope (if you have 25 or 50 feet it’s more fun), a piece of tape (colored if you have it), a slinky, and a partner. Are you ready?
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Click here to go to next lesson on Motion of Waves.
This is a recording of a recent live teleclass I did with thousands of kids from all over the world. I’ve included it here so you can participate and learn, too!
Sound is a form of energy, and is caused by something vibrating. So what is moving to make sound energy?
Molecules. Molecules are vibrating back and forth at fairly high rates of speed, creating waves. Energy moves from place to place by waves. Sound energy moves by longitudinal waves (the waves that are like a slinky). The molecules vibrate back and forth, crashing into the molecules next to them, causing them to vibrate, and so on and so forth. All sounds come from vibrations.
Materials:
- 1 tongue-depressor size popsicle stick
- Three 3″ x 1/4″ rubber bands
- 2 index cards
- 3 feet of string (or yarn)
- scissors
- tape or hot glue
Click here to go to next lesson on Energy of a Wave.
Some waves need a medium to travel through while others do not. Mechanical waves need a medium for the wave to travel through to transport energy. Ocean waves, jump ropes, pendulums, sound, and waves in a stadium are all examples of mechanical waves.
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The words particle and wave are two words you’ll see in nearly every area of physics, but they are actually very different from each other. A particle is a tiny concentration of something that can transmit energy, and a wave is a broad distribution of energy that fills the space it passes through. We’re going to look at particles in more depth later, and instead focus on understanding waves.
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Sound is a type of energy, and energy moves by waves. So sound moves from one place to another by waves; longitudinal waves to be more specific. So, how fast do sound waves travel? Well, that’s a bit of a tricky question. The speed of the wave depends on what kind of stuff the wave is moving through. The more dense (thicker) the material, the faster sound can travel through it.
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Sound moves faster in solid objects than it does in air because the molecules are very close together in a solid and very far apart in a gas. For example, sound travels at about 760 mph in air, 3300 mph in water, 11,400 mph in aluminum, and 27,000 mph in diamond!
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Ever gotten sea sick? It’s usually because the motion of what your body detects is different from what your eyes see. Let’s take a look at how you can calculate the wave speed by watching two boats bobbing up and down (without getting sick).
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If a wave can travel through mediums like air, water, strings, rocks, etc., then it makes sense that as the wave moves through these mediums, the tiny particles that make up the medium will also vibrate. In order for this to happen, the medium has to have a way for energy (both potential and kinetic) to be stored, so the medium has both inertia and elasticity.
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