## Day 110: Doppler Effect and Standing Waves

How does the Doppler effect work when the source is accelerating? We couldn't really explain it, so I made the Desmos animation above.

We also talked about standing waves. But we found music more interesting. It's so interesting to see octaves and resonance and frequency all interrelated.

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## Day 109: Wave Interference, Applied

What happens when wave pulses go from one medium to another? What happens at the boundary? It took many trials, and this video, to get everyone in class on the same page.

We also talked about what happened when two waves of very similar frequencies occur at the same point in space. Thanks again, Desmos:

We also talked about what happens to the frequency of a wave when the source of the wave is moving relative to the observer.

Last, we started talking about what happens to a wave that is confined to a fixed space. It seemed that, with the bounce at the end, you could get constant constructive interference. Cool!

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## Day 108: What Happens When Two Waves Overlap?

So, what happens when a wave hits a boundary? We went to lab and saw that pretty quickly.

What happens when two waves overlap? That was more difficult. We went to lab, but we couldn't really see it that well. We had to go to simulations to understand what was going on.

I made an answer key for one of the practice sheets I found in the Modeling materials. I won't post the answer key, for obvious reasons, but I've never made an animated answer key before:

Desmos can be so cool sometimes.

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## Day 107: Starting Waves, and a Lot of Vocabulary

"This looks like a biology worksheet."

"This looks like US History homework."

Yes, for the first time this year, I had my students do matching. ﻿Matching﻿.

It was after we did a quick lab trying to figure out whether period or amplitude affects the speed of a wave. It wasn't a great lab. I've never gotten great results from the long springs with just pulses. But, still, it's good to play around with the wave, to see the pulse in real life, and to try to calculate its speed.

Then I had many vocabulary words to introduce, like longitudinal wave, direction of disturbance, and trough. I wish I could get around this, but the unit of waves doesn't make coherent sense to me yet like the rest of the AP curriculum. I'm not sure what the overarching theme is. So getting through the vocabulary that isn't really that important, and doing so quickly, seemed like an OK idea.

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## Day 106: Taco Party

Today was the taco party. Four years ago, one of my students left a bottle of taco sauce in my room. The bottle was brought for a fiesta in Spanish class but was never opened. The students had opened up a jar of salsa instead. So the bottle sat in my room for a while. Then I started having students turn in their labs right next to the taco sauce. Later, the taco sauce ended up on a top shelf. This year, we realized it expired in February of this year, and we planned for a taco party.

Today was the day. We had lots of sugary treats, some nacho chips and taco meat, and no plates. We managed.

We also took a test, whiteboarded the last, relatively straightforward problems about pendulums, and played a little bit with the long slinky. We're moving on to mechanical waves tomorrow.

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## Day 105: Finishing the Restoring Force Particle Model

We worked on our graphing skills for the spring-mass system. Graphing the kinetic energy and the spring potential energy was a bit difficult. Also, finding the amplitude of the velocity-time and acceleration-time graphs requires us to go back to all our old models.

We then talked about what restoring force means. For a restoring force situation can happen, the object must be in stable equilibrium. The time the object takes to return to equilibrium depends both on the inertia of the system and the strength of the restoring force.

Oh, we also planned for a taco party tomorrow. The taco sauce that's been in my room for three years expires tomorrow.

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## Day 104: The Period of a Mass-Spring System

We started with the equation for the sinusoidal functions. We know the position-time function for the mass-spring system looks like a sine curve, and we know, from pre-cal class, that we can write a sinusoid as y = A sin (Bx + C) + D. We quickly listed what each of these four variables would do to the graph. It looked like the only one we didn't understand was ﻿B﻿What would affect the period of the sinusoid? We brainstormed some ideas (mass, spring constant, friction in the system, amplitude) and went into lab.

The final model was surprising; how could it be so complicated and yet so simple? So we talked a little about and did some practice. We should be done with this unit quickly, probably by the end of tomorrow. The restoring force particle model seems so easy now that we have so many other models under our belt.

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## Day 103: Starting Restoring Force

We finished magnetism today. The change in the magnetic flux can be hard to see, so I made skid some flash cards after school today. We had pictures of various situations on one set and descriptions of possible changes to the flux on the other set. One from Set A, one from Set B, then go! What is the induced current?

We then moved to a mass on a spring. The graphs were pretty, and we could explain them well.

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## Day 102: Ferromagnetism

Today was a hodgepodge of things. We started by whiteboarding what was mostly a review of right-hand rule problems, and we got to clarify what was going on. Then we did the classic "why is the object going so slowly down the pipe" demonstration.

Then it was on to the three types of materials: ferromagnetic, paramagnetic, and diamagnetic. We, of course, had to see the levitating frog video. Then we practiced some last Faraday's law questions.

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## Day 101: Practicing Magnetic Induction

We went slow and steady today. Magnetic flux is not an easy concept, and we want to make sure we understand it and the long, causal reasoning that goes into electromagnetic induction.

We started by debating a lot about how the flux changes. One student in particular mentioned an interesting point about what happens when you push the magnet in through the loop. Doesn't the magnetic field change direction? It took a while to clarify the question, and in that clarification, we got to talk about how the magnetic flux doesn't really have a direction. It's just positive or negative.

We spent a lot of time talking about the reasoning we'd use to figure out the direction. We got to hear many ways to explain our thinking. At the end of class, we even got to see magnetic induction using the function generator, and the current in the second loop only happened when the current in the first loop changed. We can explain transformers and wireless charging now.

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