Lerner Physics

Have you seen the Veritasium video about what happens when you drop a Slinky? If you haven't, you must go there now. (And make a guess, watch the answer video, and make sure you watch the amazing slo-mo video that's linked to the answer video.) It's a great little piece of physics. 

You can do force diagrams of both the top of the Slinky and the bottom of the Slinky and the top of the Slinky accelerates at greater than 9.8 m/s². So we talked about that, and then I got out my foam apple. 

Where do I have to hold the foam apple so when I drop the Slinky and the foam apple at the same time, they hit the ground at the same time?

As soon as I asked the question, it was pandemonium. Kids started grabbing whiteboards. Heated discussions broke out everywhere. One student started counted the number of turns in the Slinky. There was much arguing. 

But what they all knew, what they all seemed to get, is that the center of mass fall at 9.8 m/s². (I think this lesson really helped.) Here were their whiteboards after 25 minutes of discussion:

They wanted to keep going. It was such an interesting lesson. Their answers weren't that bad; we only had to tweak their answers a little bit to get the great trial you see above. (Don't expect your students to get it perfect; it takes some pretty hard-core calculus to derive the theoretical value.)

Here are the fragments of a broken Petri dish and their calculations to break it.

Without much time spent on the mathematics of projectiles, we seem pretty comfortable using UAPM and CVPM to calculate the kinematics variables. We did lots of practice in TIPERs to make sure we understand the concepts behind projectiles. 

Tomorrow, we'll talk about the Slinky Drop Experience and start the next unit.

We started by going over some conceptual problems on projectiles, and students seemed comfortable with the shape of the velocity-time graphs. At the end of whiteboarding, after doing the last question about calculating where a horizontally-launched projectile would land, a student asked me how to calculate where a projectile would land if it were launched at an angle. He started mentioning components of the initial velocity vector, and I had to hold my tongue. I didn't want to get lost in the math of projectiles this year. We used PHeT instead to simulate the launching of many projectiles. We came up with some pretty powerful conclusions, most of which were suggested my more than one student before I summarized them on the board.

Now that we've talked about the four major models of mechanics, we started projectile motion through video analysis. We immediately saw that the difference in the x-component of the motion and the y-component of the motion. Ut made sense using forces. Energy seemed like an easy way to figure out velocities. And momentum didn't seem useful at all.

We tried a problem about hang time. Two solutions are presented below. Almost every student liked the graphical way and didn't like the equation (which worked & which they learned in math class).

We went over our applications of our model of friction, which really went better than I expected. Then it was time to introduce a new force, the force of friction.

But it isn't really the time. Usually, I move from unbalanced forces to projectile motion. I had all those packets ready. But, somehow, I thought it was time to talk about the spring force. 

It wasn't what I planned, but it seemed to make sense. There's no reason we can't go back to projectiles when we know more about other tools to look at motion and force. And it was a great lab for where we are in our lab experience; I could let the students loose to use all the different tools at our disposal. (They all tried such different ways to do the lab; you can see that in the pictures.) It actually might be better to do projectile motion later, when we haven't just been focusing on kinematics for so long. It'll be a good way to reinforce and spiral back.

Or at least I hope. My subconscious made that decision, and now I'm going to try it.