Teaching the physics of projectile motion

Sometimes there is a bit of a gulf between what the students learn in Algebra 1 and their 8th grade physical science class. To get through problems involving position of falling objects, a physical science teacher might be tempted to give equations of motion that must be used, but may not necessarily feel like taking the time to explain the derivation of the equations.

I’m a big fan of understanding WHY an equation is the way it is, intuitively if possible, so that if forgotten, it could be re-derived from one’s basic understanding at any moment. Well, that’s the goal at least. To that end, I’ve created a four page student handout on projectile motion that I give to my “Honors” 8th grade physical science students.

It’s attached to this post, and is also available, with the free teacher’s key, here:  http://www.teacherspayteachers.com/Product/Projectile-Motion-Notes

I’ve also made a few videos on Youtube that explain these notes, so you can share them with absent students:

Hope you like it!

Let me know if you have suggestions for improvement, comments, questions, etc.

Happy New Year! Projectile MotionNotes

Tale of the Rejected Laminator

So, there’s been this bulky, heavy laminator stored on a cart in our back room for years. It’s broken, I’m told. I gainfully plug it in and press “ON”, and sure enough, nothing happens. Clearly this machine needs professional help.

I call up my Electrical Engineer friend. He arrives in my classroom with his tool bag of esoteric pokers, prodders, capacitors, bulbs, who knows what all these things are. He unscrews, pulls, prods, replaces, examines, tests. “Hmmm, I can’t really find anything wrong… is there some kind of safety mechanism on this that prevents it from turning on if the alignment isn’t correct?”

“Oh, I have no idea… what do you mean?” It all looked pretty straight to me.

Input tray must click in for machine to turn on.

Input tray must click in for machine to turn on.

He slides the output tray one centimeter deeper into the machine over a little hump and Click! it settles into a new place.  Pokes “ON”, and the lights turn on, machine is humming happily. Brilliant!

What’s the moral of this story to me?  This machine had sat for years in our storeroom. It had come precipitously close to being scrapped as “junk” since, after all, “It doesn’t work and none of us have a budget to get it fixed.”  A tool is only as useful as the knowledge and skill of the craftsman who wields it. The mind, not the material, is key.

Liquid Nitrogen demonstrations….

We had a full day of liquid nitrogen demonstrations today. Merged 2-3 classes of 8th graders all day in the auditorium for observations and inferences of the following demonstrations:

1. “Fried Balloons”.

Prep:  Pour a 3 cm layer of LN2 into a very large styrofoam container out of sight from the audience. Inflate and tie off 10 latex balloons, and store them in a very large garden trash bag. Before students enter the room, place the styrofoam container onto a table with the lid on the container. As students come in, do not draw attention to the box. (There will be some noise of LN2 bubbling and it’s best if students don’t notice.)

The “Patter”: “You know, the math department at our school has told us they’re very concerned that people just don’t understand basic geometry any more. They’ve asked us to incorporate some math lessons into our science, so here we go: look carefully at this balloon (hold it up), and estimate how many balloons of this size could fit into this box.”   Depending on the size of your styrofoam container, people will typically respond anywhere from 2 to 6.

“OK, let’s see.”

Lift the lid slowly and slowly place the first balloon into the box. Try to do this without swirling the vapors over much. Hold the balloon down in the LN2 until it has shrunk to about half size. Repeat until you’ve got at least 8 balloons inside. You can push them down with your bare hands, just be careful not to let the liquid nitrogen drip onto your skin.

More “patter”: “I forgot to eat breakfast this morning, so I think I’m going to need to fry up something to eat. Hmm…. what do I have around… OH! I know! I’ll just use this NICE, HOT table (pat the surface of the table) to FRY some balloons!”

One by one, pick the balloons out of the container and ‘stir fry’ them on your table surface.



Warm Ups… a helpful tool

In my 8th grade classroom, warm ups are a helpful routine. I will often have a few questions on the board for students to work on in the first 5 minutes of class while I sort out attendance & other duties.

The only way I’ve found this works is if I collect the warmups at least once a month. I tell the students to write 3 warm ups per page of paper, use both sides so there are 6 per physical piece of paper. It *really* helps to have them draw 2 lines ahead of time on the paper to subdivide the sections, otherwise they end up getting really sloppy.

I tell the students to save their warmups in the front of their science section in their three-ring binder.

Kids often want to take the paper out of their binder to write. I try to discourage the less-organized students from this, since half the time when they are done writing, they just stuff the paper randomly back into the backpack abyss. I tell them just to open their binder and write on the paper as it stays securely inside the three rings.

Topics for warm ups…

  • review of previously learned concepts. Particularly helpful to reinforce main standards. A little review, frequently, is better than a big chunk all at once.
  • quantitative reasoning (math connections to the science curriculum)
  • preview of new ideas with a motivating ‘hook’
  • problems involving ambiguous answers or frequently misunderstood concepts that can inspire a quick “Think – Pair – Share” classroom discussion

The most important thing to remember about warm up problems is to keep them interesting, at least mildly. How? I try to use sports, movies, or references to common or interesting personal experiences in the topics (as in, “The mass of Harry Potter’s wand is 20 grams. If it is placed in a graduated cylinder filled with water to 55 mL, the water rises to 78 mL. What’s the density of the wand? Malfoy’s wand has a mass of 24 grams and displaces the same amount of water. If they both drop their wands in a lake, which wand will sink? Why?”)