Physics Blows?

When I think back on high school physics at Northside, I remember three things off the top of my head.

(1.) A competition to see who would create the best egg catapult that fit inside a 1 ft cubed box and weighed less than 1 kg (2.2 lbs): I came in second place to Jamie Close. (His father, a mechanical engineer, designed and built his... it was INCREDIBLE [It took Jamie, a HUGE football player, all his strength to cock the damn thing!]) Mr. Simmer's didn't think a single one of us would be able to break 50 yards with his design requirements, but Jamie's catapult, if I remember correctly shot his egg 54 yards.

(2.) Mr. Simmers had jokes or small puzzles at the beginning of each class. Example: "What do you call a millionth of a mouth-wash?" Ans: A micro-scope. (Get it? Micro = 1e-6 = 1 millionth, and scope is a mouth-wash!). Another example: "What is a Joule per second?" Ans. A WATT (get it? What = Watt!) Jokes are always better when explained.

(3.) One day, Mr. Simmers brought a glass tube with a plunger on it to class. He put a small piece of paper inside the tube, inserted the plunger and with a massive pressing motion forced the plunger downward. What happened? The paper caught on fire!!!

Two thirds of the things I remember of physics in HS happened in a total of about 10 seconds. The time it took to tell 2 jokes plus the time it took to slam a plunger down. And sure, the jokes were corny as hell, but I've never forgotten that "micro" means "millionth" or that a watt, a unit of power (i.e. rate of change of energy), has a unit of a Joule per second. And I'm never without a picture of what an Adiabatic Compression process can result in!

That information STUCK!

My point: sometimes, it is the small stuff, the things that teachers could look at and think my god that is trivial, that really lodge bits and pieces of information into kids heads.

Many times in the last year-plus that I have been here in Uganda, I've gotten completely wrapped up about the amount of material that I have to cover. I get so stressed, so frazzled, that I focus more on the derivations than presenting the principles in a way that makes my kids ABLE to remember them, in a way that makes them worth remembering ... that makes them fun.

Classes like that suck. I hate them. Surely my kids do to.

So I've tried to incorporate more 5 minute demos here and there to illustrate more complex topics we've been studying. I present them as "magic" tricks, and afterwards, I discuss the science behind them.

In the next few blog posts, I'll try to post a few of the better demo's I've done along with some pictures. We'll start with Bernoulli's Principle.

Have you ever seen the ball above the blow-dryer trick? You turn a blow-dryer on and place a light, round ball in the air stream. One who has never seen the trick before when asked might suggest that the ball will simply blow away when dropped into the stream. What happens? It levitates! Why?

Bernoulli's principle!

Air passes around the ball with approximately equal streams of air on all sides. Because the total pressure along a stream line is constant and all the stream lines are roughly the same, the pressure and thus the force acting on the sides of the ball cancel out (thereby keeping it above the stream of air). The ball levitates above the stream because of a static pressure at the bottom surface of the ball. The static pressure creates an upward force that eventually equates with the balls weight, so the ball floats within the air stream.

I didn't have a blow-dryer here in Uganda (though absolutely need one after 18 months of not cutting my hair). So my air supply was limited to what I could generate with my own body. It needed to be constant over a longer period of time, so the tube I blew through could not be to large (large area = more space to air escape = one breath escapes a lot faster... it takes less time to blow a chest full of air through an open mouth than lips puckered to whistle). A STRAW!

Now, what type of ball could I use? It had to be small and light enough to be balanced by my measly puffs of air. A PING-PONG ball! (The problem of course is that these are hard to come by here. I had to wait until I passed through Kampala at mid-service to get one).

Well, I finally got the supplies, and last week I was able to do my experiment. The following pictures show how it went.

Proper execution requires that you hold the straw perfectly straight up and down so the air flow is vertical AND drop the ball on top once you've begun blowing. Clearly, this was not a one man job for Mugisa.

... so Moses stepped in to help. The next important step is to coordinate when you drop the ball onto the air stream. If you drop the ball to soon, you get hit in the face with a ping-pong ball. If you drop it too late, there isn't enough air flow to keep the ball up. We ended up doing an "Ok, on the count of three, you start blowing and I will set the ball down then." Of course then it was a "Is it one, two, three...?" or "is it one, two, three, go?"

Here I am marveling at our fist successful levitation. Moses is holding the straw, and Mugisa is acting like a bag of hot air.

Moses gets a turn. Suzy, the only girl in the class does the drop while Ivan (at first completely against the experiment because he was just 'too cool') holds the straw ("Vertical Ivan, VERTICAL!").

Suzy, wearing a skirt, is unable to lay on the three stools we had set up, but the table worked just fine.

All in all, the experiment worked our perfectly. Each kid got a chance to try. Afterwards I was able to go through the physics behind the magic, and before leaving each of them, almost at once, they asked me if they could keep the straws and borrow a few ping-pong balls so they could do the trick for their families.

"Of COURSE!" I replied.

I am still stoked by their excitement.

Thanks for reading!

I love you all (but especially you, Michelle!),

Devon.