A simple demo, a deep result

Sometime Simple is Better

Every physics teacher has a favorite way of demonstrating that a horizontally projected object falls toward the ground at the same rate as a dropping body. Some show a video of shooting a monkey falling from a tree. Some have a device that drops a ball at the same time as it pushes a ball outward.

I used to use such a device to introduce projectile motion.  However, I have recently have started to use instead a simpler way thanks to something I learned from from a meeting of the Western New York Physics Teachers Alliance. Using a popsicle stick and two washers or coins, you can have your students do the same.

Start with what they think will happen 

I like to start with what students "know" will happen. They "know" that an object dropped straight down will hit the floor sooner than an object flicked out horizontally. They also "know" that the faster the flicked object is moving, the longer it takes to hit the ground. I then give each team of two one stick and two washers (you can use pennies or student-supplied quarters). Their task is to figure out how to use this stuff to make one washer fall straight down at the same time as the other washer. I am still trying to figure out how to guide my guys to the proper setup. I give a hint about videos they may have seen about ripping a tablecloth out from under expensive china and fragile glassware. Some take the hint, however, for sake of time, I have to stop the exploration time and lead them to the best setup.

Once we have this setup, I let my guys experiment. It doesn't take long before most teams realize that both washers hit the ground at about the same time. We then get to the basics of projectile motion.

Bring it home

At the end of class, I tell my guys to "confound your parents with this". We figure out that a ruler can be used along with pennies or quarters. I know that most will not do this at home, but a few might. This year, a couple of days later, one guy came up to me after class and said, "I showed my mom this and she didn't believe it so we spent about an hour doing this." One small success.

Saying goodby to a treasured question

The Lance Armstrong Question

Sometimes, you get a physics question that really works, whether as a in-class group discussion, a homework assignment, or an assessment question. One of mine was the Lance question.

When we cover work and power, I have my guys measure their own power output. While some do this by bench-pressing or squatting weights, and once one guy pushed a car, most do it the simple way by running up a flight of stairs. Most get around 1 hp for for this brief burst (I love the ones who get 12 hp or even 100+ hp. I try to make it a lesson on checking to see if your calculations make physical sense). On the subsequent test, I ask the question:

In the 2001 Tour de France, Lance Armstrong rode up (at a constant speed, equivalent to running up) the mountain L’Alpe Duez (which is 3500 feet high) in 38 minutes.  His weight (with bicycle) is 180 pounds.  What work did he do in lifting himself up L’Alpe Duez?  What was his average power (in horsepower)?  

The answer is around 0.5 hp. I would then during the test review compare his power output to the student's. Most would recognize that their output was for only a few seconds, whereas Lance's was for more than half an hour. I would then tell the class that this was the third climb of the day, the previous two almost as high. Then I would tell them that the climb occurred at end of a 100+ mile race and that it was the 11th day of racing. Some students would be impressed by that athletic feat.

It was a nice way to relate a calculation that we do in class to real-life. However, with recent revelations, I can not with good conscience include this question in future classes. I recognize that performance enhancing drugs will give only about a 10% boost in output for athletes at that level, but most students would not recognize that distinction. I will miss using this classic example.

Post Script

This post has my replacement for the Lance question. 

Bill Gates, fund this experiment

My guys want to try this experiment, we just need the money.

It is time for the projectile motion labs. While I have the guys in the regular physics class use photogates to determine the speed of a marble at the bottom of a ramp and from there predict how far away the marble will land on the floor, I try to make things a little more challenging for my AP guys. I have them make the prediction based on the vertical drop on the ramp. The day of, we spend the first few minutes making sure everybody has the right equation. This year, after we go to this point

one guy blurted out, "So, gravity doesn't matter!" Another chimed in "We could do this on the Moon." Now we just need to raise the money.