So this week I am going to talk about the lab we just finished. It is a lab that takes at least 2.5 lab classes or 5 regular periods. It is the typical projectile lab that physics teachers do, but I do it as an inquiry style lab. I don’t tell the students how to do the lab. I divide it up into sections where the students have mini problems which they have to solve.

# THE GOAL

The student’s ultimate goal is to shoot a water balloon out of a water balloon launcher and hit me at a specified distance they have just learned about 10 minutes before the launch. They must actually hit me with the balloon to “win.”

# Task #1

Their first task is to figure out how to launch the water balloon with a consistent launch velocity and figure out that launch velocity. Now in class we have launched rockets in the air at a 90 degree angle to the ground and figured out it’s launch velocity, but I don’t mention it at all when I hand out the lab. Most students go in different directions – even after I mildly suggest that we may have done an easier way. They all want to launch at angles straight off. To me this shows that they really have not learned what I thought I had taught during the rocket launch. It is heartbreaking as I do my best not to tell them where they should turn in the lab. I might as well as pierce my tongue for all the tongue biting I do, as I hint at the solutions to their problems. I keep hearing, “Just tell us what to do!” from the students. But I let them struggle hoping they will see the point.

# Task #2

Once they have the initial velocity, they must now come up with a table of launch angles and horizontal ranges. Again, I don’t tell them how to do this. We have solved individual angled projectile problems which most eventually can solve. But ask them to do the same problem with different angles causes most students to start bleeding from the ears. Why this happens, I have never been able to figure out. Eventually though, they get there after much angst and arguments. I know they get it when they realize, by themselves, that there is a pattern in the horizontal and vertical velocities and eventually they see the pattern in the ranges.

# Task #4

Once the table of angles vs ranges is completed I have the students decide on one specific angle to test launch a water balloon. What they discover is that there is a whole lot of error in this lab. How they use this information I leave up to them. Many ask me what to do with it. My answer is always the same,”What does it mean to you? How do you think the error can be minimized or utilized when you really shoot at me?”

# Task #5

I look at the student’s table of angles and ranges and then pick a range distance that is not on their table but between two angles. They have ten minutes to shoot the water balloon at me.

# The Outcome

IN the end, I have never been hit by a water balloon. However there have been teams that would have hit me if they had not had a problem with shooting to the left or right side instead of straight ahead. About half the groups don’t ever come close to hitting me, but half do come close. There are two main reasons that students miss me(other than not knowing at all what they are doing)

- They have no clue how to use a protractor and plumbob for finding angles. I have only had one group in the entire 15 years i have done this lab ever create their own protractor to measure up from the ground. Almost always they use a protractor with hanging weight to figure out the angle, and no amount of my explanations ever seem to be enough for these groups to understand this device.
- Another group error is the group that decides to “gut it.” They give up on their data and decide they can guess the angle and pull. They change all of their knowns(pull length, pull angle, distance between “holders”, etc) These groups never come close to hitting me, yet they are almost always the most boisterous.

# WHY DO I DO THIS ??????

This lab is the most aggravating lab I do all year long from my point of view. The students feel that way during the lab, but they view changes throughout the year. I know this from the comments I get later. This is the lab everyone asks about when they come to visit after graduating – well this and my Mystery Box Lab but I will talk about that later. I am told constantly that this was the most aggravating lab they have ever done – but it sets the stage for the class. Failure happens, Errors happen, and they can’t count on me to tell them what to do every minute. During the entire lab, I see what they are doing wrong and I want to tell them, but I don’t. I know they will learn important lessons in their failure. But they don’t know how to deal with failure. Yet another lesson they must learn.

Some students don’t get over this lab. They are mad at me for not telling them what to do. They have always had a helper in everything they do. Yet I am smiling. Even if they don’t get it, they have learned something. They have failed at “the goal” of hitting me, but they have succeeded at real hidden goal of the lab – to put things they have learned to take things from class and utilize them in a real life activity. They fail, not because of their work, but because real life has errors involved in each and every aspect of it.

What kind of device do you use to launch the balloons? Did you build something?

WE used a commercially available water balloon launcher. Kind of like