Physics of playthings: Pump rocket

The third day of the physics of playthings intended to look at the number of pumps versus height for a couple of pump rockets.

I mistakenly thought that like the water rockets oa smallf my youth, one pumped a chamber and then released the rocket. I intentionally had not opened the packages so the students would know that I did not know what the rockets would do. This makes the exploration feel more real, and is certainly makes the activity unpredictable for me.

Upon opening I discovered that the rockets do not work that way. You simple pump once, the harder or farther one pumps, the higher the rocket goes. Only at the end of the period did I discover a small hole in the handle that should be covered to maximize the amount of air pushing the rocket. That hole is an intake when the pump handle is extended. Leaving it open bleeds air on the pump action and decreases the air pressure launching the foam rocket.

Before we went outside I outlined the data gathering. I knew we would be doing this rather roughly. I did not want to use the plumb bob on the protractor approach because I know that adds in a 90° minus the angle on the protractor complication. This was going to be very rough and ready measurements.

The basic layout can be seen here. I am 500 cm from the launch point with a yard stick and a protractor. I am holding the protractor level by guess and golly. I attempt to site the maximum height and then check the angle. The class has no background with angular measurements in the lab, and the calculation will involved the tangent function, which only a few have seen before. 

The pump has a maximum extension of 13 cm, so I instructed the class to start with a 1 cm pump extension. I had already argued that a zero pump pull distances yielded a height of 0 cm. This procedure undoubtedly contributed to problems. I could see that the rocket was exceeding 100 cm, but the students were still calling out heights of less than 100 cm. By 6 cm the rocket was clearly exceeding the top of the meter stick, and the students were still calling out values below 100 cm, so I told them to let me get heights with my rig.

In this shot a rocket can be seen just falling back down.

The data can be seen above, with notes on the angle in degrees on the left of the table.

The graph makes clear the problems associated with using two different systems of measurement. The graph also suggests that the heights were being "throttled" by the height of the meter stick. Especially when one considers that the 5 cm pump pull distance was clearly above a meter, up around 120 cm. Whether the data, once gathered consistently, would be linear, remains unclear.

In retrospect I suppose I could have used a plumb bob equipped protractor and simply handled the angular conversion myself. Sighting rings from paper towel tubes on the yardstick wouldn't hurt either. But the point was that the lab was rough and ready, invented on the fly, and yet a relationship emerged from the chaos.