Physical Science 15

“If you thought that science was certain - well, that is just an error on your part.” - Richard Feynman

Your discussion post about the lab questions is due by Tuesday, 9/9 on Blackboard by 11:55 PM PDT.

Your lab report is due at our next on-campus lab meeting (Wednesday, 9/17 or Thursday 9/18).

Our second lab is also short, also hopefully fun, and definitely *full* of science. Last week we observed a phenomena, and then came up with possible experiments to explore. This week, you'll observe again, but add *measurement* to the process. In most experiments, it isn't enough just to observe what happens; we often want to know details, including how fast, how far, how high, how big, how loud, how bright, etc. We'll see, though, how every measurement has some uncertainty in it, and then consider how one might design an experiment that minimizes that uncertainty.

Remember our list of elements included in doing science:

observation of data, and developing questions about trends, differences, and unique occurrences seen within that data,
research into previous work on similar questions,
building models and creating testable hypotheses,
designing, and gathering new data through, experiments,
testing and refining the models based on the analysis of new data collected,
modifying or discarding incorrect hypotheses,
developing interconnected hypotheses into theories,
sharing data and results with peers for independent evaluation and testing, and
publishing findings in peer-reviewed journals so that the community of science can advance.

This lab involves observation, experiments, sharing data, and publishing findings.

Lab #2 Process:

1. Observe the StompRocket demonstration. (If you aren't present, you can see another short YouTube video at http://www.youtube.com/watch?v=ctg-PYZAh7I&feature=related ) Make as thorough as possible a record of exactly what you observed. Attention to details here is important!

2. In this lab, you need to answer the question, how high does the rocket go? You'll try two different observation techniques, one based on angles, and one based on time. We'll do this in teams. And we'll compare the answers to see what we can learn. But before you can start the experiment, you must also try to fix other variables so that multiple attempts produce approximately the same results. For this lab, what must you you *control* in order to measure the height of the rocket over multiple launches? How can you control that (those) variable(s)? What can you not control? What can you measure before you start? What are those measurements?

3. The TIME method. Using electronic timers or your watch or cell phone (if it has a timer!), estimate the time it takes from launch to landing of a rocket. Repeat your experiment at least FIVE times, record the results, and compute the average time. As you'll find in Chapter 4 of our book, the height of a freely-falling projectile can be calculated from:

Height = 1/2 g (t/2)²

where t = average total time of flight going up and down, and t/2 is one half of that time, representing how long it took the rocket to fall from its maximum height.

and g = the acceleration of gravity on the surface of Earth, 9.8 meters/second / second (or about 32 feet/second/second).

Example:

Your average flight time was 9.9 seconds. One half of this is 4.95 seconds, and so the average height must have been:

1/2 x (9.8 m/s/s) x (2.4975 s)² = ~ 31 meters.

Note that even though your calculator gives you more digits in the answer ("30.56378063") your least precise measurement of time had just two digits of precision (9.9 seconds). That least precise measurement controls the precision of your answer, so you round to the same number of digits. In this case, that is two.

4. The ANGLE method. Have one person stand about 20 meters away from the launch site, holding an "astrolable" or angular measuring device. Measure as precisely as possible this distance, which will be recorded as "d". As the rocket is launched, the person should try to spot the rocket just at the top of its flight, and record the approximate angle of view on the astrolabe. Again, make at least FIVE observations, recording the angle each time. Compute the average angle, recorded as q. From this data, you can use trigonometry to find the height of the rocket:

Height = (d) x (tan q)

where " tan " is the tangent function on a calculator.

Example:

You stand exactly 18 meters from the launch site. The average flight angle was 60 degrees. The average height must have been:

= (d) x (tan q) = (18) x (tan 60)~ 31 meters.

Note that even though your calculator gives you more digits in the answer ("31.1769") your least precise measurement of angle had just two digits of precision (60 degrees). That least precise measurement controls the precision of your answer, so you round to the same number of digits. In this case, that is two.

5. Questions you need to answer in your lab review include:

How did the height calculations compare between the two methods? (Were they close, or far apart? What do YOU define as "being close"? :)
Which method has more uncertainty in its measurements of its key variable? Why?
Does the angle the projectile is launched affect the results of either method? Would it be easier if you launched the rocket at an angle rather than straight up? Why or why not?
How did you end up controlling the height of the rocket between trials? What could you do to control the launch pressure even more precisely?

Your lab report should include responses to Parts 1-5 above. For this lab, while you are welcome to collaborate with others in the class, please submit your own individual report. Aim for at least 2 pages here, typed. Bring the lab report to our next on-campus lab, or if you prefer, email it to me within Blackboard as a message, not as an attachment. Please don't post your lab report in the discussion forum.

6. POST your response to this final questions in the discussion area for the labs, and read and comment upon the posts of others:

Richard Feynman, a Nobel-prize-winning physicist, said "the test of all knowledge is experiment. Experiment is the sole judge of scientific "truth." If every experiment contains uncertainties, based on the procedures and equipment used, is there real "truth" in science? How does having two independent ways of determining height in this experiment help to establish the validity of the theory of gravity causing everything, regardless of weight, to fall at the same rate? If you read that scientists claimed global warming to be true based solely on measurements of CO2 levels in Hawaii over 50 years (carried out by Charles Keeling of UC San Diego, starting in 1958), what might you say?

Going Further: Extra Credit Post your essays for any of these in the Blackboard discussion forum for this Lab.

Explore making your own stomp rocket.

College of Physics. (2005) Stomp Rocket . Purdue University. http://www.physics.purdue.edu/outreach/physics_on_the_road/rocket.shtml

Do an experiment on your own, exploring this phenomena, and investigating one or more questions you created in step #3 above. Record your data for posterity, either in a digital video you can upload, or at minimum, a sequence of digital images we can upload for the class to view.
Read more about the Keeling Curve: Berger (2002) Keeling Curve. Global Change & Global Warming. University of California San Diego. Available at http://earthguide.ucsd.edu/globalchange/keeling_curve/01.html