When you measure something in an experiment, there is always some degree of uncertainty in the value you might record.  Measure your height with a ruler - are you 60.5" or just 60.4"?  Are you 22 years old or 22 years 6 months?  Or really 22 years 6 months, 18 days?  Every number associated with a measurement - no matter how many decimal places the number might use - still has with it some uncertainty in how it was determined.  To be precise in experimental measurement is to measure and record a quantity with as little an uncertainty as possible.  A measurement of your height as 60.4" is more precise than 60", and a measurement of 60.35" is even more precise still.  Sometimes the instrument you use to measure something limits your precision.  A ruler, for example, marked to millimeters restricts your ability to measure a length more precisely than perhaps 1/2 a millimeter (the distance between the smallest marks).  A digital scale showing pounds and ounces will restrict your ability to weigh something more precisely than about 1 ounce.

But precision isn't accuracy!  You could measure something, but measure it precisely wrong.  If you measured your height while in high-heels, or your weight while holding a gallon of water, both measurements could be done precisely, and both would be inaccurate. 

In this lab, you'll see how careful, precise measurement is important, as is estimating how precise you really are (your uncertainty in your measurements) if you want to also be accurate!

Lab #4 Process:

1. I'll demonstrate the ballistic launcher, and the computer program you can use to collect and analyze data.

2. Locate your own launcher on a lab table, and secure it with a clamp ( Why?)  Decide on a direction to launch your ball so that it won't interfere with another group.  Measure and record the vertical height of the ball from the floor to its launch point.

3. Load and fire your launcher 5 times.  Measure and record the "horizontal" range of the ball from its launch point to its impact point for each trial.  (Why are you doing five trials?)

4. The computer program will display the average velocity of the launcher.  Record this value. Include the uncertainties in your measurements in the computer program to see what affect your uncertainty has on the average launch velocity.  Record this range.

5.  Now for the test!  Take your launch without ammunition up to the second floor balcony overlooking the campus courtyard.  Locate where you will launch it, tilt it to approximately 20 degrees, and measure and record the height of the ball from the ground to the launch point. How will  you do this?  What assumptions must you make?  How uncertain are you of this measurement?

6. Enter this height into the computer program, and predict where the ball will land.  Write down your prediction, and your group names on a piece of paper, and give it to me.  When all groups are ready, you'll be given a set of 3 balls to launch, at me.  I'll stand at the spot you have calculated, with a trash can.  You have to get your ball into the trash can without hitting me. 

Rules:

If you miss me by firing to one side or another, and show that it would have hit the trash can (or come close), and you can reshoot (after re-aiming!) without penalty.  If you miss me substantially forward or backwards *and* off to one side, and that will count as a miss.

Get the ball into the can on the first try, and you get a bonus!  Do it on ALL three trials, you get an additional bonus. 

Missing on the first or second trial should cause you to re-measure something before just firing again.   You get no more than three trials.

7.  After the lab, think of all the uncertainties you found in your measurements, and any other uncertainties you might believe would affect your results.  Discuss these online in the lab discussion forum.  How could you minimize the uncertainties here (which is to say, how could you improve your precision in measurement?)  Also discuss whether you were accurate or not (based on missing me!) and how your measurement uncertainties affected your accuracy.  POST your response to this question (only) in the discussion area for the labs, and read and comment upon the posts of others.

8. Your written, typed, and spell-checked lab report should include responses to Parts 2 through 6 above.  Discuss in a brief 1/2 to 1-page essay your sense of precision and accuracy as demonstrated in this experiment.  Discuss your results!  For this lab, while you are welcome to collaborate with others in the class, please submit your own individual report.  Aim for no more than 2 pages total.   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.