Physics Activity - Measurement

Target Practice¹

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Discussion:

In this activity, you will drop a ball and see how accurately (or should I say precisely?) you can hit a target. You will need a group of 3 bright young physicists (a "shooter", a "catcher", and a "coach") for this activity.

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Equipment:

Each group will need:

7 paper targets (2 per person, plus an "extra")	1 sheet of carbon paper
1 small plastic sphere	tape

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Procedure:

Part 1 - Eyes Open:

1. In your lab notebook, write a brief description of this lab, and a sketch of the lab setup. Remember that your lab book is a record of the lab as it happens - not an after-the-fact report.
2. Tape 2 target sheets together by one edge, both face up. Place the sheet of carbon paper between them, printed side up. This way, when the plastic ball hits the upper target, it will make a permanent mark on the lower target.
3. Place the target sheet assembly on the floor.
4. The "catcher" kneels down near the target and catches the ball after it hits the target the first time, since subsequent bounces will also leave a mark on the target and we only want to see the results of the first hit.
5. The "shooter" holds the plastic ball about 2 meters above the floor (approximately level with the top of your head), and drops it on the target. The goal is to score a "bulls eye".
6. Repeat step 4 for a total of 10 trials (drops/shots).
7. Label the lower sheet with the shooter's name and "Eyes Open".
8. Replace the lower sheet with a new target. The next person in the group becomes the shooter.
9. Repeat until each person in the group has been the "shooter".

Part 2 - Eyes Closed:

1. This part of the activity works pretty-much the same as the first part, except that the person dropping the ball must keep their eyes closed at all times. (!). One person in the group drops, one catches, and the third person acts as the "coach". The coach can direct the person dropping the ball, but they must do so from at least 5 meters away (across the room).
2. After the you take 10 shots at the target, label the bottom target with your name and "Eyes Closed".
3. Switch roles so that each person in the group is a "shooter", a "catcher" and a "coach", in turn.

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Results & Conclusions:

Suppose that each of your "shots" in this activity represents a measurement of some quantity, and that the target's bulls eye represents the accepted value of that quantity. Physicists have two concepts to express the quality of measurements - accuracy and precision.

1. Like any other physics terms, accuracy and precision are carefully defined, and may not mean the same thing to a physicist as they do to a non-physicist. Before you go on, be sure that you read the material on accuracy and precision and understand the meanings of these terms.
2. For each of your (two) targets, write a brief description of the pattern of "shots" using the terms "accuracy" and "precision". (Were your results accurate? Were your results precise? Why do you think so?) You may write these descriptions directly on the target page. Then tape or staple the target pages in your lab notebook. Scientists always keep their original data.
3. For each of the descriptions below, make a sketch (in your lab notebook) of a target and a group of "shots" that could be described as:
   1. very accurate and very precise.
   2. very accurate, but not very precise.
   3. precise, but not very accurate.
   4. neither precise nor accurate.

   (Be sure to clearly label each diagram.)