Lab 1: Accuracy and Precision (How Good of a Shot are You?

Lab 1: Accuracy and Precision (How Good of a Shot are You?) 

This lab does not require your lab kit, as many of you may still be waiting on yours to arrive. Please be diligent about checking on its shipment so that you do not fall behind next week. If/when you have yours, be sure to register it by following the directions in Lesson 1 that were before the link to this procedure. 

Introduction 

In any scientific experiment, there is uncertainty. The reason is because our measurements depend on man-made instruments. Thus, our results can only be as certain as the limitations associated with the measuring device we are using. For example, the weight given by a bathroom scale that measures to one decimal point is nowhere near as certain as a scale in the chemistry lab that measures to four or five decimal points. Two words often used to describe this certainty are accuracy and precision. Those outside the scientific community often incorrectly use these words interchangeably, but they have very different meanings. Accuracy is how close the measured value is to the true, actual, or accepted value. These accepted values are agreed upon by scientists and are the result of many carefully controlled experiments with very sophisticated instrumentation (but even so, there is still some uncertainty in the values since they were measured). So if something has a true value of 10.00, and you measure it as having a value of 9.99, you are very accurate. Precision, on the other hand, is simply how close you measurements are in agreement with one another. For example, if you take three measurements of 7.98, 7.96, and 7.99, you are precise. If the true or accepted value is 10.00, however, even though you are precise, you are not accurate. So you can see how it is possible to be both accurate and precise, neither accurate nor precise, or one without the other. The purpose of this lab exercise is to give you some applied practice with these terms, as well as to just have a little fun. It will also give you some practice with reporting uncertainty in the form of percent error. 

Procedure 

Part 1 

1. Find three sheets of paper. It doesn't matter what size they are, as long as you have three pieces that are the same size. Wad each of them up into a "ball." These will be your projectiles for this experiment. 
2. Mark a small target at which to throw your projectiles (i.e. a bull's-eye). It should be on a flat surface that makes it easy to mark their position. The floor works well, or maybe a table top. 
3. Take three shots at your target from a distance of your choice (make it challenging, and it will be more fun!). Measure and record the distance in inches (in Table 1) from the center of your target to each of your projectiles. This reflects your accuracy. Do not move your projectiles, as you need leave them where they are for the next step and the next measurement. 
4. After measuring your accuracy, make a label with your name, the lab title, and the date, and place it next to your bull's-eye. Unless all three—your name, the lab title, and the date are on the label, in focus, and legible, you will not receive credit. Take a picture of your results (the location of your shots relative to the bull's-eye), and make sure your label can be read clearly in your picture, your bull's-eye is clearly visible, and all three shots are also clearly visible. Save this single picture in a PDF document in order to submit it in the Lab 1 Assignment. Follow the link if you need assistance. If you do not submit your pictures in PDF format on the Assignment, you will not receive credit. 

1. Use a piece of string or a ruler (or anything you want, really) to find the smallest circle that will enclose the final position of all three projectiles. Measure and record the diameter of this circle in inches (in Table 1). The circle will have at its center a point between the three projectiles, and it will likely not be the center of the bull's-eye. The size of this circle reflects your precision. 
2. After measuring your precision, make a label with your name, the lab title, and the date, and place it next to your circle. Unless all three—your name, the lab title, and the date are on the label, in focus, and legible, you will not receive credit. Take a picture of your results (your shot grouping), and make sure your label can be read clearly in your picture, and the grouping of all three shots is also clearly visible. Save this single picture in a PDF document in order to submit it in the Lab 1 Assignment. Follow the link if you need assistance. If you do not submit your pictures in PDF format on the Assignment, you will not receive credit. This applies to all labs going forward. 
3. Find two other people to do the experiment as well—each taking three shots from the same distance as you did. Record the same measurements for accuracy and precision (in Table 1). You do not need to take pictures of these trials. 

Part 2 

1. Find (2) 20 oz. bottles of water. Alternatively, find (2) empty 20 oz. bottles and fill them up with water (any kind of water will do—drinking, tap, river, rain, etc.). 
2. Take your full 20 oz. bottles of water and place it on a scale. Any scale will work, regardless of its accuracy— kitchen, bathroom, etc. Record your weight (in pounds, lbs.) in Table 2. If your scale does not produce a reading, it may be necessary to stand on the scale yourself and weigh yourself holding the bottles of water, then weigh yourself without holding the bottles of water, and subtract the difference to find the weight of just the bottles of water. 
3. Take a picture of your bottles of water on the scale being measured (and you, if that was necessary). The bottle and the scale readout or measurement value must both be clearly visible in the picture to receive credit. Include in the picture a label with your name, the lab title, and the date, and place it next to your bottle on the scale. Unless all three—your name, the lab title, and the date are on the label, in focus, and legible, you will not receive credit. Save this single picture in a PDF document in order to submit it in the Lab 1 Assignment. Follow the link if you need assistance. If you do not submit your pictures in PDF format on the Assignment, you will not receive credit. This applies to all labs going forward. 
4. All things being equal, 20 oz. of water should weigh 1.304 lbs. That means two bottles should weigh 2.608 lbs. (the accepted value). How far off was your measurement using your scale? Use the following formula and present your answer as percent error in (Table 2): |???????????????????????????????? ????????????????????−???????????????????????????????? ????????????????????| ???? 100% = ???????????????????????????? ???????????????????? ???????????????????????????????? ???????????????????? 
5. Clean up your lab and put away your materials. 
6. Answer the post-lab questions below in preparation for the Lab 1 Assignment in the Test & Quizzes area. 
7. Create Tables 1 and 2 below (completed with your date) as separate Microsoft Word documents, then save them as separate PDF files. Alternatively, you can create the tables by hand, and save each as single picture in a PDF document in order to submit them in the Lab 1 Assignment. Follow the link if you need assistance. If you do not submit your pictures in PDF format on the Assignment, you will not receive credit. This applies to all labs going forward. You should have 5 total PDF files to submit for this lab. 
8. Go back into the course classroom, click on Tests & Quizzes located on the left side menu, and complete the Lab 1 Assignment posted there. Remember, it is in a quiz format, but it is not a quiz. You can access it as many times as you wish while entering your answers, and it is not timed. Just be sure that you do not actually click "Submit" until you are ready to be graded. 

Table 1. Accuracy and Precision 

Shooter #1 (You) 

Shooter #2 

Shooter #3 

Try 1 

Try 2 

Try 3 

Avg 

Try 1 

Try 2 

Try 3 

Avg 

Try 1 

Try 2 

Try 3 

Avg 

Accuracy (inches from bull's-eye) 

Precision (diameter in inches of enclosing circle) 

_______________ in. 

_______________ in. 

_______________ in. 

Table 2. Accepted and Measured Values of Bottle Water 

Measured weight of (2) 20 oz. bottles of water: 

___________________ lbs. 

Accepted weight of (2) 20 oz. bottles of water: 

___________________ lbs. 

Calculated percent error: 

Post-Lab Questions 

1. You have been chosen from the crowd to be the assistant to a knife-throwing demonstration. The thrower has asked you to stand very still with an apple sitting on top of your head, and he is going to throw a knife into the apple, removing it from your head. For whatever insane reason, you have agreed. Would you rather the thrower be accurate, or precise? Explain why. 
2. Which resulted in more accurate and more precise results—your three shots or the three shots from different individuals? Explain why you would or would not have expected this result. 
3. List and discuss two potential sources of error in this experiment. 

NEED ALL PICTURES AND ANSWERS.