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3.A Writing Experimental Procedures

Science Practice 3 is about scientific questioning and argumentation. Skill 3.A is to create experimental procedures that are appropriate for a given scientific question.

This skill will not be tested on the multiple-choice section of the AP Physics 1 exam and will be only part of 35-45% of the free response dedicated to all of science practice 3. The individual skills do not have a % breakdown provided for the free-response section.

How will you recognize when to use this skill?

This will be needed on the Experimental Design and Analysis (LAB) free response question (Q3) on your AP exam. We will also need this skill when we do labs in class.

You can sense this is happening when the prompt starts describing a question about a scenario that students need to answer by collecting data. The part of the question that says "Describe an experimental procedure for collecting data . . . Includes any steps necessary to reduce experimental uncertainty . . ." is where you will use this skill.

How to approach writing data collection procedures

In describing the LAB questions, CollegeBoard states their expectation: "The experimental procedure is expected to be scientifically sound: vary a single parameter, and measure how that change affects a single characteristic. Methods must be able to be performed in a typical high school laboratory. Measurements must be made with realistically obtainable equipment or sensors." Keep that in mind as you plan, draw, and describe with the following tips.

Most of the points on the FRQ are tied up in the data collection but see the section at the bottom of the page on data analysis too.

Plan - read to understand and plan the math

Read and Re-read:

First read the prompt for understanding to get an idea of what the scenario is. Then go back and double check you understand what the variables are you are investigating, conditions, and limitations.

In the example prompt 1 the goal is to find the acceleration due to gravity and we are very limited on materials: books, stopwatch, cart, meterstick. This limits us to distance and time data we can collect.
In example prompt 2 we have all normal lab equipment (mass sets, spring scales, force probes - anything we have used in class is available), but our goal is more confusing. When they say force per centimeter stretched, that is a description of the spring constant k for each spring. So, our goal is to figure out the relationship between k and the diameter of the springs.

Plan any math needed

Once you know your goal and what you can collect, identify any equations or math processes you will need to use to answer the question. It is very common for us to ask you how to create a graph that can use your data to answer the question, so have that in mind as you plan what data to collect. There are several ways this experiment could be carried out, so the following are only examples, not the only approach.

Example 1: With the books you can prop the table at an angle and measure the time it takes the cart to travel a measured distance. We can find acceleration using Δx = 1/2at^2 if the cart is released from rest each time. Since acceleration on a low friction ramp is found by a = ΣF/m = mgsinθ/m =gsinθ, in order to get g from this we would need the angle of the ramp, which can be found with the change in height of the table and the length of the table. In summary, we need to collect the unchanging height change Δy of the ramp and Length of the ramp, as well as combinations of distances and times for travel down the ramp.
Example 2: You can hang the springs vertically on a hook and place a mass on the bottom of the spring. Once at equilibrium the spring's force will be equal to the weight, mg. Beyond expressing that k=F/x., that is all the math needed.

Take your time to plan well because skim reading the question and moving ahead with the wrong idea of what is being asked or not having a path to the solution will end up with a low score or will take much more time to rewrite if you figure it out later.

Example prompt 1 - Find a quantity:

Students are given a dynamics cart, a stack of books, a stopwatch, and a meterstick and are asked to take measurements to create a graph that could be used to determine the acceleration due to gravity.

(a) Describe a procedure for collecting data that can be graphed to determine the acceleration due to gravity. Students have access only to the dynamics cart, books, stopwatch, and meterstick. Include any steps necessary to reduce experimental uncertainty.

Example prompt 2 - investigate a relationship:

Students are given a set of springs all the same length, but of varying thicknesses. Student A claims the force per centimeter stretched for the springs is directly proportional to the diameter of thickness. Student B claims the force per centimeter stretched for the springs is directly proportional to the diameter of thickness squared.

(a) Describe a procedure for collecting data that can be graphed to determine which claim, if either, is correct. You may use equipment typically available in a high school classroom. Include any steps necessary to reduce experimental uncertainty.

2. Draw a diagram of how equipment is used

For each quantity you need to use in your analysis (outlined in your plan) decide a piece of equipment and assign a symbol.

These are most often outlined in a chart before the procedures are to be written.
Not all the rows need to be used and you do not need to list every single iteration of a measurement.

Diagrams help clarify how tools are to be used and where measurements are to be made.

Sketch where any equipment will be placed in the initial setup and label the most important parts.
If any measurements are to be made at different places or between places, indicate them with arrows and symbols you will use in your table.

Note how in example 2, the diameter of thickness could refer to the whole coil thickness or just the thickness of the wire, but the diagram makes it clear.

3. Describe collecting the data

Note, that the free response question will nearly always separate the data collection and the analysis description into different sections of the question. Look for the words data vs. analyze. These tips are about collecting data. Do not fill up the space for data collection by describing the calculation to be made or the graphs to be produced.

For every measurement made, use the O-AIM technique as fully as possible to ensure clarity.

- Object being measured
- Action being taken (or definition of beginning and end of the measurement)
- Instrument making the measurement
- Measurement being made

Examples:

Use the motion sensor to record velocity of the mass as a function of time as the mass falls to the ground.
Use the ruler to record the position of the spring after the mass has come to rest.

Double check that you describe measuring all the measurements needed from your table & math plan. There should be 2 measurements that vary (one dependent on the other) and pay be a number of constants needed.

Reduce error by repeated trials and variation over a range when possible. Ideally, we will do lots of repetitions, so write it that way, even if in lab we don't have time to do them all.

Note some specific things that should not be included:

Do not state to "gather materials" as this makes you feel you have written something of substance, but you have not yet. No points have ever been awarded for this.
Do not get overly detailed with something students should generally understand, like "have one student slowly slide the cart up to the top of the ramp and get ready to release it on a cue established. As that student is moving it up the track, have another student open the clock app on their iPhone and select the stopwatch. They should count down, like '3 . . .2 . . .1 . . . release!' to be sure they start the stop watch and release the cart at the same time. If the first student accidentally pushes the cart forward or backward, restart the trial. As the cart travels down the ramp have the second student get their thumb ready to press stop at the exact moment when the cart reaches the designated location." Assume that the audience knows how to use the tools. Clarify with a well annotated diagram and you can be efficient by saying "use the stopwatch to time the cart as it travels from rest at the starting position to the final position."

Example 1:

Prop books under one edge of a table to create an angled ramp for the cart to travel down.
Use the meterstick to measure the distance from the floor to both the high end and low end of the table. Record these as h1 and h2.
Use the meterstick to measure the length of the tabletop. Record this as L.
Set the cart at the top of the ramp and release it from rest, using the meterstick to measure the distance traveled to the end of the table and a stopwatch to measure the travel time.
Repeat step 4 for 5 trials for the same release position and average the travel times recorded to reduce random error.
Repeat steps 4 and 5 for 8 different distances to further reduce error.

Example 2:

Record the mass of a hanging weight by placing it on a digital balance.
Measure the diameter of a spring using the caliper.
Hang the spring on a stand and use the ruler to record the position of the bottom of the spring.
Hang the weight on the bottom of the spring and record the new position of the bottom of the spring, ensuring the weight is at rest in equilibrium. (note: no need to say to use the ruler again)
Repeat steps 3-4 for 5 trials for the same spring diameter and average the position recorded to reduce random error.
Repeat steps 2-5 for each of the spring thicknesses.

How to approach describing data analysis procedures

In describing the LAB questions, CollegeBoard states their expectation: "Students will be expected to describe a method by which the collected data could be analyzed in order to answer the posed question, by either graphical or comparative analyses." The math you planned in step 1 before writing procedures needs to be described here.

Try and force your data into a useful graph
- For problems that are seeking to find a quantity by experiment you nearly always want to create a linear graph. If the relationship between your data collected is not linear, then you need to linearize it by calculating a new column of data. See my page on
Describe any calculations to be done.
- Not looking for you to use only words, like, "multiply half the mass times the velocity squared," but be clear about how to use the data and equation , like, "use the masses and velocities collected to calculate a column of kinetic energies using K=1/2mv^2."
Make sure you answer the question of the lab.
- Using the shape of the line on a graph is usually the easiest way to get credit. You can sometimes describe how results of calculations compare. Be careful with calculation comparisons, though, because sometimes students expect answers to be the same, but experimental error make it more complicated than that.
- For example, if the question of the lab is to tell if mechanical energy is constant within a system, make sure to explain what about the data collected tells if it is constant. You might plot the initial energy on the x axis and final kinetic energy on the y axis and if the slope of the line is approximately 1, then that would support a claim that there was no loss of mechanical energy.

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