Students writing lab reports in CSULA physics courses tend to make the same few mistakes every semester. Some of them learn what to do after a few middling grades, and others continue to struggle with the same issues throughout their class. This page tries to give example of the most common issues I’ve experienced in my classes. Other professors may have different preferences about details of formatting, but all of them should generally accept the “what to do instead” examples as correct. I’ll start with graphing problems, and then go on to other report issues.
One common mistake is to display the information you gathered in lab in a line graph. This might seem to make sense for the experiments where you are supposed to show how some number changes over time, but it is not what you should do. Each line in your data table is a separate measurement, so you should always use scatter plots when writing your report. There is no need for connecting lines or an average, only a (usually) linear fit line and its equation.
It is simply not acceptable to draw a graph in a lab report. That worked for teaching concepts in math courses, but it is not good enough to analyze data. This is an extremely common issue, and unlike some professors I heavily penalize reports with drawn graphs, because the conclusions from them are not reliable. Make your graphs with a computer, typically in a spreadsheet program. You need to print out the result and attach it to your report, not just draw a copy of what’s on the screen.
The reason to make a graph is to use it to analyze data. The most common analysis we do is construct equations where the slope of a graph is a number you are trying to find. Most of the labs in undergraduate physics are built around some linear equation. That is where the number you are looking for depends on two numbers you have measured, and the equation for your variable includes one number divided by another. If you put the numerator on the y-axis of a graph and the denominator on the x-axis, then a line drawn through the data points will have a slope that is equal to the answer. How you add the line depends on your software. The instructions for using Microsoft Excel are at https://support.office.com/en-us/article/add-change-or-remove-a-trendline-in-a-chart-fa59f86c-5852-4b68-a6d4-901a745842ad?ui=en-US&rs=en-US&ad=US&fromAR=1#bmaddtrendline
If you make the graph with a computer and make a trendline on it you still aren’t done, because you need the numerical value of the line’s coefficients. The coefficients of an equation are the numbers in front of the variables, like the m and the b in y=mx+b. Most of the time we only need the slope, the number before x. You can measure it on the graph with a ruler, but that throws away all the precision the computer gives us. Display the equation for the trendline in your software. Instructions for Excel are at https://support.office.com/en-us/article/add-change-or-remove-a-trendline-in-a-chart-fa59f86c-5852-4b68-a6d4-901a745842ad?ui=en-US&rs=en-US&ad=US&fromAR=1#bmdisplaytrendlineequation
Sometimes a student will create a graph correctly, but they misunderstand the purpose of the equation. You may notice that in any lab where you are supposed to make a graph and find a linear fit, you can divide your y-axis data by your x-axis data to get a number with appropriate units and a size near what your theoretical expectation tells you. This is no coincidence, a linear fit to a graph is basically the same division. But a linear fit includes contributions from every data point, not only one of them. The answer you calculate from any pair of data points will not match what the slope of a fit tells you, because the fit is a sort of average across your data.
This problem is less common than the others, but it happens at least once every semester. If you make a graph with the wrong data selected or with only one column selected, most software will not throw an error. It will just assume that each thing you’ve selected is a point on the y-axis, and that the x-axis values should count from 1 up. Look at the values on your graph’s scale before doing anything with the numbers shown.
A lab report should communicate what information you have collected and how you are analyzing it. All your graphs and all your calculations should have a description. For graphs, that description needs labels on your axes that say what they measure in what units, and a title that tells what the data is for. “Velocity over time” is not a title, you need to say "Acceleration of a sled".
The purpose of the title is to show what you’re analyzing. There may be many graphs in a report, and they might all have the same axes. A reasonable title could be the title of that part of the procedure, or an identifier of what was different in that part compared to the others.
There are many phrases used in the instructions to indicate what the axes of a graph should be. The key to understanding those phrases is knowing that the y-axis is always labeled first. So "period over length", "period vs length", etc all mean period on the vertical axis and length on the horizontal. If you reverse the axes, the slope of a linear fit will be the reciprocal of the expected number, because you’ve switched divisor and dividend.
Sometimes you will mess up in class. You write a number wrong in your table or during your arithmetic after. So when you graph out the data, there will be one or a few data points very far from the others. Those sort of data points are called outliers. When you see them, first look over all your arithmetic again to look for a mistake. If the numbers in the data table don’t look like there are any particularly big jumps between two lines, then the mistake is in your math for sure. But if the table shows the jump you see in the graph, the mistake happened in class and probably can’t be fixed. But you should make another copy of your graph from a data table that doesn’t include the outlier. Do your analysis a second time from the new graph. Make sure to show both original and modified results in your conclusion, and talk about why points were excluded. This should happen very rarely, most students won’t need to do this for any lab.
After you’ve done all the work to make a graph, it needs to get into your report. Since your reports need to be in a composition book, this means attaching a printout to the book. You need to cut the paper around the graph so that it fits neatly into the book without any folding. Everything in your report needs to be readable without moving anything, so do not cover existing text and do not stack multiple attachments. Size the graph small ehough that it fits into the book with the text facing the same direction as your writing. Graphs don’t need to be big when you use a computer, because the precision comes from its calculations and not the size on the paper.
This is by far the most common problem. All physics calculations need units on every number. Some numbers are "dimensionless", meaning they do not have a unit. Other than π, this will rarely come up. You need to include the units on your numbers and use them in your math because that is part of what makes an exercise "physics", by grounding it to the reality of measurements. Without units, math is much harder to understand when reading a report, and lots of mistakes can slip in that are impossible with units. When you know your answer should be a velocity in meters per second and the number you’ve calculated is in seconds per meter, you can immidiately see a probblem and how to fix it. Remember that units are just like variables in algebra. You can multiply or divide them, and you can add and subtract numbers that have the same units, but not different ones. Many units have a "prefix", a number that indicates a change in the size. These also have to follow along in the math. If you are ever unsure of what a math operation does to a prefix, just convert the number to the base unit. Usually that is the same unit with no extra symbol. like converting cm to m. Note that the base unit of mass is actually kg, not g.
Related to the problem of not using or incorrectly using units is angles. Angles in physics are a confusing idea for many undergrads, because previously you’ve always seen units labeled in degrees and not had to think about what that means. In physics, degrees are a unit, and not a very useful one in some contexts. We often prefer to label angles in "radians", which are called the “natural units” of angle. They are dimensionless, though they are sometimes given the symbol "rad". In any equation that involves multiplying or dividing by an angle, the angle has to be in radians. Degrees are only acceptable when they are used inside a trigonometric function like sine or tangent. Be careful when using a calculator, they usually have a mode button to change between degrees and radians. If you aren’t sure which mode you are in, use it to find cosine of 90. If the result is 0, you are in degrees mode. If it is -0.448, you are in radians.
Labs don’t just make you calculate things for fun, you always need to compare something you’ve measured to some other number you calculate or measure. So if you look over your report and see a calculation that wasn’t compared with anything, it either wasn’t necessary or you’ve forgotton something. Usually you’ll compare a “measured” or “experiemental” value to a “theoretical” or “expected” value. A measured value is one that is somehow found from the whole set of numbers you’ve collected in lab. The expected one comes from some equation with just a few measurements or is given to you without explanation. The most common comparison is the “percent difference” between experimental and expected, but don’t assume every comparison in every lab will be that.
For ease of reading the report, make different sections for your data and your calculations. Data should mean numbers you found with a tool in the classroom. That should be earlier in the lab report. Calculations are everything that you found by math from the data. Put that all afterward, and use sentences to explain what you’re doing. Write down the equations you use in symbolic form before putting numbers in those them, both for me to read it and yourself to think about what you’re trying to do.
Lots of labs will involve repeated measurements and repeated calculations. I don’t need to see the equation and arithmetic every time, but I do need to see at least one case of each type of calculation you perform in the report.
This was already in the graph section, but it is important. It also applies to typed tables and text, or anything else you want to put in a report. The lab report book needs to stay readable as a book, without turning it or unfolding or lifting anything. You need to cut the paper around any attachment to fit neatly on the page. Everything in your report needs to be readable without moving anything, so do not cover existing text and do not stack multiple attachments. Make attachments small ehough that to fit into the book with the text facing the same direction as your writing.
A good habit that few students learn is to evaluate the validity of their math and fix the mistake when the result doesn’t make sense. You always have some idea what a number you are calculating should be before doing the math, so if you are calculating the mass of a ball and see "900 kg", it is obvious there is a mistake. You can’t just accept the presence of a mistake, find it and fix it. That will make a big difference in your grade. I don’t grade based on how accurate results are, but I will deduct points for incorrect math. If I can see a problem in a few seconds without a calculator, you could have fixed it while writing the report and you will lose points.
Each instructor may have their own particular policy about late work. Mine is simple, I don’t take it. Anything not on-time is a score of 0. So be on-time. Reports are due at the start of class the week after you complete a lab. Don’t ask for help with a report during the next class, you need to ask your questions during the week bewtween collecting the data and turning your report in.
Students come in late to classes quite often. It is a part of the university teaching environment that professors do not need to wait for you. You need to be on-time, there before the start of every class, every time. If you are not, you will miss instruction on the concepts and execution of the day’s lab, and should not hold up the rest of your group due to not understanding something.
Listen to the introduction lecture for each lab. Your lab manual tells you what to do, but the point of an instructor is to adapt to teach in ways that a book cannot. If you don’t listen and just follow the instructions, you’ll miss any changes I’ve made to make procedures easier or faster, work around misbehaving equipment, or any other changes. If you follow the analysis instructions in the book in a case where I said something different, do not expect to get full credit.
The procedure around unplanned absences is "let the student deal with it". You are responsible for learning whatever you missed, ideally by attending the lab on another day that week. Ask the instructor in the room if you can work with a group to perform the experiment. You cannot just ask your normal group for their collected data, doing the hands-on work is part of the class and required to turn in the report. Your report will still be due at the normal time. If you don’t get it done on-time then that report scores 0. It is a department standard that missing multiple classes will fail the course.
Bring your reports to the locked boxes by the elevator of biological sciences, on the first floor. Read the labels for your class section, and turn in to the slot directly above the label.