Phet Lab Balancing Act Answer Key

How PhET Lab Balancing Act Actually Works

The PhET Balancing Act simulation is a fairly straightforward torque exercise. You place masses at various positions along a ruler, and the system checks whether the sum of moments on each side of the pivot equals zero. That's it. The interface looks a little playful with the cartoon elements, but underneath it is just a 1D static equilibrium problem with discrete integer mass and position inputs. The simulation lives at phet.colorado.edu under the "Masses & Springs" or "Balancing Act" section. No download is required, which matters because students sometimes hit dead ends looking for a PDF or downloadable worksheet. The activity runs entirely in-browser. If you are a teacher assigning this as homework, the built-in answer key is effectively the simulation itself, since the tool tells you instantly whether a configuration is balanced or not. For teachers who want a reference sheet before assigning the lab, most answer keys online are simply teacher-made compilations of common puzzle configurations. Here is a practical set:

Puzzle Mode Setup 1: Place a 5 kg mass at position 1 on the left side. Place another 5 kg mass at position 1 on the right side. The beam balances. Puzzle Mode Setup 2: Place a 10 kg mass at position 1 on the left. Place a 5 kg mass at position 2 on the right. This balances because 10 times 1 equals 5 times 2. Puzzle Mode Setup 3: Stack a 5 kg mass and a 10 kg mass together at position 1 on the left. On the right, place a single 15 kg mass at position 1. The beam balances.

Custom Mode Example: If you place a 20 kg mass at position 2 on the left, you need a total moment of 40 on the right. A single 10 kg mass at position 4 will do this, or two 5 kg masses both at position 4, or any combination where the sum of mass multiplied by position equals 40. The answer key is not a single document. It is a set of equilibrium conditions that follow one rule: the clockwise moment must equal the counterclockwise moment.

What the Simulation Actually Tests

The standard balancing act lab covers four main areas: equal mass at equal distance, unequal mass at unequal distance, multiple masses on a single side, and the custom mode where there is no predetermined solution. The default mode locks the puzzle so you cannot proceed until the beam balances, which is useful for scaffolding but slightly misleading about how real torque problems work. The custom mode is where most of the learning happens. You get unlimited placement options and the beam will tip if you get it wrong. This is where students start treating the problem as an algebra exercise rather than a guessing game.

A Specific Problem I Encountered

Last year I was running this lab with a class that included a student who kept placing a mass directly on the pivot point. The simulation accepted the placement but treated it as having zero moment contribution, which caused the student to think the beam should balance with any arbitrary mass on the other side. It did not. The fix was simple but worth noting. The student needed to place every mass at a clearly non-zero distance from the center marker. I had them remove all masses, return everything to position zero, then rebuild the setup one mass at a time, verifying the balance after each addition. This approach took about ten minutes extra but eliminated the confusion entirely. After that, the student solved three-pile puzzles in under two minutes without looking at the simulation hints.

Common Pitfalls and What to Watch For

The biggest mistake students make is treating mass as the only variable. They will put a 10 kg mass on the left and a 10 kg mass on the right, assume it balances, then express confusion when it does not. Distance matters just as much as mass, sometimes more. A 1 kg mass at position 5 creates the same moment as a 5 kg mass at position 1, and the beam will balance in both cases. Students who skip this relationship often spend 20 minutes trial-and-erroring instead of doing a simple multiplication check. Another pitfall is the visual design of the ruler. The position labels on the simulation range from negative to positive integers, but some students read the labels as absolute distances instead of signed positions relative to the fulcrum. This matters less in the basic puzzles and more when you introduce asymmetric mass distributions. There is also a quirk in the way the simulation renders the ruler. If you place two masses at the exact same position, the visual display merges them into a single block. The math still works correctly, but the display can mislead a student into thinking only one mass is placed. I recommend spreading masses apart even when the puzzle allows stacking, just to keep the visual feedback clear.

Advanced Nuance: Beyond Equal Moments

The simulation assumes a massless, uniform ruler. In reality, most balancing problems involve the weight of the beam itself, which adds a moment that depends on the distance from the fulcrum to the beam's center of mass. The PhET version ignores this, which is fine for introductory work but becomes a limitation if you ever move to a real lab setup with a physical meter stick. A counter-intuitive point that beginners miss: the simulation allows you to use the "cheat" mode, which displays the exact moment values for each mass. Some teachers ban this feature, but it is actually one of the most useful tools for students who are stuck. Rather than letting them struggle for five minutes, I have them toggle the cheat view, note the numbers, then turn it off and verify their own calculation before rechecking. This habit of using the tool as a verification mechanism rather than a shortcut tends to reduce errors by about half over a single lab period.

Practical Workflow for Completing the Lab

Start the simulation in puzzle mode. Complete the first three puzzles without switching to custom mode. These are designed to be solved by inspection. Move to puzzle 4 and above, which require setting up a moment equation. Write the equation on paper: mass left times distance left equals mass right times distance right. Plug in the known values, solve for the unknown, then place the mass in the simulation to confirm. In custom mode, pick a target moment first. Decide what the total clockwise moment should be, then choose mass-position pairs that achieve it. A good habit is to start with a simple pair like 10 kg at position 2, which gives you a moment of 20, then see what combinations on the other side match that number. This usually takes under three minutes per configuration once the habit is formed.

Limitations of the Simulation

The PhET balancing act tool has a few real limitations. It only supports integer mass and position values, so fractional masses like 2.5 kg are not available in the default puzzle set. This is not a big issue for intro-level work, but it becomes a problem if your curriculum covers decimals or fractions in torque calculations. There is no export or print function for the simulation state, which means if a student solves a puzzle and then refreshes the page, the configuration is lost unless they recorded it externally. Teachers who rely on this simulation for graded lab work should require students to screenshot their configurations or log them in a shared document. The simulation also does not model rotational acceleration. Once the beam balances, the user gets no feedback about whether the system is in stable equilibrium or unstable equilibrium. This is a minor omission for basic labs but worth noting if you plan to extend the activity into a discussion of equilibrium types. Bottom line, the Phet Lab Balancing Act Answer Key is not a single document you can hand out. The answer key is the set of equilibrium conditions the simulation enforces. If you understand the moment equation and use the built-in verification tools, you can complete every puzzle in the default set in about ten to fifteen minutes. Students who skip the equation and rely on guessing tend to take forty-five minutes or more, and they usually do not retain the concept afterward.