Navigating the PhET Universal Gravitation Simulation
I've been guiding students through this lab for years, and it always goes the same way. Someone downloads the simulation, opens the worksheet, and immediately gets stuck on the data table portion. The simulation doesn't hand you an answer key directly, which trips people up. What exists is a set of expected results based on Newton's law of universal gravitation, and knowing how to derive those values yourself saves a lot of headaches. The PhET simulation titled "Gravity Force Lab" or "Universal Gravitation" lets you place two objects on a track and measure the gravitational force between them. The core equation is F = G(m1*m2)/r², where G is 6.674 × 10¹¹ Nm²/kg². When the worksheet asks you to fill in force values for different mass and distance combinations, the expected answers come directly from plugging those numbers into that equation. Here's the practical part. In the simulation, you typically set mass 1 and mass 2 to values like 50 kg each, then place them at distances like 4 m, 5 m, 6 m, and so on. The simulation displays the force in the readout. Copying those values directly into your lab report is straightforward, but the trick is understanding why they change the way they do. Double the distance and the force drops to a quarter. Double one mass and the force doubles. Triple the distance and you're looking at one-ninth the original force. That inverse-square relationship is the whole point of the exercise.
One thing I ran into recently that catches people off guard: the simulation sometimes displays forces in scientific notation that rounds differently than what you'd get calculating by hand. I had a student last semester who spent twenty minutes convinced her calculations were wrong because the third decimal place didn't match. It was just rounding. The simulation rounds to two or three significant figures depending on the values displayed. I told her to trust the simulation readout for the lab but keep four digits when doing manual calculations to avoid compounding rounding errors across multiple steps. Another counter-intuitive detail beginners miss is that the simulation shows force vectors on both objects, equal in magnitude and opposite in direction. Students often think the larger mass should exert a greater force, but Newton's third law applies here just like anywhere else. The force on each object is identical. I make them look at both arrows in the simulation and confirm they're the same length before moving on. There's also a common pitfall with the distance measurement. The simulation measures from center to center, not edge to edge. If you're manually calculating and you accidentally use the gap between the objects instead of the center-to-center distance, your numbers will be wildly off. I've seen this mistake at least once per cohort. Measure from the middle of one sphere to the middle of the other, period.
If you need the actual lab worksheet or answer key document, those are usually distributed through instructor channels rather than posted publicly. The PhET site itself provides the simulation and some teacher resources, but the specific answer key for a particular lab version depends on which worksheet your course is using. Check your learning management system or ask the instructor directly. The values themselves, though, are straightforward to compute. One more practical note: if your simulation seems stuck or the force readout isn't updating when you drag objects, refresh the page. The HTML5 version occasionally has a rendering glitch where the force vector stops redrawing even though the numerical value updates. A simple reload fixes it. I've found that happens more often on Chrome than Firefox, but it's not consistent enough to pin down.
Get the Full Details
