On Earth, or any other body with enough gravitational force, people can use a number of different tools to weigh themselves, including balancing scales and spring scales. In outer space, astronauts need to always know their mass so they can record muscle loss and any data points for human research experiments. Astronauts can’t measure their mass using the same tools on Earth because there’s no gravity in outer space, so experts have designed devices to calculate mass with springs and the Second Law of Motion.

Using Newton’s Second Law and a spring, NASA developed the Space Linear Acceleration Mass Measurement Device (SLAMMD) to calculate mass while in outer space. The spring pulls the astronauts less than a meter at a specific acceleration, according to their mass. Astronauts with more mass will pull and accelerate slower, while the spring and distance remains constant, so they can then determine their mass using Newton’s Law, F=ma, where force is the constant spring and distance. Russia also designed the Body Mass Measurement Device (BMMD) to measure mass in outer space with springs.