The International Space Station (ISS) is an ideal environment in which to conduct experiments, especially those concerning motion. As the interactive activity explains, the space station, and the astronauts and objects onboard, orbit Earth in a constant state of freefall. This creates the appearance that there is little to no gravity—a condition called microgravity. In the absence of gravity, astronauts can demonstrate subtle actions that can help scientists better understand the motion of celestial bodies, such as planets, nebulae, and black holes, and even inform engineers as they design future spacecraft.

Rotation occurs when every point in an object moves in a circular path around an internal axis that runs through its center of gravity—the point around which the object’s weight is evenly distributed. (Rotation should not be confused with revolution, when an object moves in a circular path around an external axis—like Earth around the Sun or the Moon around Earth.) All objects that move in a rotating or spinning motion seek stability. With objects like the can in the video, which are symmetrical and whose mass is more or less evenly distributed, a stable rotation results almost immediately once a rotational force is applied. But things are different with an object whose shape and/or mass distribution are uneven. The spinning object eventually finds stability around a rotational axis but in the process may appear to wobble and turn.

People can measure the amount of spin a rotating object has—known as angular momentum. Ordinary momentum is a measure of an object's tendency to move at constant speed along a straight path. It is calculated based on speed and mass. An object’s angular momentum depends on its speed of rotation, its mass, and the distribution of its mass relative to its axis of rotation. According to Newton’s second law of motion, this quantity remains constant for any object in motion. It can be transferred to other objects but never destroyed.

This important concept can also be applied to more complicated systems. If the distance between the mass of a rotating object and its axis of rotation decreases, the entire system must spin faster in order to conserve angular momentum. Thus, an ice skater will spin faster with the arms drawn into the body, and slower with the arms held away from the body.

In the video, when the arms of the can opener spread wider apart, the opener straightens out, so its mass is brought closer to its axis of rotation and it begins to spin faster. With respect to astronomy and the formation of galaxies, as a spinning gas cloud collapses inward, the spin speed increases. A disk forms around the spin axis, and the denser parts of the disk will eventually form stars.

So why does rotation matter to objects like satellites that are launched into space? When engineers design orbiting spacecraft, they choose the shape according to their understanding of how it will behave in space. The goal is to distribute mass evenly across the spacecraft so that motion around any rotational axis will be stable—in other words, to avoid a design that produces an unpredictable response like the can opener in the video. In this way, mechanical adjustments to the spacecraft’s motion—for example, changing its attitude or its orientation with respect to Earth or the Sun—can be made based on simpler calculations.

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