Astronauts float around the interior of the space shuttle because they’re in a gravity-free environment. This creates a unique problem for the astronauts trying to get work done up there: how to turn around.
Because they’re floating, when astronauts needs to turn around, they can’t do it as easily as you can on earth. On earth, if you’re standing on the ground facing one way and you want to turn around, you use the muscles of your legs to pull on the segments of your leg to create a force from the ground that pushes you in the direction you want to turn. To turn around in space, the floating astronauts can’t push off the ground because they aren’t always touching the ground! The astronauts could wait until the random floating motion brings them over to a wall, or the floor, or the ceiling, and then push off that surface, but this waiting is wasted time.
One technique used to turn while floating in space is similar to the technique used by a cat. Cats, as the saying goes, always land feet first. As they do with all movement, Newton’s laws provide the explanation for turning cats (and astronauts).
The angular versions of Newton’s laws are related to the turning effect of a force, called torque. Newton’s first law says that an unbalanced torque causes a change in the angular motion of a body, an angular acceleration. Newton’s second law says that the size of the acceleration depends directly on the size of the torque applied to the body — a larger torque causes a larger acceleration, and a smaller torque causes a smaller acceleration. But Newton’s second law also says that the size of the acceleration is inversely related to the resistance of the body to change motion — greater resistance means less acceleration, and less resistance means more acceleration.
The resistance to changing angular motion is called the moment of inertia. The moment of inertia depends not just on the mass of the body, but on how the mass is distributed around the axis of rotation. Humans, and cats, can manipulate the moment of inertia by moving body segments closer to, or farther from, an axis of rotation. Moving segments farther from the axis increases the moment of inertia and increases the resistance to changing angular motion. Bringing segments closer to the axis reduces the moment of inertia and decreases the resistance to changing angular motion.
The axis of rotation when an upright person turns around to face the other direction is called the vertical axis of the body. It’s an imaginary axis running the length of the body from head to foot (or foot to head). When a person is standing upright with her arms in close to the body and her feet together, the moment of inertia around the vertical axis is at its lowest value.
Mechanically, a person consists of two separate bodies — the upper body (head, arms, and trunk, including the vertebral column, or backbone) and the lower body (pelvis and legs). The upper body and lower body can rotate independently around the vertical axis (like when you stand upright and twist from side to side — your upper body twists, but your feet stay planted on the ground), and each has its own moment of inertia.
Consider an astronaut in an upright position facing to the right while floating in space. To turn the entire floating body to the left, the technique used by the astronaut involves the following movements:
Raising the arms above the head while at the same time raising the legs in front to create an L position of the body: These movements reduce the moment of inertia of the upper body and increase the moment of inertia of the lower body, around the vertical axis.
Twisting the upper body toward the left: This twisting motion is caused by muscles in the abdomen and lower back. One end of the muscles attaches to the lower body on the pelvis, and the other end attaches to the upper body on the vertebral column and ribs. The pull of the muscles is equal at both ends. When the muscles pull the upper body toward the left, they pull the lower body toward the right. The rotation of the upper body toward the left is more than the rotation of the lower body toward the right because the moment of inertia of the upper body is less than the moment of inertia of the lower body.
Lowering the arms so they’re straight out in front of the body while at the same time lowering the legs. These movements create an upside-down L position to the body, increasing the moment of inertia of the upper body and decreasing the moment of inertia of the lower body around the vertical axis.
Twisting the upper body toward the right: The pull of the muscles cause the upper body to rotate to the right and causes the lower body to rotate to the left. The lower body rotates more because it has the smaller moment of inertia. The body is now aligned in the original starting position.
Repeating the sequence of movements until the astronaut faces the intended direction.
Astronauts must learn the technique of manipulating the moment of inertia to turn while floating in space, although cats seem to be born with their version of the technique wired into their neuromuscular system (even kittens almost always land feet first). A similar version of twisting can be performed on the trampoline, negating the need to go to outer space to see how manipulating the moment of inertia can allow rotation while airborne.
