Cats have the seemingly unique ability to orient themselves in a fall allowing them
to avoid many injuries. This ability is attributed to two significant feline
characteristics: A 'righting reflex' and a unique skeletal structure.
The 'righting reflex' is the cat's ability to first, know up from down, and then
the innate nature to rotate in mid air to orient the body so its feet face
downward. Animal experts say that this instinct is observable in kittens as young
as three to four weeks, and is fully developed by the age of seven weeks.
A cat's 'righting reflex' is augmented by an unusually flexible backbone and the
absence of a collarbone in the skeleton. Combined, these factors allow for amazing
flexibility and upper body rotation. By turning the head and forefeet, the rest of
the body naturally follows and cat is able reorient itself.
Reports of cats surviving falls of several stories in height have coined the
expression of cats having 'high rise syndrome.' Like many small animals, cats are
said to have a non-fatal terminal falling velocity. That is, because of their very
low body volume-to-weight ratio these animals are able to slow their decent by
spreading out ' flying squirrel style. Simply put, animals with these
characteristics are fluffy and have a high drag coefficient giving them a greater
chance of surviving these falls.
Stephen Portz, Technology Teacher, Space Coast Middle School, FL
Moment of inertia is important ...
To understand how a cat can land on it's feet, you must first understand some
concepts of rotational motion, since the cat rotates as it falls.
Rotational motion is similar to motion in a straight line (linear motion), however
the constants are slightly different. For example, instead of the mass of an
object, we use what is known as the moment of inertia.
The moment of inertia of an object is determined by the distance it's mass is
distributed from the rotational axis. Think of a ball tied to a string that you
swing around your head, the greater the length of the string, the greater the
object's moment of inertia. Relating this to the cat, if the cat stretches out it's
legs and tail, it increases it's moment of inertia; conversely, it can decrease
it's moment of inertia by curling up. You can prove this by extending your arms
while spinning around on a swivel chair.
Just as a more massive object requires more force to move, an object with a greater
moment of inertia requires more torque (which is the angular equivalent of force,
and is proportional to the distance from the axis of rotation) to spin. Therefore
by manipulating it's moment of inertia, by extending and retracting its legs and rotating its tail, the cat can change the speed at which it rotates, giving it control over which part of it's body comes in contact with the ground.
If a cat is dropped they almost always tend to land on their feet because they use
the conservation of angular momentum to change their orientation. When a cat
falls, as you would expect, its centre of mass follows a parabolic path. The cat
falls with a definite angular momentum about an axis through the cat's centre of
mass. When the cat is in the air, no net external torque acts on it about its
centre of mass, so the angular momentum about the cats centre of mass cannot
change. By pulling in its legs, the cat can considerably reduce it rotational
inertia about the same axis and thus considerably increase its angular speed.
Stretching out its legs increases its rotational inertia and thus slows the cat's
angular speed. The conservation of angular momentum allows the cat to rotate its
body and slow its rate of rotation enough so that it lands on its feet safely.
Dan Summons, Physics Undergrad Student, UOS, Souhampton
'Restlessness and discontent are the first necessities of progress.'