This week’s question was asked by: a cat lover.
QUESTION: Why do people say that cats have nine lives?
ANSWER: Such cat tales are the result of cats falling a great distance and surviving. According to a veterinarian in New York City, a cat named Sabrina fell from the 32nd floor of a building and lived to meow again. Sabrina walked away with a broken tooth and a few other minor injuries.
When people fall from great heights, their injuries are severe or fatal. Backs break and skulls are caved in, or more likely, internal organs are ruptured and they bleed internally. Medical people call it blunt force trauma, an injury that is different from a penetrating object entering the body, such as a knife or bullet wound. People rarely survive a fall from more than a few floors.
Cats have only one life, of course, but they’re very good at falling. They strike the ground with much less force than we do. Cats have several things going for them that give them an advantage over people or even dogs.
As soon as a cat starts to fall, it will twist around so that it lands on all fours. The excellent balancing mechanism in the inner ear allows a cat to quickly determine what position it is in and right itself. It’s as if the cat carried a gyroscope. The cat can land on all four feet to absorb the impact. The cat’s legs are bent so when it lands, the force of the fall doesn’t just travel through breakable bones but is spread through muscles and joints. In addition, the cat, by bending it legs, is taking more time to come to a halt. You and I (but mostly children) do the same thing if we jump off a chair or ledge of any kind. We bend our legs to increase the time of impact, so there is less force on the legs and joints.
There’s another factor involved of why Sabrina can fall 32 stories and move on to life number 2. It involves this idea of terminal velocity. Any falling object will accelerate, go faster and faster, increasing its speed 22 mph every second or 32 feet per sec every second (9.8 meters/second/second). If there wasn’t air (vacuum), any two objects, such as a feather and a truck, would hit the ground at the same time.
In the real world of falling through air, the faster the object falls, the more air molecules it encounters each second, and that air resistance increases until force of gravity down is equal to the air resistance in the opposite direction. When those two forces are equal, the object no longer accelerates but falls at some constant speed, a speed called terminal velocity.
For a person, terminal velocity is reached after falling about 1,000 feet, in about 10 seconds, and going about 120 mph. But a cat is smaller and weighs less. It has more surface area proportional to its weight. A cat’s terminal velocity is around 60 mph.
If a cat has a long fall, it can spread out its legs after reaching terminal velocity and shape its body more like a parachute. The uprushing air has a bigger area to push against, air friction increases, and the cat is able to slow down. That’s why Sabrina was able to survive a 32-story fall.
In everyday life, we don’t deal much with terminal velocity. Objects just don’t fall very far. Try this neat experiment at home using coffee filters. Coffee filters have a large surface area and relativity little mass or weight, so they reach terminal velocity rather quickly. Hold a coffee filter above your head and let go. The shape of the filter will insure it will not tumble or turn over. A dropped coffee filter will fall faster and faster but will reach terminal velocity within a few inches of fall. After those first few inches, the filter will have the same speed until it reaches the kitchen floor.
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Larry Scheckel is a retired Tomah High School physics teacher.