How Could a Banana Peel Cause You to Slip Up?
The favored fruit of slapstick comedy, bananas have a theatrical way of taking down even the largest linebacker. But even though banana gags rank among the lowest common denominator of comedy formulas, there's a lot of physics involved in the action of slipping and falling on a banana peel.
Basically, you have friction — or a lack thereof — to blame when you slip on something like a rotten banana peel. No surface is perfectly smooth, and whenever we come into contact with one, such as sitting in a chair or stepping on an icy sidewalk, two different surfaces interact, causing friction. The less friction between an object, such as a banana peel or slab of bacon fat, the more slippery it is.
Two kinds of friction are at work in a slip-up scenario: static and dynamic (aka kinetic). Static friction refers to the resistant force that prevents something from moving when external force is applied. Think about the difference between pushing a heavy box along a linoleum floor and a gravel road. The static friction, a downward force, is exerted less on the smooth linoleum than on the rocks. Dynamic friction is the resistant force acting on a moving object. Imagine the difference between sledding down a hill after a snowstorm and trying to do so in the middle of the summer. The kinetic friction between the moving sled and the grass is greater than that between the sled and the snow.
In order for you to slip on a banana peel, or any other obstacle on the ground, the force of forward linear velocity must overcome downward static friction. Once the peel is in motion, it requires less force to continue moving since kinetic friction is always less than static friction. That explains why after you step on something and begin to slip, it can be difficult to regain your footing.
Newton and Bananas
Though best known for his apple, Sir Isaac Newton can also help explain the mechanics of slipping on a banana peel. Newton's first law of motion states that any object in motion will stay in motion unless acted on by an outside force. Friction is one of those outside forces that halts inertia. Newton's third law holds that for every action, there's an equal and opposite reaction. When we run into a banana peel while walking, the forward force of our intended velocity overcomes the peel's static friction and sets it in motion. In reaction to that accelerated forward force, we experience an equal, backward force on our bodies that can send us to the ground with a little assistance from gravity.
Understanding the interaction of static and dynamic friction also allows engineers to make the world a safer place. By measuring the friction between a tire and a wet road, for example, the manufacturers know how much tread is needed to prevent skidding. The bottoms of the shoes we wear are also designed to provide adequate friction between our feet and the surfaces we traverse.
Consequently, banana peels are more of a hazard on the comedy stage than real life thanks to the strong static friction between a shoe and the tough outside skin of the peel.