|MadSci Network: Physics|
Greetings, Nancy: The main force involved is friction. Without the friction associated with the rubber tires contacting the road, the motorcycle would go only in a straight line. This is, of course, in accordance with Newton's First Law of Motion. And if you have ever encountered a patch of sand or gravel while making a sharp turn, I am sure you noticed what happens when that essential rubber/road friction becomes suddenly greatly reduced: The bike falls onto one side and slides in a fairly straight direction. (Ouch.) The reason you lean while performing a turn is entirely due to something that you may have neglected to remember. I therefore invite you to get on your motorcycle, and to drive around a curve WITHOUT leaning -- but I don't recommend it! For safety, let's consider an automobile for a moment: If you are belted down into your seat, and the car goes quickly around a curve, you will note a tendency for your upper body to lean in the direction AWAY from the center of the curvature. This is again the First Law of Motion in action; the inertia of your body IS the tendency to maintain a straight course, while the car is using its rubber/road friction to turn away from that course. More regarding this can be found in another Answer, involving balloons in a car. So...if you were on a motorcycle, going around a curve without trying to lean, inertia would tip you over and lead to a wipe-out. (Ouch again). We don't worry about this in a car, because it has four wheels on the ground; they provide so much stability that the car leans very little, while the passengers can be caused to lean a lot. When motorcycling around curves, the reason you deliberately lean toward the center of the curvature is to counterbalance that inertia-caused outward lean. Furthermore, the faster you go around a curve, the greater the inertia-caused lean would be, and so the more you must counter-lean (and the more you must hope that your rubber/road friction will suffice to keep you on your chosen curve). That's all.
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