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Why can a human being sense some accelerations, such as in a car or airplane, but not others, such as free fall? - MDJ, Ozark, MO

freefall

Loch Ness Monster. Busch Gardens Williamsburg. ©2001 Busch Entertainment Corporation

Actually, you can sense all accelerations. Free fall is no exception. Whenever you accelerate in one direction, you feel a gravity-like sensation pulling you in the opposite direction. That sensation is sometimes called a "fictitious force," despite the fact that it isn't a force at all. The feeling of acceleration is really just your inertia acting to impede your acceleration. You feel stresses inside your body as its parts push on one another to make them all accelerate together and you interpret those stresses as a weight-like feeling in the direction opposite your acceleration. With this idea in mind, it's easy to understand what you feel when you speed up or slow down in a car, or when you turn hard to the left or right. But what about when you step off a diving board? In that case, you are in free fall and are accelerating downward at 9.8 m/s2 (32 ft/s2). You feel this downward acceleration very acutely: the fictitious force is upward and that upward weight-like feeling balances the downward feeling you have from your real weight. Even though you haven't actually lost any weight, you feel weightless and it's a terrifying feeling. The thrill of that first big hill on a roller coaster is this same weightless feeling. You can definitely tell when you're accelerating downward.

Actually, you can sense all accelerations. Free fall is no exception. Whenever you accelerate in one direction, you feel a gravity-like sensation pulling you in the opposite direction. That sensation is sometimes called a "fictitious force," despite the fact that it isn't a force at all. The feeling of acceleration is really just your inertia acting to impede your acceleration. You feel stresses inside your body as its parts push on one another to make them all accelerate together and you interpret those stresses as a weight-like feeling in the direction opposite your acceleration.

With this idea in mind, it's easy to understand what you feel when you speed up or slow down in a car, or when you turn hard to the left or right. But what about when you step off a diving board? In that case, you are in free fall and are accelerating downward at 9.8 m/s2 (32 ft/s2). You feel this downward acceleration very acutely: the fictitious force is upward and that upward weight-like feeling balances the downward feeling you have from your real weight. Even though you haven't actually lost any weight, you feel weightless and it's a terrifying feeling. The thrill of that first big hill on a roller coaster is this same weightless feeling. You can definitely tell when you're accelerating downward.

Answered by Lou A. Bloomfield of the University of Virginia.