Finding a net value is something like cleaning up a mathematical mess (or at least reducing a mess).Ī person sitting in a chair is acted upon by two forces whose sum is zero. The word net is related to the word neat. It's what you get when all things are considered. The word "net" in the phrase "net force" means total, combined, or overall. This is my personal favorite (which is why I put it first) because it relates back to the statement of the first law (which is also why I put it first). The system is in translational equilibrium. There are several ways to describe this situation. There are two forces acting on you (weight pulling down and the normal force pushing up) and no acceleration. Sit in a chair, stand on the floor, or lie in bed. So what about the case of two forces, or three, or more? This again is easy to test. Extrapolating to the point where weight is the only force acting on an object does not make acceleration go away. When it's smaller still, it's only more obvious. When drag is small, acceleration is obvious. When drag is large, free fall acceleration can be hard to perceive. True, but we could think about the limiting case. That means drag is present as a second force. Well, maybe it's not so easy to test, since we seem to drop nearly everything in air near the surface of the Earth. This is not quite true, however, because it overlooks the last bit of the law - "unless compelled by a net external force to act otherwise".Ī single force will accelerate an object. "An object at rest tends to remain at rest and an object in motion tends to continue moving with constant velocity." This seems to imply that when one or more forces do act on an object, it must accelerate. This is a consequence of Newton's first law of motion. When no forces act on an object, it does not accelerate. The idea is to see what it's like to solve such problems so you can recognize them when they pop up later. The acceleration in every problem will be zero in all directions. The purpose of this section of this book is to serve as a repository for problems in statics. When we are at rest on the moving earth, we feel as if we are at rest with respect to the entire universe. The Earth is faithfully following the Sun through the Milky Way at an inconceivable speed ( 250,000 m/s), but because our acceleration is nearly zero ( ~10 −10 m/s 2) this motion is essentially undetectable. There is no physical distinction between being at rest and moving with a constant velocity. Moving at a constant speed in a straight line for an extend period of time is another way. Sitting still for an extended period of time is one way to not accelerate. Statics seems to imply being stationary, but this is not necessarily the case. In contrast, dynamics is the study of forces and motion or more formally, the branch of mechanics that deals with the effect that forces have on the motion of objects. More formally, statics is the branch of mechanics that deals with forces in the absence of changes in motion. Informally, statics is the study of forces without motion. It's a page about solving a particular (and common) kind of problem in mechanics. This is not a page about some fundamental principle of physics.
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