![]() To be crystal clear, that means that when you say things like The rest of Newton's laws, however, simply don't work, because they talk about the trajectory of a given particle and in QM particles simply do not have trajectories, and it is counter-factual to try and speak about them. In that form, the first law is absolutely still valid in QM, and indeed it is a core part of the background that allows QM to work. A proper understanding of Newton's laws will re-shape them quite a bit from the form in which they're presented at high-school level the re-shaped version is explained in detail in this answer, and in that scheme the first law readsįirst Law. It's also important to note that your understanding of Newton's laws is rather flat, and that there is a lot of nuance there that you're completely trampling over. the classical mechanics of a particle with an uncertain position and momentum, where an ensemble of particles with uncertain momentum will still spread in position even though the newtonian First Law still holds. its propagation does not involve too much interference between different components of the matter wave), then this spreading will be describable using liouvillian mechanics, i.e. ![]() Generally speaking, though, if the wavepacket isn't "too quantum" (i.e. The relationship between mass and weight is explored later in this chapter.For an electron in the quantum regime (here defined as a particle whose position and momentum uncertainties $\Delta x$ and $\Delta p$ have a produce $\Delta x \, \Delta p$ that is of the order of $\hbar$), Newton's first law still holds in a quantum sense (basically, if there are no forces on the particle then its momentum distribution will not change), but the Heisenberg Uncertainty Principle demands that the particle must form a wavepacket with a finite momentum uncertainty $\Delta p>0$, and that uncertainty in the velocity means that the wavepacket will spread in position space. In other words, the inertia of an object is measured by its mass. It is more difficult to change the motion of a large boulder than that of a basketball, for example, because the boulder has more mass than the basketball. As we know from experience, some objects have more inertia than others. Newton’s first law is often called the law of inertia. Mass is also related to inertia, the ability of an object to resist changes in its motion-in other words, to resist acceleration. The magnitude of this attraction is your weight, and it is a force. Gravitation is the attraction of one mass to another, such as the attraction between yourself and Earth that holds your feet to the floor. Roughly speaking, mass is a measure of the amount of matter in something. ![]() Regardless of the scale of an object, whether a molecule or a subatomic particle, two properties remain valid and thus of interest to physics: gravitation and inertia. The genius of Galileo, who first developed the idea for the first law of motion, and Newton, who clarified it, was to ask the fundamental question: “What is the cause?” Thinking in terms of cause and effect is fundamentally different from the typical ancient Greek approach, when questions such as “Why does a tiger have stripes?” would have been answered in Aristotelian fashion, such as “That is the nature of the beast.” The ability to think in terms of cause and effect is the ability to make a connection between an observed behavior and the surrounding world. Identifying these laws is like recognizing patterns in nature from which further patterns can be discovered. The idea of generally applicable or universal laws is important-it is a basic feature of all laws of physics. Experiments have verified that any change in velocity (speed or direction) must be caused by an external force. ![]() Newton’s first law is general and can be applied to anything from an object sliding on a table to a satellite in orbit to blood pumped from the heart. When the air is off, friction quickly slows the puck but when the air is on, it minimizes contact between the puck and the hockey table, and the puck glides far down the table. \): An air hockey table is useful in illustrating Newton’s laws. ![]()
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