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fxdung

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In summary, Newton's second law is essentially a definition of force, with mass being informally defined as a property of matter that resists acceleration. This circularity in logic is due to the fact that physics is an empirical science and the best that can be done is to define and prove experimentally. Newton's laws also introduce key concepts such as inertial mass and force, which require a standard unit for accurate measurement. This is achieved through the practical world of weights, measures, and equipment calibration. In conclusion, science works because its theories and definitions match observation and experimentation, making circularity a non-issue.

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fxdung

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fxdung said:

The second law is essentially a definition of force. Mass could be defined informally as a property of matter that resists acceleration.

Physics isn't logic and there is a certain circularity in Newton's laws.

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fxdung

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fxdung said:

I'm sorry to say I cannot make any sense of what you've written.

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fxdung

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I do not understand why a science subject not follow a principle of logic?

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fxdung said:I do not understand why a science subject not follow a principle of logic?

Because it's an empirical science. It's not a branch of logic.

Physics is logical, in the sense that statements must follow logically from each other. But, there is no way to define (inertial) mass or force empirically without reference to each other. In an empirical science, that's the best you can do.

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fxdung

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Yes, the true test is that it matches experiment.fxdung said:

When Newton devised his laws, he was trying to explain physical phenomena. Previously it was assumed (Aristotle) that objects had a natural tendency to slow down and stop. Newton recognised that this was not true and that, instead, he gave his first law:

1) An object will remain at rest or in uniform motion in a straight line unless acted upon by an external force.

And, that there was a proportional relationship between the force and the acceleration:

2) ##F = ma##

These laws don't define "velocity" or "acceleration" or "straight line". And, they introduce two key concepts: "inertial mass" (##m##) and "force".

You could, of course, try to introduce physics from a more abstract point of view. And, in fact, in something like quantum mechanics the Schroedinger equation is much more abstract.

But, that's not what Newton's Laws are trying to do. There's an underlying assumption that we know what we mean by "object", "straight line" and that we can measure acceleration etc.

The next issue is that, because force and mass are introduced together, we need a standard unit for one or the other. The solution for many years was a platinum cylinder in Paris that defined ##1kg##. That provided the means to

This is how an empirical science works.

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fxdung

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fxdung said:If we define force as product of mass and acceleration,then when we compare the test mass with standard mass(1kg) we must use the same force and measure accelerations.But how do we define the same force acting on standard and testing masses?

Gravity would do as a standard force, for example. You can balance two masses on a set of scales. That takes you into the practical world of weights, measures and equipment calibration.

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PeroK said:Gravity would do as a standard force, for example. You can balance two masses on a set of scales. That takes you into the practical world of weights, measures and equipment calibration.

PS One point to note is that this works. We're able to build aircraft or whatever based on classical physics and a practical approach to measurement.

Your point would only be valid, really, if this stuff didn't work. If we couldn't work out how much stress an aircraft wing could sustain, because there was some logical impasse at the heart of classical physics. There isn't a logical impasse. We can and do test equipment and build planes using Newton's laws. We know the mass of things and can measure the forces on them.

Physics works, in other words. It's not a branch of logic, as I've said before.

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A.T.

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Yes. We postulate several quantities and the relationships between them. If it matches observation we use it.fxdung said:

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fxdung said:Which part of Newton's second law is definition and which part is law content?

Newton's second law is law content only. All related terms are defined seperately:

mass: definition 1

momentum: definition 2

force: definition 4

However, these definitions alone are not suficcient to use the terms. They just clarify what Newton is talking about in general. For a full understanding the laws of motion and additional restrictions (e.g. isotropy) are required as well.

fxdung said:It seem that there is a violation in logic, because we define the notion of force through the notion of mass, then we define the notion of mass through the notion of force when we consider the second law.

No, there is no violation of logic. Definitions don't need to be explicit - not even in mathematics (see Hilbert's axioms).

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Mister T

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It is possible to take mass as a fundamental entity and use Law II as a definition of force. It is also possible to take force as a fundamental and use Law II to define mass. Different textbooks have tried the different approaches, with those textbook authors and others arguing in the literature as to which is better. I would say that the former approach has tended to win the day. Especially since the SI defines the unit of mass as fundamental and the unit of force as derived.fxdung said:

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fxdung

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So are there (possibly) two types of definition: theoretical definition(e.g definition force as through acceleration and mass) and experiment definition(E.g definition of force as through the deflection of spring using in experiments)?(But we consider them equivalent)

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Except I see no reason to refer to one or the other as either theoretical or experimental.fxdung said:

The definition of mass in terms of logic is the measure of an object's resistance to acceleration when a force is applied to it. It is a fundamental property of matter and is often described as the amount of matter an object contains.

In logic, the notion of mass is defined as a quantitative measure of an object's inertia, or its tendency to resist changes in motion. It is typically measured in units of kilograms (kg) or grams (g) in the metric system.

Yes, the definition of mass in logic is the same as its definition in physics. Both fields define mass as a measure of an object's resistance to acceleration when a force is applied to it.

The concept of force is directly related to the definition of mass in logic. In fact, mass and force are two of the most fundamental concepts in physics and are closely intertwined. Force is defined as any interaction that causes an object to accelerate, and mass is the measure of an object's resistance to this acceleration.

Yes, the notions of mass and force can be defined without violating logic. These concepts are based on empirical observations and are defined in a way that is consistent with the laws of logic. While there may be ongoing debates and refinements in their definitions, they do not inherently contradict logical principles.

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