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Basic Concepts in Physics

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Hi! I'm about to go nuts trying to understand the basic concepts which highschool physics is founded upon.

What are these things, exactly?

  • Mass
  • Force
  • Energy
  • Charge
  • Field

I'm not looking for the cause or explanation for these phenomena, I just want to understand what books and teachers are saying.

Mass as I understand it is a measure of an objects inertia, or resistance to being accelerated. Feel free to correct as you like.

The one I have biggest problem with atm. (and perhaps the reason for my confusion on the rest of them) is the concept Force (F).

It is said that force is the cause of movement. a "push" or "pull". But if i push a glass on the table next to me (applying OPAR reasoning here) isn't the cause of the glass moving my hand, and not some mysterious force? Second example: If I throw a ball it will fall to the ground. My teacher says the ball falls because of the force of gravity. But isn't force just a way of explaining motion? If that is true then saying that it falls because of the force of gravity is like saying that it falls because it falls. Can a force really be the cause of movement..isn't it just a way of describing movement (or acceleration)? I don't understand how to apply this concept.

Could someone help me understand these concepts, and relate them to Objectivist epistemology? That would be great. I bet there are lots of confused students in need of this discussion.

I've carefully read OPAR and ITOE so I have some basic understanding of Objectivism.

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The one I have biggest problem with atm. (and perhaps the reason for my confusion on the rest of them) is the concept Force (F).

It is said that force is the cause of movement. a "push" or "pull". But if i push a glass on the table next to me (applying OPAR reasoning here) isn't the cause of the glass moving my hand, and not some mysterious force?

No, you are not paying attention to what is different between simply putting your hand in contact with the glass by touching it and same hand when pushing it. The hand is the same, so it cannot be the cause. The force is different, it is the cause.

Second example: If I throw a ball it will fall to the ground. My teacher says the ball falls because of the force of gravity. But isn't force just a way of explaining motion? If that is true then saying that it falls because of the force of gravity is like saying that it falls because it falls. Can a force really be the cause of movement..isn't it just a way of describing movement (or acceleration)? I don't understand how to apply this concept.
I don't understand how "just a way of explaining motion?" is to be considered a failure, or in any way inadequate.
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Be careful: a force doesn't cause movement, but a change in movement, or an acceleration. It is true that any explanation is in fact a description of what happens. In physics we look for the regularities in those descriptions, which we call physical laws. Those enable us to make calculations and to predict what will happen in certain circumstances, so that we don't have to find a new explanation for every new phenomenon that we see, but we can catalog them according to the existing system that we already derived. But the basis of those laws is always experimental evidence, a systematic description of what we see happen in specific circumstances, of which we assume that this is universally valid (until proven otherwise). For example that objects on Earth always fall with a constant acceleration (at least in vacuum or when we can ignore the air resistance), which we describe by introducing the notion of a force (something that causes accelerations) that we call "gravity", which is approximately constant on Earth. Later we learn more about that gravity, its relation to the mass and the mutual distance of objects, etc. We also see that there are other kinds of forces, like the electrostatic force, that also can cause accelerations, etc.

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I was under the impression that only things can cause. When one billiard-ball strikes another, the reason for the movement of the second one is the first one. (That's Objectivist causation). So how can an abstraction (Force) cause movement?

Using Objectivist epistemology, I can come up with this. A concept means it's units - and wherever those units are observed the concept applies to reality. The unit of Force is Newton (N), which in turn means kgm/(s^2). So a force is whenever something with mass accelerates. That is the time when you can properly say you observe in reality the phenomenon which is Force. Have I understood it correctly if I put it like that? I think so.

Now if this criterion applies and I observe a force somewhere, how can it be proper to say that I now know the cause of the acceleration? I have merely identified the units of newton, I do not know why the thing accelerated. I just know that it accelerated. I don't know if someone can see my dilemma here.

I don't understand how "just a way of explaining motion?" is to be considered a failure, or in any way inadequate.

I meant that there is a difference between saying for example how fast or in what direction something is accelerating, and for what reason something is accelerating.

Be careful: a force doesn't cause movement, but a change in movement, or an acceleration.

Good point.

Could someone also comment on the idea that Mass is a measurement of an objects "amount of matter", while it is in fact a measurement of intertia?. My physics textbook jumps between these as if they were the same thing.

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Objectivism holds an entity based ontology if only implicitly....

I like HBLs words here reproduced with permission from a private list

" "I am also sympathetic to the idea that in the final analysis forces are

only collisions. This would mean that attractive forces, such as

gravity, have to be reduced to repulsive forces, based on collisions.

Schemes for doing this reduction have been put forward in history,

but I'm not qualified to assess them. Note that I recognize that the

idea of reducing all forces to collisions smacks of an over-

simplified, overly mechanistic, even "a priori" prejudice. But I

would still look to that explanation first, since it is the simplest and

the closest to what we observe in the cases where the causality is

perceptually observable, as in pushing things to move them."

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I was under the impression that only things can cause. When one billiard-ball strikes another, the reason for the movement of the second one is the first one. (That's Objectivist causation). So how can an abstraction (Force) cause movement?

Force is not an abstraction in the fundamental case of you forcing a glass to move across the surface of a table. Forces are existents. Do not make the error of assuming only entities exist.

Also, while an acceleration is the result of a net force, forces may also net out to zero producing no acceleration. Nevertheless the forces still exist.

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"Forces are existents."

Mechanics can be formulated without the concept of force. Newtonian mechanics (F=ma etc.) can be derived from the more general Lagrangian mechanics (principle of stationary action, Noether's theorem etc.). Moreover, the concept of force as the cause of and proportional to acceleration does not hold in relativistic mechanics. In fact, in relativistic mechanics, the force and acceleration vectors are not, in general, in the same direction.

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Be careful: a force doesn't cause movement, but a change in movement, or an acceleration.

To be even more careful, force is the change in momentum. In Newtonian mechanics, F = dp/dt = d(m*v)/dt = m*a and so force is proportional to acceleration.

In relativistic mechanics however, (relativistic) momentum is not proportional to velocity (p = gamma*m*v) so force is proportional to acceleration only in the limit of vanishing velocity (gamma -> 1).

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Mass as I understand it is a measure of an objects inertia, or resistance to being accelerated. Feel free to correct as you like.

In Newtonian physics, the term mass can refer to either gravitational mass or inertial mass.

Inertial mass is the m in F = ma, i.e., it is the constant of proportionality between force and acceleration. Inertial mass is the measure of inertia, the "resistance" to changes in momentum.

Gravitational mass is the m in Newtons universal law of gravitation. Gravitational mass is, in a sense, a measure of the amount of matter that creates the gravitational field in analogy with how the amount of electric charge creates the electric field.

There is nothing in Newtonian physics that requires the inertial mass and gravitational mass of an object to be numerically equal and yet, empirically, inertial mass and gravitational mass are equal. That's why objects of different inertial masses accelerate equally in a gravitational field - the inertial and gravitational masses exactly balance. Why this is so is unexplained in Newtonian physics.

In General Relativity, there is no gravitational force, there is only one concept of mass, and that is as a measure of the invariant (or "rest") energy of an object. Different objects fall at the same rate because each is simply following the shortest (in a special sense - a geodesic) path through a curved spacetime.

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"Forces are existents."

Mechanics can be formulated without the concept of force. Newtonian mechanics (F=ma etc.) can be derived from the more general Lagrangian mechanics (principle of stationary action, Noether's theorem etc.). Moreover, the concept of force as the cause of and proportional to acceleration does not hold in relativistic mechanics. In fact, in relativistic mechanics, the force and acceleration vectors are not, in general, in the same direction.

The OP is attempting to put the fundamentals of high school physics on a sound first level conceptual basis. Lagrangian and relativistic mechanics are irrelevant to that task, nor will he find that they are contradictory to the classical model if he ever reaches that context and level of understanding.

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The OP is attempting to put the fundamentals of high school physics on a sound first level conceptual basis. Lagrangian and relativistic mechanics are irrelevant to that task, nor will he find that they are contradictory to the classical model if he ever reaches that context and level of understanding.

The OP is, as I read it, quite perceptive and my responses go to that. OP asks "Can a force really be the cause of movement..isn't it just a way of describing movement (or acceleration)? I don't understand how to apply this concept."

This is a deep question, what is force? Is it a cause or a description? I think the OP's question is related to the question asked here: http://www.physicsforums.com/showthread.php?t=117898

"I'm not surprised you don't understand what I'm asking, since I keep wondering if I'm just chasing my own tail here.

OK: So, F=dp/dt is a "law"? If so, then it is stating a fact about the motion of bodies in the physical universe. It is placing a constraint upon the motion of an object, out of all the possible paths that it could take, the 'laws of physics' must tell us which one it actually does take. F=dp/dt does not do this, as it stands. You've got to replace F with something. Say, 'mg' for a simple projectile problem, or 'GmM/r^2' for an orbit problem.

Here's what I'm getting at: if F=dp/dt is a law, what is the definition of a force?"

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The OP is attempting to put the fundamentals of high school physics on a sound first level conceptual basis.

Yes! Well put. So how do we do that?

Force is not an abstraction in the fundamental case of you forcing a glass to move across the surface of a table. Forces are existents. Do not make the error of assuming only entities exist.

Also, while an acceleration is the result of a net force, forces may also net out to zero producing no acceleration. Nevertheless the forces still exist.

I looked in the Principia Mathematica itself and here is Newtons definition of a force:

"An impressed force is an action exerted upon a body, in order to change it's state, either of rest, or of uniform motion in a right line."

I was thinking that forces can't really be said to exist because they aren't things. But I suppose it is better to regard them as actions.

This is what OPAR has to say about actions:

"Action" is the name for what entities do.

And this can be found on the first page of the first chapter in ITOE:

"The building-block of man's knowledge is the concept of an "existent"-of something that exists, be it a thing, an attribute, or an action."

(I found a good thread discussing the concept 'Action' here: http://forum.ObjectivismOnline.com/index.p...&hl=action)

I also asked around on physics chat-rooms and they said that they identify a force whenever something is accelerated, much like I said above. I then asked how they know there is a force acting on a rock lying on the ground since the rock isn't accelerating and I didn't get much of an answer. They also said this, which might be of interest:

From a definition point of view, we only define what a net force means.
Which can be taken to mean that the only forces that do exist, are net forces. the rest are theoretical constructs. Take or leave.

I have yet to form a worthwhile conclusion on all of this.

Grames would you like to elaborate a little more on your definition of force and in what way a force exists?

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I was under the impression that only things can cause. When one billiard-ball strikes another, the reason for the movement of the second one is the first one. (That's Objectivist causation).

I know this is the Objectivist position, but in my opinion this is an impractical viewpoint. The cause of the movement of the second billiard-ball is not really the first ball, but the fact that the first ball is moving and strikes the second ball. It's much more useful to consider that event as the cause of moving the second ball. After all the first ball may lay still till doomsday and nothing will happen to the second ball. When we consider events we can see also the causal chain: why did the first ball move? Because I hit it with a cue, which was caused by the movement of my arms etc. All those objects may be instrumental in what happens, but it is the highly specific events that create a chain of causes and effects. I could also have used a hammer to make the billiard ball moving by giving it a certain impulse, but what is really useful is to know what happens in those events, that is also the way to understand them, to calculate the effects, to predict how the ball will move. Merely stating that the first ball or the hammer is the "cause" is not very informative. It is the impulse transfer from these objects (or from any other object that might be used) to the second ball that determines its motion.

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I also asked around on physics chat-rooms and they said that they identify a force whenever something is accelerated, much like I said above. I then asked how they know there is a force acting on a rock lying on the ground since the rock isn't accelerating and I didn't get much of an answer. They also said this, which might be of interest:

Which can be taken to mean that the only forces that do exist, are net forces. the rest are theoretical constructs. Take or leave.

I have yet to form a worthwhile conclusion on all of this.

Grames would you like to elaborate a little more on your definition of force and in what way a force exists?

"Theoretical construct" is completely wrong. Even when there is no motion there is deformation of a material which then resists further deformation with a greater counter-force. The physics of springs do not apply only to springs.

I do not have my own private definition of force. This formulation at Wikipedia is complete and avoids using Newton's second law as a definition:

"Force can also be described by intuitive concepts such as a push or pull that can cause an object with mass to change its velocity (which includes to begin moving from a state of rest), i.e., to accelerate, or which can cause a flexible object to deform."

My only quibble with this is that pushing and pulling is not intuitive but ostensive, shown as in a demonstration and known by direct perception.

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Merely stating that the first ball or the hammer is the "cause" is not very informative. It is the impulse transfer from these objects (or from any other object that might be used) to the second ball that determines its motion.

There is no such thing as a disembodied impulse transfer or other event either, so it is senseless to claim events cause events.

What is important to keep in mind is that physics is relational. A force is a relation between two things that exist, and without one or the other there would be no force. Attributing a force entirely to one side of an interaction and not both is technically an error. However, when doing causal analysis using the methods of difference and similarity one thing is held constant while another is varied; a subsequent change is attributed entirely to the new factor. This analysis is valid for as far as it goes, but it usually stops at that point because the relational nature of existence is left implicit.

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There is no such thing as a disembodied impulse transfer or other event either, so it is senseless to claim events cause events.

What is important to keep in mind is that physics is relational. A force is a relation between two things that exist, and without one or the other there would be no force. Attributing a force entirely to one side of an interaction and not both is technically an error.

Thank you. I was just about to jump in on that very point. It's important to remember that not only did the second ball's motion change but also the first, i.e., the second ball "acted" on the first too. This is essentially the content of Newton's 3rd law. The collision of the two billiard balls, the event, is the interaction (action and reaction) of the two (composite) entities.

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There is no such thing as a disembodied impulse transfer or other event either, so it is senseless to claim events cause events.

Not at all, no one claims that the impulse transfer is disembodied, but it is the essential factor that determines the movement second ball. You may replace the first ball by countless other objects and still get the same result for the second ball, as long as those objects can generate an impulse that is large enough. So the event of movement, collision and ensuing impulse transfer is the determining factor, not the identity of the first object, therefore it's much more sensible to call that event the cause of the movement of the second ball. Moreover, if that object lies still, nothing will happen, it doesn't cause anything in itself.

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Since this thread is about the first level validation of physics concepts, I invite you to explain how an event is first level and not a higher level compound of more fundamental elements.

In the first post the following basic concepts are mentioned: mass, force, energy, charge and field. These are all introduced to explain regularities in the movement of objects that we observe. We observe for example an accelerating object, therefore we introduce the concept of a force (similarly for inertial mass). So the movement (acceleration) is in fact the basis of the concept of force, even if we may later reverse the hierarchy for convenience and say: "this force causes the acceleration of that object with a certain mass, etc." But we don't observe forces or masses or charges, we observe movements, things that change, in other words, events, so these form the primary basis of all later theorizing.

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In the first post the following basic concepts are mentioned: mass, force, energy, charge and field. These are all introduced to explain regularities in the movement of objects that we observe. We observe for example an accelerating object, therefore we introduce the concept of a force (similarly for inertial mass). So the movement (acceleration) is in fact the basis of the concept of force, even if we may later reverse the hierarchy for convenience and say: "this force causes the acceleration of that object with a certain mass, etc." But we don't observe forces or masses or charges, we observe movements, things that change, in other words, events, so these form the primary basis of all later theorizing.

This is quite mistaken. Force is caused by your own muscles and external forces are felt by your own skin, that is the fundamental referent of the concept force. Masses are inferred from the force of weight, and charges are inferred from the electrical forces.

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This is quite mistaken. Force is caused by your own muscles and external forces are felt by your own skin, that is the fundamental referent of the concept force. Masses are inferred from the force of weight, and charges are inferred from the electrical forces.

This seems quite absurd. You claim that force from one's own muscles is the fundamental referent of the concept of force. Is the force of weight not a fundamental referent? Electric force? Magnetic force?

You claim that masses are inferred from the force of weight. Would that be gravitational mass? Inertial mass? Both? Neither?

If [electric] charge is inferred from electrical forces, would not magnetic charge be inferred from magnetic forces? Maybe a magnetic fluid instead?

It seems to me that Tensorman is on the right track here. The mathematically precise definitions of force, mass, energy, etc. used in high school physics are indeed a far cry from the colloquial notions of force, mass, charge that are commonly used and that you seem to be referencing here.

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This seems quite absurd. You claim that force from one's own muscles is the fundamental referent of the concept of force. Is the force of weight not a fundamental referent? Electric force? Magnetic force?
Feeling the force of weight of an object in your own hand is precisely the kind of fundamental perception that is at the root of all observation and measurement. The force is experienced directly, not the mass. The same goes for any other force.

The mathematically precise definitions of force, mass, energy, etc. used in high school physics are indeed a far cry from the colloquial notions of force, mass, charge that are commonly used and that you seem to be referencing here.
The referents of the concepts are the same, so the meaning of the concepts is the same.
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The force is experienced directly, not the mass. The same goes for any other force.

The referents of the concepts are the same, so the meaning of the concepts is the same.

Does one experience the "force" of gravity directly if one is in free fall?

Does one experience the force on one's body from the atmospheric pressure directly?

Moreover, can it be said that the meanings of the concepts that go by the name of force in the above and in your "weight of object in hand" example are the same?

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