# Basic Concepts in Physics

## 58 posts in this topic

I think you are looking at physics with a strange frame of reference. you are treating motion like some sort of primary and matter and forces as being just describers of motion. Think about it like this without matter there can be no motion. without a force acting upon matter there can be no motion, else what was the cause of that motion and there must be causation. So, definitionally a force is some manner in which matter interacts to alter the movement of some said matter.

That is not what a force is. Whatever a "said matter" is intended to mean, forces on objects exist that do not cause their motion when the forces balance, and objects can be in uniform motion without forces. A net force causes acceleration, not "motion". This has been discussed here previously.

For if matter did not interact what would be the point of it existing? You would have a boringly static universe.

Matter does not exist for a purpose, "non-boring" or otherwise. This has nothing to do with physics.

In summation, motion is simply an attribute of matter that is determined by a strange set of forces, Matter and forces are not simply ways of describing motion.

Motion is not limited to motion of matter, and whatever you think about forces being "strange" it has nothing to do with physics.

Kimematics is the study of description and measurement of motion without regard to forces causing it. Dynamics is the study of motion in relation to forces.

The original poster is looking for discussion of several basic concepts of physics in terms of Objectivist epistemology, not pre-Newtonian medieval scholasticism.

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Back again.

They obviously didn't joke when they said university physics was hard. Boy...

I've found some satisfying answers to the initial questions here. I'll try and list them for "scrutiny" here later if I have the time.

For now I'll just say that I keep running into these conceptual problems every now and then, an example being with "energy". My book on thermodynamics says it is about to define energy (Two times it proudly says this), then in the following section it that energy is "such a rich concept" that it therefore can have "many different meanings" and then proceeds to not define it but say that "It can change it's forms" and then lays out the the old formulas for kinetic and potential energy - and expects the reader to now know what energy is. (Or not? I'm starting to think maybe both authors and teachers don't really care if there is any actual understanding.)

I asked my teacher, after he had "Defined" energy the same way on a lecture: "What is energy?" (The thing which he'd been talking about for two hours) and the answer he gave was "It's hard to say precisely what it is". I mean come on... Makes me angry just thinking about it.

Anyhow I got from Harry Binswanger the definition of energy as "The ability to do work", and perhaps you said so here also. That sounds right and that's what I'm using.

At the moment I'm trying to figure out the difference between Energy and Exergy. And I'm also trying to learn the two first laws of thermodynamics. And thermodynamic reversibility - that one is hard because I almost think the definition of it looks like a blatant contradiction.

Btw what do you mean by this?

Motion is not limited to motion of matter.

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For now I'll just say that I keep running into these conceptual problems every now and then, an example being with "energy".

An exercise you may find useful is this.

Imagine you have an object at rest on a frictionless surface, and you briefly apply a constant force on that object for a distance until it acquires some velocity. If you use the kinematic equations, you'll find that the product Fd (force times distance) equals 1/2mv^2 (0.5 times the mass times velocity squared).

Now imagine your friend pushes with a force F' against the object over a distance d' until the object comes to rest again. You should be able to find again that F'd'=1/2mv^2

So regardless of the magnitude of the force you push on this object, to get an object of mass m to a velocity v you must exert this force over a distance such that Fd=1/2mv^2, and to bring the object to a rest again your force applied against the motion must again be such that F'd'=1/2mv^2

This is important, because 1/2mv^2 from this is apparently some quantity describing the state of motion of the object is that invariant with respect to the applied force, provided that the product Fd remain the same. More importantly, this original quantity is apparently conserved throughout the evolution of motion of this object. I started it moving by supplying an Fd, and once it was moving and the applied force ceased the velocity was such that Fd=1/2mv^2. When my friend stopped the object by applying a different force F' against it, the distance d' required for it to stop was F'd'=1/2mv^2=Fd. So we are dealing with a quantity that is conserved throughout the motion. The "Fd" is "Work", and by doing work on the object we gave it a kinetic energy 1/2mv^2. To reduce its state of motion to a standstill, an equal work was applied against the motion. This would be a very rudimentary display of the Work-Energy principle. http://hyperphysics.phy-astr.gsu.edu/hbase/work.html

Work then is clearly a higher concept than force, but it is enormously useful, because in practice it liberates us from having to analyze step by step all the complicated forces an object undergoes in motion, and instead simply analyze the net-Work done on the object. (considering the complicated and changing forces acting on an object falling down a curvy slide, vs simply considering the total work done by gravity over the duration of the fallen height, is an excellent example of this).

My book on thermodynamics says it is about to define energy (Two times it proudly says this), then in the following section it that energy is "such a rich concept" that it therefore can have "many different meanings" and then proceeds to not define it but say that "It can change it's forms" and then lays out the the old formulas for kinetic and potential energy - and expects the reader to now know what energy is. (Or not? I'm starting to think maybe both authors and teachers don't really care if there is any actual understanding.)

Many new science textbooks aren't that good. The university library probably will have many, many, good physics textbooks. Perhaps check one out and use it as a supplement for your lousy textbook.

I asked my teacher, after he had "Defined" energy the same way on a lecture: "What is energy?" (The thing which he'd been talking about for two hours) and the answer he gave was "It's hard to say precisely what it is". I mean come on... Makes me angry just thinking about it.

Many, if not most, physics professors are not teachers. They are researchers who must put up with teaching... There's a difference, and unfortunately that difference means you'll probably have your share of lousy professors.

At the moment I'm trying to figure out the difference between Energy and Exergy. And I'm also trying to learn the two first laws of thermodynamics. And thermodynamic reversibility - that one is hard because I almost think the definition of it looks like a blatant contradiction.

How can you understand the relation between two concepts when you admittedly still don't understand some of those concepts on their own? Without first understanding energy, trying to understand it's relation to other concepts will be reduced to trying to find the connection between floating abstractions in your mind.

Btw what do you mean by this?
Motion is not limited to motion of matter.

Who is "you"?

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And for moral support, patrik if you are having trouble learning Physics then join the club! For me it by no means was easy, but the important thing was that it was always fun

You'd probably be surprised to learn the number of good physics researchers in a typical department who weren't hotshots in physics classes back when they were undergrads either. Learning Physics is a long and painstaking path, but ultimately worth it if you have the proper motivation.

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Btw what do you mean by this?
Motion is not limited to motion of matter.

Velocity can pertain to movement that is not the motion of a hunk of matter.

• A travelling wave down a string tied at both ends has a longitudinal velocity even though the particles in the string are only moving up and down with transverse velocity.
• In travelling waves of energy in the ocean the water cycles back on forth as waves break and recede on a beach, but a net energy is propagated continuously towards the shore.
• Phase and group velocities of waves in general: Phase velocity is the velocity of the wave component for a single frequency in the mathematical spectral decomposition, not necessarily a physical velocity in the wave, and group velocity is the "signal velocity" of the shape of a wave packet. Neither need correspond to the velocity of a particle of matter.
• Electromagnetic waves do not correspond to mass of matter moving. Photons travelling at the speed of light have no rest mass and are not matter.
• Matter is comprised of atoms and molecules, which themselves have independent motion, as do atoms within a molecule and subatomic particles with atoms.

Maybe Carlos will elaborate or add more examples.

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For now I'll just say that I keep running into these conceptual problems every now and then, an example being with "energy". My book on thermodynamics says it is about to define energy (Two times it proudly says this), then in the following section it that energy is "such a rich concept" that it therefore can have "many different meanings" and then proceeds to not define it but say that "It can change it's forms" and then lays out the the old formulas for kinetic and potential energy - and expects the reader to now know what energy is. (Or not? I'm starting to think maybe both authors and teachers don't really care if there is any actual understanding.)

I asked my teacher, after he had "Defined" energy the same way on a lecture: "What is energy?" (The thing which he'd been talking about for two hours) and the answer he gave was "It's hard to say precisely what it is". I mean come on... Makes me angry just thinking about it.

Anyhow I got from Harry Binswanger the definition of energy as "The ability to do work", and perhaps you said so here also. That sounds right and that's what I'm using.

Yes, use that. It is the basic, introductory concept. But to understand the meaning of even that you need more than a definition. The meaning of the concept is what it refers to, not the defintion.

Understand how energy and work are mathematically and physically related in simple mechanics: how potential energy -- energy associated with position or state, is related to kinetic energy -- energy associated with mass in motion, through the conservation of energy, as for example when work is done on an object moving down a frictionless inclined plane or a pendulum swings back and forth. You add many more kinds of energy to the concept as you progress, including at an elementary level dissipation of energy through friction as it is converted to heat energy.

Read the chapters on work, energy and conservation of energy in vol 1 of the Feynman Lectures on Physics.

You may also get something out of this brief thread on conceptualizing "potential energy" started by Nate here on the Forum.

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Btw what do you mean by this?
Motion is not limited to motion of matter.

Velocity can pertain to movement that is not the motion of a hunk of matter.

• A travelling wave down a string tied at both ends has a longitudinal velocity even though the particles in the string are only moving up and down with transverse velocity.
• In travelling waves of energy in the ocean the water cycles back on forth as waves break and recede on a beach, but a net energy is propagated continuously towards the shore.
• Phase and group velocities of waves in general: Phase velocity is the velocity of the wave component for a single frequency in the mathematical spectral decomposition, not necessarily a physical velocity in the wave, and group velocity is the "signal velocity" of the shape of a wave packet. Neither need correspond to the velocity of a particle of matter.
• Electromagnetic waves do not correspond to mass of matter moving. Photons travelling at the speed of light have no rest mass and are not matter.
• Matter is comprised of atoms and molecules, which themselves have independent motion, as do atoms within a molecule and subatomic particles with atoms.

Maybe Carlos will elaborate or add more examples.

This is a good list and I couldn't add anything useful to it.

Generally though it worth stating a point for patrik. If you can ascribe a position to an entity (whether it be the discrete position of an idealized point particle, or the more delocalized, "smeared out" position of a traveling wave or disturbance) implicit to that is stating the entity can be in a state of motion. This is true regardless of whether the entity is massless or massive. To have a definable position in space is to have motion in space, period.

The motion of an entity in space is only capable of being defined by its relation to other entities in space and their states of motion. Therefore any object with a definable position is by definition moving in other reference frames, whether the entity is moving with respect to you or not. Motion is intrinsic to position.

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As a general suggestion patrik, avoid excessive arguing or questioning of physics concepts with your professors (unless they are obviously nice guys) because many professors can be shockingly unprofessional and non-objective with how they deal with and grade students; you can very easily piss-off people who wield essentially infinite power over you for innocent reasons.

In the academic world, in any exchange with someone higher above you, you must think "will this guy ever be in a position to hurt me academically/professionally in the future?".

Not all professors are like this, and you will encounter many nice and endearing ones, but you still need to be careful.