How does mechanical motion start at the deepest, smallest level?

The first molecular motions of gears and levers in a machine.

I can make an educated guess about how a seemingly unrelated type of motion starts... how a magnet's magnetic fields can cause a metal to move: first you move the magnet nearer, and its magnetic field will touch the electrons of a great many of the metal's molecules almost at the same time, so a whole bunch of tiny pushes to each molecule's momentum will then combine into a macro level motion we observe.

That small start to the metal object's motion has now entered it deeper into a stronger part of the magnet's magnetic field, adding a fresh momentum to the one in progress, so the acceleration piles up until 'snap'! The metal hits the magnet.

If that's accurate, then I'm having difficulty trying to figure out how the motion starts in a machine at the molecular level to spread and become a full motion of the entire machine part that then moves the rest of that machine's parts.

The difficulty in logic is because in a magnet vs metal, all the molecules in the metal are in essence starting to move at the same time, all together. But a machine's parts aren't magnetic as far as I'm aware.

Similar conundrum when trying to figure out where mechanical action starts in our human bodies.

You compress or stretch or shear the material. This produces a compression or tension or shear wave that propagates through the material.

At the deepest, smallest level, it is the Pauli's exclusion principle that starts the motion, isn't it?

P.S. I misunderstood the OP question. I thought that it is about how a moving body makes another body to move when pushing on it. I see in the rest of the thread that it is something completely different.

Dale said:

You compress or stretch or shear the material. This produces a compression or tension or shear wave that propagates through the material.

Good to know there's 3 types of motion that can be started (had to look up shear wave, which looks like a transverse wave). Logically, an object would have to start to compress or stretch or shear the material at the spot that then propagates. But then how did that triggering object start to move? (it's motions all the way down!)

I see how an object at a steady inertial motion can move another entire object by directly transferring its kinetic energy in a collision, but am failing to grasp how an object can start to accelerate at will from an inertial position of rest.

It's the difference between a whole bunch of molecules at steady motion randomly slamming into another whole bunch of molecules (macro vs macro), and some vague inner part of an object purposely kicking the rest of itself into motion. (micro into macro?)

syfry said:

But then how did that triggering object start to move? (it's motions all the way down!)

Well, motions, forces, and fields all the way down to be more complete.

syfry said:

I see how an object at a steady inertial motion can move another entire object by directly transferring its kinetic energy in a collision

In a collision the same thing happens. You get a compression at the point of impact. That sets up a compression wave which propagates as described above.

syfry said:

am failing to grasp how an object can start to accelerate at will

Me too. I didn’t know objects have free will. That is pretty encouraging. If ordinary objects have free will then people probably do too.

I think if you have a physics question then perhaps a concrete example that embodies the question may be helpful. Right now with all the free will and purposeful comments I am unable to proceed. Try to avoid such distracting embellishments

Dale said:

I didn’t know objects have free will.

Meaning in an automated sense. Like when a machine has a timer to activate. I said at will only to include human motion that we can consciously start, like jumping or yelling.

Asking with a sense of humor: did you really think I meant objects could think?

syfry said:

Meaning in an automated sense. Like when a machine has a timer to activate. I said at will only to include human motion that we can consciously start, like jumping or yelling.

Asking with a sense of humor: did you really think I meant objects could think?

This is not helpful.

I foresee this thread as likely to degenerate into philosophical mumbo-jumbo. I wish that the OP would learn from other bad thread starts and try to start making them better,

My answer would be that there is no difference between motion and rest. Zero is a velocity like any other, and everything is at rest in some frame and moving in some other. How velocities change has been abstracted to the concept of "force". If this is not satisfactory, the OP needs to put substantially more time into asking the question that he wants answered.

Vanadium 50 said:

My answer would be that there is no difference between motion and rest. Zero is a velocity like any other, and everything is at rest in some frame and moving in some other.

Yep! I can see all the steps when a motion starts from a collision of two inertial objects, whether they're perceived as being at rest or moving at a steady unchanging pace. The collision will transfer some kinetic energy so that'll now accelerate the objects.

That's clear to me, because a lot of molecules at the same time transferred the kinetic energy and a lot of molecules then started accelerating in response at the same time.

It's similar to the magnet vs metal.

To be clear, I'm trying to explore the start of an acceleration in a machine, and I'm making an educated guess that our human bodies start their own accelerations in a similar mechanical manner.

The reply was that any of 3 types of waves propagate through the material. I'm still having trouble in seeing what then accelerated the previous object that caused the waves

There's zero reason to fear that we'd go down a path of philosophy because there's zero philosophy in the question. I retract the 'at will' wording which was simply an error of using an analogy to rephrase my question.

But the error is understandable because when I ask everyday people this same question, they don't know the answer, but they certainly don't think that I meant an object is alive or whatnot... usually either we know what a person meant, and work with them instead of assuming an absurd conclusion.

Hopefully we can move past that and realize I was saying that purely as a figure of speech. If not I'll go back edit that part out if that's what's necessary.

Dale said:

This is not helpful.

When objects at steady motion or at rest (in some reference frame) collide, it's more obvious to me how one will change the motion of the other. Or, like you replied, how one object will start waves that lead to a change of motion in the other object.

When we turn on a machine, somewhere the waves you mentioned will start a series of motions that lead to the machine parts moving.

What object or physics started those waves?

The steps aren't obvious to me like in the case of macro objects changing their motions by colliding when they weren't accelerating. So purely inertial objects.

Because like with the magnet vs metal, I'm seeing that in both cases there's a lot of molecules moving at the same time as each other (or nearly so). So all of their momentum should logically add up into a macro level motion. Unless my thinking is incorrect.

How does acceleration start? (that's the better question)

I see how a magnet can accelerate a metal, since the magnetic field is acting on many or all molecules at the same time (or nearly so).

But if machine parts aren't magnetic, then I'm at a loss on how the waves of motion are starting. (for the machine to move)

syfry said:

and work with them

That is what this was:

Dale said:

I think if you have a physics question then perhaps a concrete example that embodies the question may be helpful

Now that you have mentioned a machine, perhaps you can give a specific concrete example of what confuses you. What specific motion of what specific machine are you finding confusing?

syfry said:

and work with them

That is what this was:

Dale said:

I think if you have a physics question then perhaps a concrete example that embodies the question may be helpful

Now that you have mentioned a machine, perhaps you can give a specific concrete example of what confuses you. What is an example of a specific motion of some specific machine that concerns you?

Dale said:

Now that you have mentioned a machine, perhaps you can give a specific concrete example of what confuses you. What specific motion

We cross posted! Every machine in existence.

My friend says a car operates on gears, so that's a good one. Edit: so that's a good machine to begin on. (must be very, very careful to avoid being misinterpreted as implying that gears are "haha, a good one, I came to discuss philosophy at a science forum!" /s since apparently it's necessary )

Edit: For the record, I did mention machine in the first sentence of the tdlr, and also in the first sentence of the post.

syfry said:

My friend says a car operates on gears

The car’s motion starts with a chemical reaction. It rapidly increases the pressure and temperature of a gas. The gas pushes on the piston. This causes a tension wave to go through the piston and the piston accelerates. It pushes on the camshaft where the compression wave is turned into a torsion wave and the camshaft rotates. The rotation of the camshaft is turned into torsion in the transmission which is turned into torsion in the axle which is turned into torsion in the wheel. The torsion in the wheel places the tires in torsion, and the friction at the contact patch places the tire additionally into shear. The shear wave propagates through the tire to the wheel to the axle where the shear wave becomes a compression wave in the frame of the vehicle. This compression wave then transmits the force throughout the car producing the acceleration.

Dale said:

The car’s motion starts with a chemical reaction. It rapidly increases the pressure and temperature of a gas. The gas pushes on the piston. This causes a tension wave to go through the piston and the piston accelerates. It pushes on the camshaft where the compression wave is turned into a torsion wave and...

I'm limited by lack of knowledge about the innards of machines, on the types of examples to provide.

But your description of the car engine is readily apparent that again, many molecules are in suddenly in motion all at the same time (or nearly so), and that togetherness of a lot of molecular motions at once is the root cause of the macro motions.

It's still nice to see the mechanical step by step though.

For a car to move, the molecules of gas start the motion by moving in a chemical reaction all at the same time. For a magnet attracting a metal, it's the magnetic fields from the magnet accelerating the metal's molecules almost all at once, but you've gotta move the magnet to get the molecules going.

What I'm having trouble with is for things without a chemical reaction or without being magnetically pulled / pushed.

I don't know enough about such machines. A search will bring up talk about gears and levers, and electricity, but I'm failing to find a starting point of motion as apparent as your explanation that starts with a chemical reaction.

Was originally thinking about the displays of big gears we see for electric machines at factories or whatnot. But I don't know what they're called.

So an electric lawnmower or an electric chainsaw is probably a good example.

syfry said:

What I'm having trouble with is for things without a chemical reaction or without being magnetically pulled / pushed.

So then why did you pick as your example a car?

Please provide a concrete example of a specific motion of a specific machine that actually exemplifies your confusion. Please consider that it takes a decent amount of effort to write an analysis of a machine. So choose your example carefully and clearly so as to be considerate of the effort you are asking and not have me waste it on things that won’t satisfy you. Try for the simplest and clearest machine that embodies your confusion. I am happy to spend effort helping you, but not to spend effort to no avail.

syfry said:

So an electric lawnmower or an electric chainsaw is probably a good example.

syfry said:

What I'm having trouble with is for things without a chemical reaction or without being magnetically pulled / pushed.

Electric lawnmowers and electric chainsaws both involve magnetic forces. Are you sure that is what you want? Please spend some effort on this part.

syfry said:

Was originally thinking about the displays of big gears we see for electric machines at factories or whatnot. But I don't know what they're called

All electric motors involve magnetic forces.

Edit: the “prime mover” in machines I can think of are
1) a chemical reaction eg automobiles
2) a magnetic force eg electric motors
3) an external fluid eg wind turbines
4) an external mechanical force eg levers

I cannot think of any other kinds of machines categorized by what first makes them move

Dale said:

So then why did you pick as your example a car?

When a person lacks knowledge it's often hard to know if a question is faulty.

Yesterday a senior adult asked me how it's possible that the kids in a series (Young Sheldon) is the same person who's now 50 years old. They were also confused about how the actor in a different series being a different person than the real life story they were reenacting.

This is a person who worked in finance at a big company and yet didn't get to watch any TV until their late teens or in their early twenties, if much. Now they regularly do watch TV but they probably don't want to sound dumb so they never ask the type of questions a kid might ask about how TV and acting works. They know me to readily answer any questions without judging if they sound dumb, so they asked, but still that made me realize a lot of adults probably believe bizarre things because they lack the knowledge to piece together seemingly simple parts.

Many people who are knowledgeable about academic things seem unable to fathom an intelligent person perceiving TV in such ways, but maybe that means they can relate.

Not to be off topic, but I wanted to give that example to demonstrate a person could put a lot of thought into a question and still appear as though they hadn't.

So given my hectic schedule and slow writing, and having skipped dinner to address this, I give a lot more thought than is apparent. Technical stuff is personally really hard.

Please consider that it takes a decent amount of effort to write an analysis of a machine

thanks. For what it's worth, I had asked only about what starts the whole thing. And you did answer about the waves which helped. I appreciate that you detailed the rest, and am now stuck only at what causes the waves?

All electric motors involve magnetic forces.

My understanding, which might be correct or incorrect, is that some type of motion, like wind, or steam from heated water, etc will push a turbine, which moves a magnet to cause the flow of electricity.

For purely electric devices, such as an electric lawnmower, I'm thinking that electricity is moving a magnet, which is then doing something. Doing what, I don't know.

When we plug something in, what leads to the motion of waves that then lead to the motion of machine parts. I'm seeking only the parts that lead to waves, for now.

syfry said:

For purely electric devices, such as an electric lawnmower, I'm thinking that electricity is moving a magnet, which is then doing something. Doing what, I don't know.

It might help if you back up and start from the start.

The electric mower has either a battery or a cord. Either one is a source of electricity - something we have built in advance to store energy for when we want to operate our mowers.

The three core components are: stored electricity, a way for the electricity to do something useful (i.e. a circuit), and a human (i.e. another storehouse of energy, and a way for it to do something useful) to trigger the flow.

Electrons are sitting there, potential piled up, and gets released when a switch is closed. Electrons flow. Any kind of moving current generates a magnetic field in its locality.

Can you pick it up from there?
It might help if you did a quick check to see how an electric motor works. They convert flowing electricity into mechanical (typically rotational) motion.

This one is about a simple as you can get. It has, like, four parts - parts you can probably find in your kitchen junk drawer and put together in a half hour:

syfry said:

I give a lot more thought than is apparent. Technical stuff is personally really hard

Thanks. I appreciate the effort!

syfry said:

My understanding, which might be correct or incorrect, is that some type of motion, like wind, or steam from heated water, etc will push a turbine, which moves a magnet to cause the flow of electricity.

That is correct. An electric motor and an electric generator are the same device, just run in reverse. One takes mechanical power and converts it to electrical power. The other takes electrical power and converts it to mechanical power. Both use magnets and magnetic forces in the conversion.

syfry said:

When we plug something in, what leads to the motion of waves that then lead to the motion of machine parts. I'm seeking only the parts that lead to waves, for now.

In an electric motor you run a current through a magnetic field. A magnetic field exerts a mechanical force on a current. This is called the Lorentz force. This is ultimately the magnet force that starts the mechanical wave in a motor.

The fields and currents in a motor are not random. They are carefully designed and oriented so as to produce a torsion in the rotor. The torsion in the rotor then provides torsion to the motor’s drive shaft. And then out to the rest of the machine

syfry said:

What I'm having trouble with is for things without a chemical reaction or without being magnetically pulled / pushed.

Take two small magnets and put them near each other. They should attract or repel each other depending on the orientation. That's how things work at the molecular level, there's just a lot, lot, LOT more charges that can push/pull on each other. All contact forces are electromagnetic in nature, meaning that the way two macroscopic magnets push or pull each other is similar to the way atoms and molecules do.

Hill said:

At the deepest, smallest level, it is the Pauli's exclusion principle that starts the motion, isn't it?

P.S. I misunderstood the OP question. I thought that it is about how a moving body makes another body to move when pushing on it. I see in the rest of the thread that it is something completely different.

syfry said:

When we plug something in, what leads to the motion of waves that then lead to the motion of machine parts. I'm seeking only the parts that lead to waves, for now.

I don't know where this talk about 'waves' comes from.
I suppose if one breaks any function down by fourier, it can be thought if as being composed of a supposition of waves, but really that is just a side discussion.
As an input, one can have an impulse, step ramp, sinusoidal, exponential, square, or any other including composites, and not all inputs will give a transient response that can be considered as normally being a wave.

One can have body forces, such as gravitational, electromagnetic that act upon the whole object.
Mechanical point contact set up all three of the compression, tension, and stresses shear within a solid body, but it is usually common to discuss them as if only pure stresses occur, as a simplification.

going back to something considered as a wave ,
Consider a sound wave - did it start as the rarefaction, or the compression segment of the longtitudinal sound wave. How would you know? Fluids, such as air or water, do not support shear waves, as once the stress ( force ) that produced the displacement is removed the fluid does not return to the 'initial' position through relaxing internal elastic bulk stresses.

@Hill gave, in my opinion, the answer to your question on how mechanical forces are transmitted from one body to another. Electrons, being fermions, are subject to the exclusion principle. So if you are wanting to push your eraser across your desk with your pencil ( a simple machine ), the surface electrons on both surfaces repel each other - you can attribute that to electrostatic repulsion or pauli exclusion, resulting in a force between the objects. If you push hard enough to overcome the static friction between the desk surface and eraser, the eraser will move.
No need to discuss the internal stresses induced within the pencil, nor eraser.

All mechanical contact systems work in a similar way.

256bits said:

I don't know where this talk about 'waves' comes from.

It comes from me. When a motor is off there is no strain. When it is running under load it is under strain, specifically torsion. When you first turn on the motor it goes from the unstrained to the strained state as torsion begins at the rotor and propagates to the shaft in an acoustic wave.

That was my response to the OP’s question on how the motion starts at one point and spreads to become motion of the entire machine.

This is not about Fourier decomposition but about the actual mechanical waves that propagate to initiate motion. The Pauli exclusion principle is an essential part of the stiffness of the material, but it is an insufficient explanation on its own.

256bits said:

I don't know where this talk about 'waves' comes from.

For non-rigid bodies, waves are how the rest of the body learns that something happened at a location.

Dale said:

the “prime mover” in machines I can think of are
1) a chemical reaction eg automobiles
2) a magnetic force eg electric motors
3) an external fluid eg wind turbines
4) an external mechanical force eg levers

I cannot think of any other kinds of machines categorized by what first makes them move

Gathering all of Dale's insightful bits about how motion starts at one point and spreads to become motion of an entire machine.

To distill the essence of the whole thread.

The 'prime mover' beautifully phrases the beginning motion.

Dale said:

In an electric motor you run a current through a magnetic field. A magnetic field exerts a mechanical force on a current. This is called the Lorentz force. This is ultimately the magnet force that starts the mechanical wave in a motor.

The fields and currents in a motor are not random. They are carefully designed and oriented so as to produce a torsion in the rotor. The torsion in the rotor then provides torsion to the motor’s drive shaft. And then out to the rest of the machine

Part of my disconnect was because I couldn't fathom how a magnet small enough to be the magnetic 'prime mover' in a machine, is also powerful enough to be a magnetic 'prime mover' in the first place.

Obviously small magnets are capable of strong magnetism, like a neodymium magnet. But I had imagined machines to have a magnet of regular variety magnetic strength, from being clueless.

And this is likely more clueless reasoning, I had considered magnets as too fragile to be a prime mover. (was imagining the current to move the magnet through fields, and then the magnet to directly, physically push the next object (the secondary mover) in the machine.

So it seems that's incorrect. The correct interpretation is possibly that the magnet's magnetic field is moving the secondary mover (without ever directly touching it).

But then logically that means the secondary mover must be a metal (or a magnetically attractive object).

So logically the primary mover and secondary mover in a machine cannot ever be wooden or plastic or stone, even if the rest of the machine is.

The quote below also throws me off:

"A magnetic field exerts a mechanical force on a current"

I'm visualizing a magnet is creating the electricity (i.e. a current), but then that makes the electricity the prime mover? And we're back to square one.

But now, maybe I should instead interpret that as: A magnetic field exerts a mechanical force from a current.

Because then the magnet is the prime mover.

Thanks for bearing with me, been trying to understand this series of motions all of my life and now seem on the cusp of starting to grasp it.

Dale said:

The car’s motion starts with a chemical reaction. It rapidly increases the pressure and temperature of a gas. The gas pushes on the piston. This causes a tension wave to go through the piston and the piston accelerates. It pushes on the camshaft where the compression wave is turned into a torsion wave and the camshaft rotates. The rotation of the camshaft is turned into torsion in the transmission which is turned into torsion in the axle which is turned into torsion in the wheel. The torsion in the wheel places the tires in torsion, and the friction at the contact patch places the tire additionally into shear. The shear wave propagates through the tire to the wheel to the axle where the shear wave becomes a compression wave in the frame of the vehicle. This compression wave then transmits the force throughout the car producing the acceleration.

From Dale's list of four prime movers, that describes the chemical reaction. So the lesson for me is that not every machine will use a magnet to start.

Dale said:

When you first turn on the motor it goes from the unstrained to the strained state as torsion begins at the rotor and propagates to the shaft in an acoustic wave.

That was my response to the OP’s question on how the motion starts at one point and spreads to become motion of the entire machine.

That almost sounds like when people are ready to run a race, stepping back will give them more room for beginning the acceleration. Like it's probably isn't as efficient to start a strained motor. An unstrained motor starts better, or something.

the actual mechanical waves that propagate to initiate motion. The Pauli exclusion principle is an essential part of the stiffness of the material, but it is an insufficient explanation on its own.

Where does Pauli exclusion fit in?

syfry said:

I'm visualizing a magnet is creating the electricity (i.e. a current), but then that makes the electricity the prime mover? And we're back to square one.

Electrical current is simply the net movement of electric charges. In the specific case of electromagnetics, electrons. Electrons are physical objects that have both charge and a magnetic field, though it is the charge that is important here. A moving charge creates a magnetic field, and a whole bunch of moving charges creates a large magnetic field, which is exactly what we get in an electromagnet. Two different electromagnets can exert a force on each other because the current of electrons generates and responds to a magnetic field. The electrons then exert a force on each other and to the lattice of positive charges in the metal, causing the whole electromagnetic to experience a force.

syfry said:

Where does Pauli exclusion fit in?

The Pauli exclusion principle states that fermions (electrons, protons, and some other particles) cannot exist in the same state at the same time. This prevents all the electrons in an atom from existing in the exact same orbital, for instance. In contrast, a boson (photon, gluon, and some other particles) CAN exist in the same state at the same time. You can effectively shove them all into the exact same spot. So a material composed of fermions, like a metal wire, always has a non-zero volume, resists compression, etc. It means that when you shove on one end of a material the whole thing moves instead of collapsing in on itself.

To be honest, I'm not sure what you're looking for here. Are you looking for a subatomic description of how motion is started? In that case there's no need to talk about 'prime movers' at all. It's simply that charged particles exert forces on each other, so, for example, when a hammer strikes a nail the particles in the hammer repel the charges in the nail, and those particles push/pull on other particles within their respective objects, leading to the acceleration and movement of both objects. In the case of something like an electromagnetic we have magnetic fields instead of close-contact forces between the surface particles, but otherwise the description is near-identical.

syfry said:

I had imagined machines to have a magnet of regular variety magnetic strength, from being clueless.

That really depends on the application. If the motor must be both small and powerful then they usually use rare earth magnets. For large motors or ones that are not particularly powerful then it is not necessary to use the expensive rare earth magnets.

syfry said:

But then logically that means the secondary mover must be a metal (or a magnetically attractive object).

Generally it is a copper wire. Copper is non ferromagnetic. The force is the Lorenz force, not ferromagnetic forces.

syfry said:

The quote below also throws me off:

"A magnetic field exerts a mechanical force on a current"

I'm visualizing a magnet is creating the electricity (i.e. a current), but then that makes the electricity the prime mover? And we're back to square one.

As I said, a magnet exerts a force on a current carrying wire. By Newton’s 3rd law a current carrying wire exerts an equal and opposite force on a magnet.

Motors and generators have a rotor and a stator. The rotor is the part that rotates and the stator is the part that stays in place. Both motors and generators also have one part that has the wire and one part that has the magnet.

It doesn’t matter if the magnet is on the rotor or the stator. Either way there is a force. If you provide mechanical power to the rotor then you will produce a current and the device is a generator. If you provide a current then you will produce mechanical power at the rotor and the device is a motor. It doesn’t matter if the wire is on the rotor or if the magnet is. Whichever one is on the rotor is the one that moves.

syfry said:

Where does Pauli exclusion fit in?

It is one of the main contributors to compressive stiffness. It is not as important for tensile or shear stiffness.

There was a fellow named Sadi Carnot (b1796)who was interested in how and why steam engines can be movers (of water mostly : pumping it from mines) and how they can work better. He developed what we know as thermodynamics and showed that any cyclic steam engine uses "concentrated" heat energy and expells less "concentrated" heat energy. "Entropy" creation is a measure of this process.
So the spontaneous ability to move certain parts of something always involves energy and usually sullies some pre-existing order to accomplish the task. I would call that the prime mover.

syfry said:

... I'm having difficulty trying to figure out how the motion starts in a machine at the molecular level to spread and become a full motion of the entire machine part that then moves the rest of that machine's parts.
...

If we leave the source of energy inducing the movement out of the discussion, and go back to the two gears example, you could see that one solid surface is resisting the penetration of another solid surface, unless the limit of strenght is reached.

This may help:
https://en.m.wikipedia.org/wiki/Strength_of_materials

The great R P Feynman gave this some thought. The first part of this interview is wonderful, in my opinion.
Please give it your attention.

hutchphd said:

So the spontaneous ability to move certain parts of something always involves energy and usually sullies some pre-existing order to accomplish the task. I would call that the prime mover.

Maybe I shouldn’t have used the term “prime mover”. It is just a very old engineering term for the thing that converts some form of energy into mechanical power. I.e. the first thing that moves

Well, it is difficult to choose which turtle! This is all a little bit difuse for sure.

Why the focus on electricity and magnetism?

An alternative is pendulum clocks that can be powered by gravity and initiated by the user moving the pendulum to one side. A key part of the mechanism is an "escapement".

Or the turbines in a hydroelectric dam that are ultimately powered by hydrogen fusion and initiated by the user turning a valve in a penstock.

jbriggs444 said:

... the turbines in a hydroelectric dam that are ultimately powered by hydrogen fusion ...

Er what?

Oh, you mean ultimately ultimately!

Sol, the great evaporator.....

Thanks for everyone's replies, gained a much better understanding, and I'll watch the videos on the way to work tonight.