BS: Newton's Laws

Subject: Newton's Laws
From: Kim C
Date: 28 May 02 - 02:49 PM

Are there any physics experts out there who can splain this to me in plain English?

Okay, so in my copious spare time, I am studying to become a personal fitness trainer. I got into Chapter 3 of the textbook, and there, staring me in the face, were Newton's Laws. (gotta know how motion affects the human body and all that)

Regarding the law of inertia.... what exactly is an "unbalanced force"?

Regarding the equal and opposite reaction... if I smack my hand down on the top of the table, I'm exerting force against the table. Presumably, the table is exerting an equal force on my hand, requiring it to stop when it reaches the table. But the table is an inanimate object and as such, doesn't "exert" force.

Help? Thanks! :-)

Subject: RE: BS: Newton's Laws
From: MMario
Date: 28 May 02 - 03:08 PM

An unbalanced force would be one that is not compensated for by an equal and oppisete force. the pull of gravity is an unbalanced forc. friction is an unbalanced force.

"exert" in the second case does not imply violition;

see this school site which has some good, basic explanations.

Subject: RE: BS: Newton's Laws
From: Kim C
Date: 28 May 02 - 03:15 PM

Thanks, MMario, that helps. This is all too heavy for me to absorb in one sitting! It may take me a little while before the light bulb goes off overhead.

Now... here's another one. Years ago when I took karate, I broke a board with my foot. Where is the equal and opposite reaction there? With the board, or with the person holding it? Or am I looking at it all wrong?

I need an aspirin. ;-)

Subject: RE: BS: Newton's Laws
From: Eric the Viking
Date: 28 May 02 - 03:40 PM

Breaking booards with the feet, and the head-I used to do that when I did karate'- the force you use is disapated, it spreads out in all directions along the fibres of the obechi wood until they cannot absorb any more force and then snap, and also into the arms and body of the person holding it.So that the energy is used up-the energy, you produced by eating foodstuffs and cellular respiration. Of course the energy originally came from the sun( the source of just about most of our energy on the earth)

When you hit the table, it resists with an equal and opposite force, this is called potential energy (energy in waiting) Like a rock at the top of a hill, sitting there quite happily until someone pushes it off and starts it rolling.

An example of an unbalanced force could be that you hit the table so hard, you put more energy into the blow than the table has as potential energy and it breaks.And as MMario says "exert" doesn't necessarily mean that it is pushing back with an amount of force.

There are loads of good physicists here, who can give you chapter and verse-clever stuff, but if you don't want to get heavy-this should explain it for you.

Cheers

Subject: RE: BS: Newton's Laws
From: Bev and Jerry
Date: 28 May 02 - 03:58 PM

Kim:

One of us is a recovering engineer and we have a daughter who is a personal fitness trainer so we can empathize with your situation. Let's see if we can help.

Aristotle thought that a force was needed to maintain motion. Remove the force and the motion stopped. This was widely accepted for nearly two thousand years until Galileo figured out that the opposite was true. He said that an object moving at a constant velocity has no net forces acting on it. This means that whatever forces are acting on a body moving at constant velocity they are exactly balanced by other (opposite) forces. Newton formalized this concept into a simple (well, maybe not so simple) equation.

If the constant velocity of the object is zero, we say that the object is at rest. If you are at rest on the surface of the earth, the net forces acting on you are zero. You are, according to Newton, being pulled toward the center of the earth by the force of gravity (this concept was later shown to be incorrect by Einstein but let's not go there). The earth (an inanimate object) must be pushing on you with a magnitude and direction exactly opposite to the force of gravity. If not, there would be an unbalanced force acting on you and you would be accelerating in some direction.

If you hold out your hand palm up and someond smacks it with their hand, your hand hurts due to the force acting on it from their hand. If you smack the table with your hand, your hand hurts due to the force acting on it by the table. Same difference!

The breaking of the board with your foot is a rather complicated process. You can't break the board by exerting forces on it unless someone (or something) is keeping it from moving by exerting a compensating force on the board in the opposite direction. If you stand the board on edge on the top of a table, you can kick the hell out of it but it will merely fly away instead of breaking. So, in the first instant that you contact the board, the force of your foot is balanced by the board pushing back on it and your foot is stationary. The board exerts a force on the hands of the folks holding it and they push back with an equal force. There is no net force on the board so it doesn't fly away. But, they are holding it at the ends and you are exerting a force at the middle. This causes the board to bend and then break.

These concepts are more easily visualized (at least by us nerds) through the concept of vector diagrams but you do not want to get involved with that.

Did this help or make it worse?

Bev and Jerry

Subject: RE: BS: Newton's Laws
From: MMario
Date: 28 May 02 - 04:09 PM

It might help if you think of the second "force" as "resistance" rather then a "push".

Subject: RE: BS: Newton's Laws
From: Les Jones
Date: 28 May 02 - 04:09 PM

It is very difficult to believe that when you push them, tables and walls push back. If you push a beach ball under water and let it go, the water will push the ball back and it will jump up. if you press down on a trampoline it will push you back. All surfaces behave like very stiff trampolines, push on them and just like water and trampolines they push back.

Most objects experience a range of forces: gravity, friction, pushes, pulls, stretches and squashes. If they all cancel each other out, ie 'balance' the object will not move, say a person sitting still on a chair, or they carry on moving at a constant speed, like a person sitting still on a chair on a planet moving around a star.

If some forces are greater, in a particular direction, ie 'unbalanced', the object will accelerate in that direction, and continue to accelerate unless some other force comes into play

Subject: RE: BS: Newton's Laws
From: Mr Red
Date: 28 May 02 - 04:10 PM

Was that the first law? or the second
conservation of momentum?
fitness training? try thinking about the difference between mass and weight. mass acts horizonatally too - if you have a forward motion and you want to stop. Weight is the easy way to guage mass because of the gravity of the situation. (It's constant)

Subject: RE: BS: Newton's Laws
From: Bev and Jerry
Date: 28 May 02 - 04:18 PM

By the way, Kim, us science types all had difficulty with these concepts when we were first exposed to them.

One of us went to a science and engineering school and this is the first thing we covered in freshman physics. After several weeks of study, restricted entirely to bodies at rest, more than 400 of us were subjected to an exam made up by the several instructors. Each instructor submited a problem and four were selected. The instructors then took the exam themselves giving themselves five minutes to complete it and they all got 100. When we took the exam, we were given 50 minutes to complete it and the class average was 32 out of 100. And we were all there because we liked this kind of stuff!

Bev and Jerry

Subject: RE: BS: Newton's Laws
From: GUEST
Date: 28 May 02 - 04:38 PM

AMOS, get in here and sort this lot out will ya mate?..

Subject: RE: BS: Newton's Laws
From: Kim C
Date: 28 May 02 - 05:02 PM

"Resistance" makes more sense - I guess resistance is technically a force unto itself.

Mr. Red, the third law is the one about the equal and opposite reaction. The first one, inertia, I sort of get, the second one is acceleration and momentum, which I sort of get, at least insomuch as it applies to weight lifting. The third one flummoxes me but perhaps I am trying to put too much philosophy into it!

Yes, y'all are helping. Like I said, it may take a little bit for the light bulb to go off. I knew I was in for the anatomy lesson, but not physics! :-)

Subject: RE: BS: Newton's Laws
From: greg stephens
Date: 28 May 02 - 06:31 PM

this is all a bit tame. How about some thermodynamics and quantum theory?

Subject: RE: BS: Newton's Laws
From: Bert
Date: 28 May 02 - 09:11 PM

Yup Les has got it right. The table does actually push back. To do this it uses strain energy.

Take a wooden ruler and place it on the table supported at each end by a book. If you press down in the middle of the ruler you will see it bend and you will feel the upward pressure increase the harder you push.

The table does exactly the same thing.

Subject: RE: BS: Newton's Laws
From: Amos
Date: 28 May 02 - 10:17 PM

There are forces holding molecules together, don't forget; and binding dissimilar elements in various ways in complex materials like wood. When that ruler stops being bent and starts to straighten up again, it starts accelerating the mass back in the other direction. The amount of force pushing back is a result of that acceleration. You're studying forces colliding and trying to overwhelm other forces (for example, all the accelerated parts of your foot versus the holding together and counter-forces of the board. At some point the force (mass*acceleration) of your body and foot overcomes the nechanical bindings of the bibers in the board, and they let go of each other and strat accelerating off in various directions inthe forms of shreds, splinters and sawdust.

But this all too complicated to see the action-reaction law clearly. Get one of those toys with steel balls on strings that can knock each other and notice what happens when ball A hits ball B -- ball A rebounds back! Why? What makes it turn back on its course and go back the other way?

Here's another one -- go to a skating rink and wearing skates in mid rink, stand face to face, palm-to-palm with someone. Then push them as hard as you can and see who goes where! If it weren't for that fundamental action-reaction thing, you'd stand still and he'd glide away. But that ain't what happens, is it?

A

Subject: RE: BS: Newton's Laws
From: Amos
Date: 28 May 02 - 10:37 PM

There are forces holding molecules together, don't forget; and binding dissimilar elements in various ways in complex materials like wood. When that ruler stops being bent and starts to straighten up again, it starts accelerating the mass back in the other direction. The amount of force pushing back is a result of that acceleration. You're studying forces colliding and trying to overwhelm other forces (for example, all the accelerated parts of your foot versus the holding together and counter-forces of the board. At some point the force (mass*acceleration) of your body and foot overcomes the nechanical bindings of the bibers in the board, and they let go of each other and strat accelerating off in various directions inthe forms of shreds, splinters and sawdust.

But this all too complicated to see the action-reaction law clearly. Get one of those toys with steel balls on strings that can knock each other and notice what happens when ball A hits ball B -- ball A rebounds back! Why? What makes it turn back on its course and go back the other way?

Here's another one -- go to a skating rink and wearing skates in mid rink, stand face to face, palm-to-palm with someone. Then push them as hard as you can and see who goes where! If it weren't for that fundamental action-reaction thing, you'd stand still and he'd glide away. But that ain't what happens, is it?

A

Subject: RE: BS: Newton's Laws
From: catspaw49
Date: 28 May 02 - 10:58 PM

The angle of the dangle is in direct proportion to the heat of the meat providing the mass in the ass remains constant.

Hope this helps.

Spaw

Subject: RE: BS: Newton's Laws
From: GUEST,ozmacca
Date: 29 May 02 - 12:05 AM

Try Flanders & Swann's "First and Sceond Laws of Thermodynamics" ....

"And all the heat in the universe is gonna coo-ool down - That's entropy, man....."

And anyway, the Newtonian Laws aren't laws as such, more sort of guidelines for the Einsteinian theories..... I always remember the cartoon strip Wizard of Id when the Wizz sat under a tree and watched an apple soar upwards... "YTIVARG!"

Subject: RE: BS: Newton's Laws
From: GUEST,Dave Williams
Date: 29 May 02 - 12:47 AM

Kim,

In the simplest terms a force is a push or a pull. Which term is used depends upon direction. If you apply a force toward you it is called a pull; if applied away from you it's a push.

The law of inertia (sometimes referred to as Galileo's Principle of Inertia [he got it first] or Newton's First Law of Motion) simply says that any mass is going to continue to do what it's already doing - if it's at rest, it's going to stay that way; if it's moving, it's going to continue to do that, at a constant speed and in the same direction - until something happens to change what the mass is doing.

What has to happen? Something must push or pull the mass. In other words, a force has to act or be applied. Depending on the direction of the force, the mass may speed up, slow down, and/or change direction. So, as stated by Newton's Second Law of Motion, any force (however small) can change the motion of any mass (however large).

The confusion arises because on earth we are stuck with an ever-present force called gravity which complicates things. There are also other forces which can muddy the waters. For example, Aristotle's conclusion that a force is necessary to _maintain_ motion seems obvious. Every little kid who's ever pushed a toy car across the floor knows that when the pushing stops the car soon comes to rest.

Galileo's argument was that forces _CHANGE_ motion, so that the car stops because the total force on it is NOT zero when the kid lets go. When the little kid is pushing the car, there are TWO forces acting on it. (Actually there are more, including gravity, but they don't affect the motion _across_ the floor.) Those two forces are the kid and one we call friction. That's the one that Aristotle and apparently everybody else missed for a looooong time. While the kid is pushing the car at constant speed his force and friction, acting in opposite directions, give a total force of zero, so there are no "unbalanced" forces, and no change in motion. When the kid lets go, the friction remains and is the "unbalanced" force which changes the motion of (stops) the car.

Newton's Third Law of Motion is the most widely misunderstood of them all. Most simply stated it means that you can't push on anything without it pushing back on you.

The key to simplifying the understanding of dynamics (Newton's Laws) is called a "free-body diagram". This is a sketch of the object we wish to observe and all of the forces which are applied TO that object. The forces applied BY the object to its environment (the so-called "reaction" forces) can have no effect on the object's state of motion.

I hope this helps.

Subject: RE: BS: Newton's Laws
From: GUEST,Dave Williams
Date: 29 May 02 - 02:41 AM

Kim,

Missed your second question re: foot and board the first time.

So, a little more on Third Law. I've always disliked the standard textbook phrasing of Newton's Laws. They seem to be designed for memorizing and parroting back rather than for true understanding. "For every action, there is an equal and opposite reaction."

In the first place that statement is abbreviated to the point where it is an oxymoron. There is no such thing as being equal _and_ opposite. What it really means to say is that there is an equal force (same size) in the opposite direction. If an abbreviation is necessary, it should omit the word equal, because opposite _means_ same size already. For example, 2 and -2 are opposites, both the same distance from zero (2 spaces), but in opposite directions (2 spaces right, 2 spaces left). But that's mathematics. Back to physics.

Part of the difficulty people have in learning physics is a result of sloppy thinking like that above. Yes, it's true that the quoted statement is from Newton himself, but he _knew_ what he meant, as eventually everyone who struggles with physics long enough does; or anyone else who is much smarter than the average bear. However, that does not help beginning students to get a clear hold.

Simplest terms, you push (or pull) something, it pushes (or pulls) back just as hard. You push the wall, it pushes back. Action: you push wall. Reaction: wall pushes you. You can always identify an "action-reaction pair" by reversing the subject and the direct object.

And the aspect which nobody ever explicitly pointed out to us non-geniuses (at least when I was a student): an action-reaction pair _ALWAYS_ acts on two different objects. This is the answer to the old paradox about the horse and wagon. The horse, at rest, begins to exert a force on a wagon. But, according to Newton's 3rd, the wagon exerts the same amount of force in the opposite direction on the horse. Ergo, the horse can never move the wagon. Explain.

This stumps most beginning physics students. (Remember the class average of 32 in Bev & Jerry's post.) So, how to understand this?

Action: horse pulls wagon. Reaction: wagon pulls horse. Notice we're interested in the wagon. Is there a force on the wagon? Yes, horse pulls wagon. So the motion of the wagon _CHANGES_. The force that the wagon exerts on the HORSE has nothing to do with the motion of the WAGON. Get it?

Action: Foot pushes board. Reaction: Board pushes foot. The rest is slightly more complicated because in this example the forces are changing. At the beginning of the kick, when the foot first contacts the board the force exerted by the foot is small but rapidly increasing. The board pushes back matching the increasing force exerted by the foot until the board can't push any harder, then it breaks. This is because the intermolecular forces which hold the board together (as mentioned by Amos) have been exceeded. So the board, by pushing on the foot has managed to slow the foot down, but because it isn't strong enough does not manage to stop the foot.

Subject: RE: BS: Newton's Laws
From: GUEST,JohnnieBGood
Date: 29 May 02 - 02:59 AM

If mass is directly related to energy, how is the sun the source of most of the earth's energy?

Subject: RE: BS: Newton's Laws
From: GUEST,Dave Williams
Date: 29 May 02 - 04:38 AM

Stars like the sun fuse 4 hydrogen nuclei together to form a single helium nucleus. The total mass of the 4 hydrogen nuclei is very slightly greater than the mass of the resulting helium nucleus, and that tiny "mass deficit" is converted into energy a la E = mc^2. A very tiny fraction of that energy reaches the earth mostly in the form of electromagnetic radiation - light, heat, radio, x-rays, ultraviolet, etc. - where plant life converts it into chemical energy by means of photosynthesis. ALL the earth's energy, except for that from radioactive decay - the spontaneous disintegration of very large atomic nuclei such as uranium - comes from the sun.

Subject: RE: BS: Newton's Laws
From: Nigel Parsons
Date: 29 May 02 - 04:52 AM

Guest,Dave Williams: surely 2 Hydrogen nuclei to form one of Helium, else you have 2 protons sat round doing nothing! This is nuclear fusion, which is energy releasing. However, it needs to be 'kick started' by a temperature of about 100 million C, so the sun is the closest source we'll get. Hence all the hoo ha whenims to have produced a 'cold fusion' reaction.

Cheers

Nigel

Subject: RE: BS: Newton's Laws
From: GUEST,jbgood
Date: 29 May 02 - 06:55 AM

You mean a big old dead rock doesn't represent nothing energy-wise? What about the molten core of the earth?

I think you may be focusing on life-related energy...

Subject: RE: BS: Newton's Laws
From: Amos
Date: 29 May 02 - 08:55 AM

Dave:

Ya know, that is a good point about the two objects beiong considered independently.

It's easy to believe, otherwise, that nothing could ever possibly get done! :>)

A

Subject: RE: BS: Newton's Laws
From: Kim C
Date: 29 May 02 - 10:23 AM

Thanks Spaw, I knew I could count on you! :-D

Dave, that makes more sense to me. I was taking "equal and opposite reaction" quite literally. Now, on a philosophical yin-yang-balance-of-the-universe level, I do believe in the equal and opposite reaction. But I suppose in the physical realm, it's a little different.

As regards the horse and wagon, though - the wagon is at rest until the horse pulls it. It will stay in motion until the horse stops. That, I can understand.

Let's say, though, that I'm curling a 10-pound dumbbell. The object is at rest until I lift it. Is the resistance provided by the weight the "equal and opposite" reaction?

Subject: RE: BS: Newton's Laws
From: Devilmaster
Date: 29 May 02 - 11:20 AM

Remember Kim, that on earth the is not often 'one' simple action and reaction force.

If you are do a standing curl of a dumbbell, the dumbbell will give a force back. But you are also fighting the force of gravity in that situation. So now your body has to act against two forces. You compensate by expending more energy.

As dave said, there are no equal and opposite forces. If the horse and wagon were in space, (bear with me :) ) once the horse got the wagon up to the desired speed, the horse would not have to expel any more energy to move it. In space, once that wagon is moving, it will keep moving until something acts upon it, like the wagon hitting an asteroid. On earth, gravity, friction resistance, and air resistance are all forces that act on the wagon when the horse pulls it. To keep the wagon going, the horse has to continually expend energy. The amount of energy a horse expends depends on all the forces the horse must overcome to pull that wagon.

You expend energy to combat the forces acting on the dumbell. If you exhert more energy than the other forces acting on it, you will move the dumbell.

Think of weight as simply a measure of gravity acting on something. You might find it easy to curl a dumbell that weighs 10 pounds. You will have a harder time curling a thousand pound barbell. Why?

In the famous E=Mc2 equation, the energy of an object is the mass of the object, multiplied by speed of light squared. The heavier an object, the more potential energy it has, and therefore, the more energy you as a person need to expel to move that heavier object.

Steve+

Subject: RE: BS: Newton's Laws
From: Devilmaster
Date: 29 May 02 - 11:23 AM

Thank you, Schoolhouse Rock!

:)
Steve

Subject: RE: BS: Newton's Laws
From: GUEST,Dave Williams
Date: 29 May 02 - 11:37 AM

Nigel,

There are also 2 positrons formed in the fusion reaction. These are particles which have the mass of an electron and a positive charge. That accounts for all 4 of the positives and as you pointed out, charge is conserved. Actually, hydrogen fusion was first produced on earth 50 years ago. It's called a hydrogen bomb, and the problem is it takes the detonation of an atomic bomb to start the reaction. So I guess our real difficulty is producing a _controlled_ hydrogen fusion reaction which can be sustained, because controlled fusion has also been produced in the laboratory. Not much help because the reaction lasts much less than one millionth of a second and takes more energy to start than it produces.

jbgood,

Good point, I was referring to _available_ energy. Of course, all mass is equivaent to energy but we can't avail ourselves of it (yet).

Kim,

This is another slightly more complicated situation (like the toy car and friction mentioned earlier) because gravity is involved. Try thinking of it like this.

Action: you push dumbbell. Reaction: dumbbell pushes you. The mass will _ALWAYS_ push back when you push it, even if there is NO gravity.

When gravity is involved, it must be overcome before an object can be lifted. You grasp the 10 pound dumbbell and start to pull - 1 pound, 2 pounds, 3 pounds ... - the dumbbell doesn't move. When your force (pull) builds up to a tiny amount over 10 pounds (we need that little bit extra to overcome the inertia of the DB), the DB starts to move, and any additional force you apply will continue to _CHANGE_ the motion of the DB - in other words it will accelerate. If you _maintain_ the 10 pounds of force, then the TOTAL force on the DB will be zero - 10# up (by you) plus 10# down (by gravity) - and it will rise at constant speed.Remember, the reaction force doesn't enter into this at all, because it is not acting on the DB.

If we take gravity away the situation is simpler (one less complication to worry about) - and only a very tiny force is needed to move the 10# weight. A larger force will simply move it faster (more acceleration). This is why astronauts orbiting the earth can't get their exercise by weight lifting. In orbit the _effects_ of gravity are negated, so even the tiniest force will move the largest mass. While working on the construction of the space station, astronauts routinely push around large structures they couldn't budge if they had to overcome the effects of gravity first.

To summarize: Applying 11 pounds of force to a 10# DB on earth produces a certain motion. If I can eliminate the effect of gravity, I can get exactly the same motion of the 10# DB with only 1 pound of force.

Subject: RE: BS: Newton's Laws
From: Amos
Date: 29 May 02 - 11:42 AM

It may help to recall that rockets work just fine in a vacuum, in zero gravity, in deep space.

This is amazing, considering there is nothing to "push against". It is a reflection of the action-reaction law.

A

Subject: RE: BS: Newton's Laws
From: GUEST,Dave Williams
Date: 29 May 02 - 12:08 PM

And, Amos, I seem to recall reading about an article printed in the New York Times in the early 1920s, while Robert Goddard's rocket experiments were making news, explaning that space travel was impossible because there was nothing to push against. It was written by a well-known physicist who should have known better. Unfortunately, I've forgotten who it was.

Just goes to show that everybody, no matter how well versed in their subject, can have lapses that make them look like jerks!

Subject: RE: BS: Newton's Laws
From: Kim C
Date: 29 May 02 - 12:56 PM

Aha! So the 11# of force required to lift the 10# weight is an unbalanced force! Right?

Subject: RE: BS: Newton's Laws
From: GUEST,Lyle
Date: 29 May 02 - 02:21 PM

As Dave Williams pointed out, the "action/reaction" statement of the third law causes all kinds of confusion, unless one uses the same meaning of action and reaction as was used at the time of Newton, which is not the same as today.

The best (and easiest) statement of the third law is very simple; "All forces occur in pairs." That's it. For a consequence of this law, see laws 1 and 2.

Nothing to ie, is there??

Lyle

Subject: RE: BS: Newton's Laws
From: Nigel Parsons
Date: 29 May 02 - 02:45 PM

Dave, the Hydrogen bomb is a special case as it required Deuterium (heavy hydrogen) which is inherantly unstable. "Bog standard" hydrogen would not react in this way! Nigel

Subject: RE: BS: Newton's Laws
From: GUEST,Dave Williams
Date: 29 May 02 - 03:10 PM

Kim,

By Jove, I think you've got it! (I thought it was time for a musical tie-in.)

And, since an unbalanced force (also called a "net" force, in the sense of "total" force) is acting, the object accelerates.

It's the TOTAL force (sum of all the forces which may be acting) that determines the nature of an object's motion.

Subject: RE: BS: Newton's Laws
From: Kim C
Date: 29 May 02 - 03:24 PM

Okay. So how does that apply to fiddle playing? The unbalanced force provided by my finger depresses the string, thus shortening the available vibrational length, then the bow against the string (bow pushes string, string pushed back against bow) produces friction which in turn produces sound waves.

Right?

Subject: RE: BS: Newton's Laws
From: Mr Red
Date: 29 May 02 - 04:47 PM

KimC
your 1 & 2 seem to be the same. Inertia and momentum are identical - they both involve acceleration (+ or - as app) and
action and reaction are the snooker balls (OK pool etc) or the recoil of a gun. Think Newtons Balls and the executive playthings of the 70's

Subject: RE: BS: Newton's Laws
From: Amos
Date: 29 May 02 - 05:01 PM

Inertia and momentum are not identical. One is a tendency, and the other is a measure.

And Kim, please stop thinking about Newton's balls!! :>)

Because the string has an elasticity to it, and the bow's horsehair has a kind of intermittent coefficient of friction as it moves across the string, the string (being held at both ends, is set vibrating, as you say, causing waves of compression and expansion in the atmosphere. These waves in turn excite membranes in ears and microphones. Those membranes ultimately are set up as electrical pulses in the nervous system. Those nervous system impulses reach the brain and cause myriad configurations of synapses to discharge. Those myriad combinations of discharge are experienced by the soul as great delight, without further mechanism.

And while the soul part ALSO operates perfectly well in a vacuum, the mechanisms from string through soundwaves do not!

:>)

A

Subject: RE: BS: Newton's Laws
From: GUEST,Dave Williams
Date: 29 May 02 - 06:03 PM

Kim,

Friction is also a type of force. It exists when two objects move (either by sliding or rolling) across each other. The size of the force basically depends on two things, the coefficient of friction (which is for the most part related to how rough or smooth the surfaces are) and how hard the surfaces are being pressed together. The reason for putting rosin on a bow is to increase the coefficient of friction between the bow and the string, so that the bow can better set the string into vibration. The vibrating string pushes (force _again_) on the molecules of gas in the air which causes them to collide with nearby air molecules which collide with other nearby molecules, and so on carrying the vibration through the air. Take away the molecules and there is nothing left to carry the vibration, which is why sound doesn't travel in a vacuum.

Sound will also travel through other materials, often better than it does through air. Everyone who has ever seen a submarine movie knows that sound travels very well through water and that this is the basis for sonar; and anyone who has ever seen a western knows that sound travels even better through steel - ear on the rail to tell if there's a train coming.

And Amos is right - inertia and momentum are not the same thing. The inertia of an object, how easy or hard it is to change its motion, is determined by its mass which is abbreviated by the symbol "m" and is measured in kilograms (or if you're an American engineer, in slugs). The momentum of an object is the product of the objects mass times its velocity, written "mv". (In physics, there is a difference between speed and velocity - speed tells you only how fast an object is moving while velocity tells you both how fast it is moving AND which way it is going. For example: Speed - 30 km per hour or 20 miles per hour; Velocity - 30 km per hour East or 20 miles per hour South)

Momentum is a quantity which is derived from Newton's 2nd Law and is more useful than the concept of force for understanding and analyzing certain events.

Subject: RE: BS: Newton's Laws
From: Amos
Date: 29 May 02 - 06:27 PM

Well, Dave, it's splitting a hair; but I woulda said that mass is what you measure in Kg, and inertia is the tendency of the mass to resist acceleration. Amounts to the same thing, really, I guess.

Welll, so, Kim... are ya sorry you asked?? :>)

A

Subject: RE: BS: Newton's Laws
From: GUEST,Lyle
Date: 29 May 02 - 10:18 PM

You guys are close, but no cigar. What you are talking about is the *measurement* of inertia, not the concept!

The *concept* of inertia is the property of matter which determines its response to a given cause of motion. From this we conclude that the greater the tendency of an object to resist a change of velocity, the greater is its inertia. And it is then and only then that we can talk about how to measure it.

Lyle

Subject: RE: BS: Newton's Laws
From: Amos
Date: 29 May 02 - 11:08 PM

Hey, Lyle, baby, that's what I just said! Gimme back that damn seegar!! And I said it in half the words you did!!

LOL!

A

Subject: RE: BS: Newton's Laws
From: GUEST,Dave Williams
Date: 30 May 02 - 01:37 AM

And I said:

"The inertia of an object, [i.e.] how easy or hard it is to change its motion, ..."

That's a definition of inertia; and:

"its mass which is abbreviated by the symbol "m" and is measured in kilograms ..."

That's referring to _mass_ and the unit in which _mass_ is measured.

So there!!! ;-)

Seriously guys, let's remember that we're trying to clarify some pretty non-intuitive ideas for Kim here. Let's save the subtleties for Physics 201!!

Subject: RE: BS: Newton's Laws
From: Kim C
Date: 30 May 02 - 10:08 AM

No, I'm not sorry. It's been kinda fun, actually. It's amazing for me to think that people go to school for years and get PhDs in Newtonian physics. ;-)

Subject: RE: BS: Newton's Laws
From: Dave Bryant
Date: 30 May 02 - 10:32 AM

Subject: RE: BS: Newton's Laws
From: GUEST,Dave Williams
Date: 30 May 02 - 11:03 AM

It takes all kinds, Kim!

But the most important question here is, "Did any of this help you out?" I hope so.

And if any other questions come up, feel free to ask. I'm sure we'll all be willing to give it another shot.

Best wishes in your studies.

Subject: RE: BS: Newton's Laws
From: catspaw49
Date: 30 May 02 - 11:11 AM

Kim, you can get an example of "Newtonian Physics" by downing an entire bag of them with two glasses of milk.

Spaw

Subject: RE: BS: Newton's Laws
From: Kim C
Date: 30 May 02 - 01:45 PM

Great, Spaw, I'll practice on Mister. Newtons is one of his favorite snacks. I don't know why.

I don't think I'll ever have a PhD, Dave, but yes, I am now enlightened. Thanks everyone. :-)