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  Topic: Basic Physics, and Bad Science Museum Explanation< Next Oldest | Next Newest >  
C.J.O'Brien



Posts: 395
Joined: Aug. 2005

(Permalink) Posted: Mar. 18 2008,15:44   

Hey all,

The last formal education in Physics I had was longer ago than I care to contemplate, so I have an (admittedly very basic) question to ask of those wiser than I.

So, we had my son's birthday party at Lawrence Hall of Science in Berkeley over the weekend. The temporary set of exhibits was called "Speed," and they had all kinds of hands-on gizmos illustrating various aspects of "speed": velocity, acceleration, drag, etc.

And one was this pneumatic thingy that shot a penny at great speed into a metal plate and then spit it out so you could see how it deformed on impact. They had some exemplars of what happens, based on composition (zinc vs. copper) and angle of impact (the pennies could "flutter" in the tube). It was pretty cool, and we fed it several pennies. But my issue is with the explanatory sign accompanying the exhibit.

It gave F=ma, talked about the force being applied to the penny. So far so good, but then it got into the "equal and opposite" reaction to the force, and then came the inexplicable talk about (paraphrased) "the plate pushes back" and said (verbatim) "the plate is the big F." (meaning the 'F' in F=ma.)

Is it me, or is this just crazily wrong? As I understand it, F is the initial force applied to the penny to shoot it down the tube, and the "equal and opposite reaction" is the deformation of the penny, i.e. the shedding of kinetic energy into heat and stresses in the material of the penny, causing it to be smashed, cracked or folded.

The way they explained it is analagous to saying that if the force of gravity is pushing you into the ground, then the opposite reaction is somehow the ground "pushing back." And that's all wrong. The ground, or the plate in the exhibit, don't exert any force at all, right? In the gravity example, the equal and opposite reaction is all going on inside your body: it's the various stresses and tensions on your skeletal-muscular system involved in keeping you upright, or, if not upright, then at least not a puddle. Right?

And if so, why does a freaking science museum at freaking UC Berkeley have signs that don't hold up to basic high school physics?

--------------
The is the beauty of being me- anything that any man does I can understand.
--Joe G

  
swbarnes2



Posts: 78
Joined: Mar. 2006

(Permalink) Posted: Mar. 18 2008,16:11   

Quote (C.J.O'Brien @ Mar. 18 2008,15:44)
Hey all,

The last formal education in Physics I had was longer ago than I care to contemplate, so I have an (admittedly very basic) question to ask of those wiser than I.

So, we had my son's birthday party at Lawrence Hall of Science in Berkeley over the weekend. The temporary set of exhibits was called "Speed," and they had all kinds of hands-on gizmos illustrating various aspects of "speed": velocity, acceleration, drag, etc.

And one was this pneumatic thingy that shot a penny at great speed into a metal plate and then spit it out so you could see how it deformed on impact. They had some exemplars of what happens, based on composition (zinc vs. copper) and angle of impact (the pennies could "flutter" in the tube). It was pretty cool, and we fed it several pennies. But my issue is with the explanatory sign accompanying the exhibit.

It gave F=ma, talked about the force being applied to the penny. So far so good, but then it got into the "equal and opposite" reaction to the force, and then came the inexplicable talk about (paraphrased) "the plate pushes back" and said (verbatim) "the plate is the big F." (meaning the 'F' in F=ma.)

Is it me, or is this just crazily wrong? As I understand it, F is the initial force applied to the penny to shoot it down the tube, and the "equal and opposite reaction" is the deformation of the penny, i.e. the shedding of kinetic energy into heat and stresses in the material of the penny, causing it to be smashed, cracked or folded.

The way they explained it is analagous to saying that if the force of gravity is pushing you into the ground, then the opposite reaction is somehow the ground "pushing back." And that's all wrong. The ground, or the plate in the exhibit, don't exert any force at all, right? In the gravity example, the equal and opposite reaction is all going on inside your body: it's the various stresses and tensions on your skeletal-muscular system involved in keeping you upright, or, if not upright, then at least not a puddle. Right?

And if so, why does a freaking science museum at freaking UC Berkeley have signs that don't hold up to basic high school physics?

The penny goes from having lots of velocity to having none when it impacts the plate.  A change in velocity is accelation, and I think it's got to be the plate that de-accelerating the penny.

Quote
The way they explained it is analagous to saying that if the force of gravity is pushing you into the ground, then the opposite reaction is somehow the ground "pushing back." And that's all wrong.


I don't think so, but you're talking about two different things.  The penny starts at rest, it gets a big momentary force on it when it gets shot, it gets another force on it to bring it to rest, and then when it is at rest, there are two forces working on it: gravity pushing down, and the ground pushing up.  They are even, so the penny has no acceleration at all.  If there were only gravity, the penny and yourself would be accelerating, and clearly you aren't.

The force of the ground pushing an object up is called "normal" force.  When an object is at rest, the normal force and the force of gravity are equal.

  
dheddle



Posts: 540
Joined: Sep. 2007

(Permalink) Posted: Mar. 18 2008,16:31   

I would say, if I understood you correctly, that they got it right. There is a force that accelerates the penny, but, as swbarnes2 points out, it is the decelerating force that deforms it. That force comes from the metal plate, and is equal but opposite to the force that the penny exerts when striking the plate. It is an "impulsive" force which is a name given to any force that is applied over a short time.

If the penny had speed v and mass m and was stopped by the plate in time dt (time in contact with the plate while slowing), then the force was mv/dt. This was the magnitude of force exerted by the penny on plate, and by the plate on the penny--although the directions were opposite.

As for gravity what you described as all wrong was in fact correct (again, as swbarnes2 points out.) Gravity does result with you pushing down on the ground with your weight. And you feel the ground pushing back with (in the simple case of standing on a horizontal surface and no other vertical forces) with exactly the same force (your weight) but in the opposite direction (up). If there is no ground to push up, you'll feel weightless (as you fall.)

--------------
Mysticism is a rational enterprise. Religion is not. The mystic has recognized something about the nature of consciousness prior to thought, and this recognition is susceptible to rational discussion. The mystic has reason for what he believes, and these reasons are empirical. --Sam Harris

   
C.J.O'Brien



Posts: 395
Joined: Aug. 2005

(Permalink) Posted: Mar. 18 2008,16:57   

Okay,
I guess it makes more sense the way you guys describe it. The whole idea of a stationary plate exerting force just struck me as strange. But I do get that there's a positive acceleration launching the penny, balanced by a sudden negative acceleration as the penny hits the plate and comes to rest.

But, again, with gravity and the ground "pushing back," I'm still befuddled. So, if I push on a solid, stationary brick wall, and nothing moves, where is the opposite reaction, required by Newton's Third Law? Does the wall push back? Or is the action, me pushing, balanced by internal stresses in my body (me throwing out my back)?

I think I understand the difference now that I write it out like that. The forces are balanced in my body, because, pushing on something, that's where the impetus comes from. In the case of gravity, the impetus is external.

Thanks, fellas.

--------------
The is the beauty of being me- anything that any man does I can understand.
--Joe G

  
dheddle



Posts: 540
Joined: Sep. 2007

(Permalink) Posted: Mar. 18 2008,17:08   

The internal stresses in your body all occur in equal and opposite pairs (bone A on bone B, and bone B on bone A, etc.), which doesnít mean they cannot cause injury (bone A might snap), but they cannot produce a net force on your body since they all add up to zero. If you push on a brick wall to the right it pushes back to the left. You generally wonít move because friction will (up to a point) produce another force on you, this one to the right, and you stay still. In that case, the total forces on you will be:

1) Gravity down.
2) Normal (the reaction from the ground) up.
3) The wall, pushing left, equal and opposite to the force you push against it.
4) Friction on your feet pushing right.

If you push hard enough, or (easier) if you do it on ice, the friction force will not be able to get large enough to counter act the wall pushing left, and you will indeed accelerate to the left.

--------------
Mysticism is a rational enterprise. Religion is not. The mystic has recognized something about the nature of consciousness prior to thought, and this recognition is susceptible to rational discussion. The mystic has reason for what he believes, and these reasons are empirical. --Sam Harris

   
Venus Mousetrap



Posts: 201
Joined: Aug. 2007

(Permalink) Posted: Mar. 18 2008,17:21   

Quote (C.J.O'Brien @ Mar. 18 2008,15:44)
Hey all,

The last formal education in Physics I had was longer ago than I care to contemplate, so I have an (admittedly very basic) question to ask of those wiser than I.

So, we had my son's birthday party at Lawrence Hall of Science in Berkeley over the weekend. The temporary set of exhibits was called "Speed," and they had all kinds of hands-on gizmos illustrating various aspects of "speed": velocity, acceleration, drag, etc.

And one was this pneumatic thingy that shot a penny at great speed into a metal plate and then spit it out so you could see how it deformed on impact. They had some exemplars of what happens, based on composition (zinc vs. copper) and angle of impact (the pennies could "flutter" in the tube). It was pretty cool, and we fed it several pennies. But my issue is with the explanatory sign accompanying the exhibit.

It gave F=ma, talked about the force being applied to the penny. So far so good, but then it got into the "equal and opposite" reaction to the force, and then came the inexplicable talk about (paraphrased) "the plate pushes back" and said (verbatim) "the plate is the big F." (meaning the 'F' in F=ma.)

Is it me, or is this just crazily wrong? As I understand it, F is the initial force applied to the penny to shoot it down the tube, and the "equal and opposite reaction" is the deformation of the penny, i.e. the shedding of kinetic energy into heat and stresses in the material of the penny, causing it to be smashed, cracked or folded.

The way they explained it is analagous to saying that if the force of gravity is pushing you into the ground, then the opposite reaction is somehow the ground "pushing back." And that's all wrong. The ground, or the plate in the exhibit, don't exert any force at all, right? In the gravity example, the equal and opposite reaction is all going on inside your body: it's the various stresses and tensions on your skeletal-muscular system involved in keeping you upright, or, if not upright, then at least not a puddle. Right?

And if so, why does a freaking science museum at freaking UC Berkeley have signs that don't hold up to basic high school physics?

The museum's explanation is accurate.

Remember the three laws:

A robot shall not harm a h-

Wait, not those laws. Newton's:

1. A body will remain at constant velocity unless acted on by a net force.
2. F = ma
3. All forces act in equal and opposite pairs.

These seem to be true, although schools have yet to teach the controversy about the failings of Newtonian gravity at speeds close to that of light.

What this means is that the Earth does indeed push up on you with the same force as you push down on it - it has to be, or it'd be violating the first law (you're not moving up or down, so the net force on you is a zero, balanced force).

Similarly, when it comes to your body, all the internal forces are balanced, by tension (which is also a force). If you go to a planet with super-gravity, then you'll find what happens when your body doesn't have the ability to balance the forces; they get balanced for you when your bones and organs puddle onto the ground.

It's the same with the penny, but horizontal. The penny is travelling at a huge velocity until it hits the plate and decelerates to zero. The first law says that a force must have applied to cause this. The third law says that a force is also applied equal and opposite to that. The second law tells you a little about the forces effect.

Since the plate is large and probably mounted, it'll shrug off the penny's force without much problem. However, when the penny takes the force in the other direction, its tiny lil body doesn't have the internal strength to decelerate and keep its shape, so it bends and crinkles.

Try to envision what happens if the plate is replaced with a piece of paper. This time, the penny wins; the paper can apply a force to slow the penny, but can't possibly take the reaction force without tearing, so the penny makes a hole in it.

  
Texas Teach



Posts: 1431
Joined: April 2007

(Permalink) Posted: Mar. 18 2008,17:37   

C. J. the explanation for why inanimate objects like the ground or the plate can "push" on you is that the bonds between the atoms stretch like the material of a trampoline when you push on them.  The force of those bonds trying to return to their rest position is what allows the ground to hold you up or the plate to resist the penny.

--------------
"Creationists think everything Genesis says is true. I don't even think Phil Collins is a good drummer." --J. Carr

"I suspect that the English grammar books where you live are outdated" --G. Gaulin

  
C.J.O'Brien



Posts: 395
Joined: Aug. 2005

(Permalink) Posted: Mar. 18 2008,17:51   

Quote (Texas Teach @ Mar. 18 2008,17:37)
C. J. the explanation for why inanimate objects like the ground or the plate can "push" on you is that the bonds between the atoms stretch like the material of a trampoline when you push on them. †The force of those bonds trying to return to their rest position is what allows the ground to hold you up or the plate to resist the penny.

There we go. Thinking this way (along with Venus Mousetrap's example of substituting paper for metal plate) makes it all much clearer.

I still think there's a better way of putting it than "the plate is the big F" but I see that the explanation is substantially correct, just using a somewhat confusing shorthand.

Sometimes I overthink these things. I appreciate you all taking the time to educate me.

--------------
The is the beauty of being me- anything that any man does I can understand.
--Joe G

  
Reciprocating Bill



Posts: 4265
Joined: Oct. 2006

(Permalink) Posted: Mar. 18 2008,17:52   

This is ridiculous.

All physical interactions were intelligently front-loaded into the universe at the beginning of time. Viewed from this intelligent, superdimensional perspective all physical motions, interactions, changes in state and form, exchanges of energy and so on are the expression of the intelligent uberwill of the designer, which creates the illusion of having been fragmented into these objects and forces. The penny in question is pleased to enter into cooperative interaction with the plate, which both experience as a kiss, and takes pleasure in conforming to new shapes for the entertainment of the onlookers. There is no application of force, because these objects and energies are willing participants that need not be coerced.

(But there's a spot on my shirt that I'm sure doesn't like me.)

--------------
Myth: Something that never was true, and always will be.

"The truth will set you free. But not until it is finished with you."
- David Foster Wallace

"Here’s a clue. Snarky banalities are not a substitute for saying something intelligent. Write that down."
- Barry Arrington

  
J-Dog



Posts: 4402
Joined: Dec. 2006

(Permalink) Posted: Mar. 18 2008,18:03   

Heddle et al:

Now please explain the forces involved in this scenario :

1.) View Ben Stein's face, and or hear his voice leading to
2.) Vomit rising in stomach and
3.) Projectile heave of massive proportions (lunch)

Is it U / V x M?

Where U = UGLY
V = Velocity
M = Mass of Vomitus

--------------
Come on Tough Guy, do the little dance of ID impotence you do so well. - Louis to Joe G 2/10

Gullibility is not a virtue - Quidam on Dembski's belief in the Bible Code Faith Healers & ID 7/08

UD is an Unnatural Douchemagnet. - richardthughes 7/11

  
keiths



Posts: 2041
Joined: Jan. 2006

(Permalink) Posted: Mar. 18 2008,18:37   

Quote (C.J.O'Brien @ Mar. 18 2008,17:51)
I still think there's a better way of putting it than "the plate is the big F" but I see that the explanation is substantially correct, just using a somewhat confusing shorthand.


Yes.  It would be more accurate to say that the plate asserts a big F on the penny, and the penny asserts a big -F on the plate (where the minus sign indicates that the second force is acting opposite to the first).

Saying that "the plate is the big F" is their way of indicating that the presence of the plate is what creates the big F that results in the penny's deformation, in contrast with the pneumatic gun, which applies a smaller F over a longer period of time, and thus does not deform the penny.

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And the set of natural numbers is also the set that starts at 0 and goes to the largest number.  -- Joe G

Please stop putting words into my mouth that don't belong there and thoughts into my mind that don't belong there. -- KF

  
Amadan



Posts: 1332
Joined: Jan. 2007

(Permalink) Posted: Mar. 18 2008,18:48   

Yeah, right.

But when you see Chuck Norris doing push-ups, that isn't him going up. It's the world going down.

--------------
"People are always looking for natural selection to generate random mutations" - Densye  4-4-2011
JoeG BTW dumbass- some variations help ensure reproductive fitness so they cannot be random wrt it.

   
dheddle



Posts: 540
Joined: Sep. 2007

(Permalink) Posted: Mar. 18 2008,20:32   

Interestingly enough, Newton's 3rd Law is sometimes violated--when fields can contain momentum.

Here is a simple example:

http://books.google.com/books?i....I&hl=en

As a partial answer to J-Dog's question, Newton's 3rd would be respected for a Stein-WAD interaction, because ID, as a field, is clearly out of momentum.

--------------
Mysticism is a rational enterprise. Religion is not. The mystic has recognized something about the nature of consciousness prior to thought, and this recognition is susceptible to rational discussion. The mystic has reason for what he believes, and these reasons are empirical. --Sam Harris

   
J-Dog



Posts: 4402
Joined: Dec. 2006

(Permalink) Posted: Mar. 21 2008,18:21   

Quote (dheddle @ Mar. 18 2008,20:32)
Interestingly enough, Newton's 3rd Law is sometimes violated--when fields can contain momentum.

Here is a simple example:

http://books.google.com/books?i....I&hl=en

As a partial answer to J-Dog's question, Newton's 3rd would be respected for a Stein-WAD interaction, because ID, as a field, is clearly out of momentum.

Heddle - I like it.  Thanks.

--------------
Come on Tough Guy, do the little dance of ID impotence you do so well. - Louis to Joe G 2/10

Gullibility is not a virtue - Quidam on Dembski's belief in the Bible Code Faith Healers & ID 7/08

UD is an Unnatural Douchemagnet. - richardthughes 7/11

  
Henry J



Posts: 4565
Joined: Mar. 2005

(Permalink) Posted: Mar. 24 2008,09:51   

Physics questions?

Why do the up and down quarks have the electric charge values that they have?

Henry

  
BWE



Posts: 1898
Joined: Jan. 2006

(Permalink) Posted: Mar. 25 2008,07:17   

Quote (C.J.O'Brien @ Mar. 18 2008,16:57)
Okay,
I guess it makes more sense the way you guys describe it. The whole idea of a stationary plate exerting force just struck me as strange. But I do get that there's a positive acceleration launching the penny, balanced by a sudden negative acceleration as the penny hits the plate and comes to rest.

But, again, with gravity and the ground "pushing back," I'm still befuddled. So, if I push on a solid, stationary brick wall, and nothing moves, where is the opposite reaction, required by Newton's Third Law? Does the wall push back? Or is the action, me pushing, balanced by internal stresses in my body (me throwing out my back)?

I think I understand the difference now that I write it out like that. The forces are balanced in my body, because, pushing on something, that's where the impetus comes from. In the case of gravity, the impetus is external.

Thanks, fellas.

No movement, no joules. (Newton meters)

Force is applied but no movement. No deceleration without acceleration.

Yoda I am making my sentences like. Yes?

--------------
Who said that ev'ry wish would be heard and answered
When wished on the morning star
Somebody thought of that, and someone believed it
Look what it's done so far

The Daily Wingnut

   
dogdidit



Posts: 315
Joined: Mar. 2008

(Permalink) Posted: Mar. 25 2008,08:33   

Quote (Henry J @ Mar. 24 2008,09:51)
Physics questions?

Why do the up and down quarks have the electric charge values that they have?

Henry

Conservation of charge.
The proton and neutron have charge +1 and 0, respectively, and since they were each posited to contain three quarks, the quarks had to have fractional charges (+2/3 for up, -1/3 for down) to make the arithmetic work out. A proton is two ups and a down, a neutron is two downs and an up.
I don't know if these fractional charge values have ever been independently verified; quarks interact with each other via the stong nuclear force, and that has peculiar properties that prevent quarks from being seen in isolation. But the quark model (or "quantum chromodynamics" as it is called nowadays noIamnotkidding) seems to work as a scientific theory, and QCD may in fact predict fractional charge from more fundamental laws. At this point you'd be better off with answers from a real physicist, and not Some Engineer with a stack o Scientific Americans...
To add to your fun, there are four other quarks (strange, charm, top, and bottom); help yourself but all the good puns have been taken.

--------------
"Humans carry plants and animals all over the globe, thus introducing them to places they could never have reached on their own. That certainly increases biodiversity." - D'OL

  
BWE



Posts: 1898
Joined: Jan. 2006

(Permalink) Posted: Mar. 25 2008,09:28   

Reminds me of the Eddie Murphy stand-up routine where his family is named after breakfast cereals.

--------------
Who said that ev'ry wish would be heard and answered
When wished on the morning star
Somebody thought of that, and someone believed it
Look what it's done so far

The Daily Wingnut

   
dogdidit



Posts: 315
Joined: Mar. 2008

(Permalink) Posted: Mar. 25 2008,09:48   

Quote (BWE @ Mar. 25 2008,09:28)
Reminds me of the Eddie Murphy stand-up routine where his family is named after breakfast cereals.

Yeah, the goofy names and the fractional charges and what-all kept quark theory from being readily accepted into the Standard Model of particle physics...until they started making predictions with it, which were borne out by experiment. Gee, what a concept.

--------------
"Humans carry plants and animals all over the globe, thus introducing them to places they could never have reached on their own. That certainly increases biodiversity." - D'OL

  
Henry J



Posts: 4565
Joined: Mar. 2005

(Permalink) Posted: Mar. 25 2008,15:20   

Quote
and QCD may in fact predict fractional charge from more fundamental laws.


That's what I was wondering - whether the -1/3 and +2/3 were implied by current theory, or determined empirically.

And if it's implied, is the reasoning expressable in language that I would understand or would it shoot over my head?

Henry

  
dogdidit



Posts: 315
Joined: Mar. 2008

(Permalink) Posted: Mar. 26 2008,07:55   

Quote (Henry J @ Mar. 25 2008,15:20)
† † † †
Quote
and QCD may in fact predict fractional charge from more fundamental laws.


That's what I was wondering - whether the -1/3 and +2/3 were implied by current theory, or determined empirically.

And if it's implied, is the reasoning expressable in language that I would understand or would it shoot over my head?

Henry

This is probably one of those instances where if you had to ask the question, you won't understand the answer. The Standard Model employs "gauge theory" and invokes "special unitary group" mathematics and if you know what either of those are, you're way smarter'n me. We never studied that stuff in undergrad physics.

I think (take the above comment as an indication of how accurate the rest of this comment might be) it is true that the fractional charge nature of quarks is still an unsolved problem, and probably one with a Nobel Prize dangling from it. The question must be maddening to the particle physicists: the electron has unitary charge (-1) and it contains no quarks or any other constituent component, so why the fractional charge nature of the quark? We can be confident that there are plenty of (competing) hypotheses that answer the question, but so far none of these have graduated to theory AFAIK.

BTW the fractional charges were indeed confirmed empirically, as of course were the existence and number (and masses) of the quarks themselves. Teh Big Physics Boyz Haz Skillz.

--------------
"Humans carry plants and animals all over the globe, thus introducing them to places they could never have reached on their own. That certainly increases biodiversity." - D'OL

  
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