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The Grand Unification Theory…
http://ffden-2.phys.uaf.edu/211.web.stuff/Lichtenberger/main.htm
Actually, the electromagnetic and weak forces have been shown to be different manifestations of the electroweak force...
The idea of a GUT seems to attract many people who think they can explain it all.
Naturally, when their ideas don’t get accepted, it’s because of some grand conspiracy against them.
Keep that in mind while browsing the web.
…so you might be justified in saying there are only three fundamental forces in nature...
…but for now let’s say our four forces are all that are needed to explain the universe as we know it. There are no more or no less.
About 1986, some distinguished physicists thought they had found experiments which necessitated a fifth force. To understand their work, we need to revisit the idea of mass…
Don't let anyone invent some new force with a fancy name and get you to invest in his (or her) new invention which will revolutionize the world. All it will do is suck up your money.
If you Google “fifth force,” beware of pseudoscience gibberish.
What is mass?
The “thing” that goes into F=ma?
The “thing” that goes into F = Gm1m2/r2?
Are these two “things” (inertial mass and gravitational mass) the same?
OR! Is there even a theoretical or experimental basis for demanding they are the same?
Hungarian physicist, Lorand Eötvös, carried out precise experiments between 1906 and 1909 to compare gravitational and inertial mass.
He concluded the two were the same to within 1 or 2 parts in 200,000,000.
A 1964 experiment1 showed the two were the same to within 1 part in 100,000,000,000.
1. P. G. Roll, R. Krotkov, R. H. Dicke, Annals of Physics, New York, 26, 442, 1964.
This experimental result is the basis for one of the postulates of general relativity:
The Principle of Equivalence: an observer in a closed laboratory cannot distinguish between the effects produced by a gravitational field and those produced by an acceleration of the laboratory.
w = mg
uniform gravitationalfield, downwards
a=g
uniform upwardacceleration = g
force
“The original Eötvös experiment was designed to measure the ratio of the gravitational mass to the inertial mass of different substances.”
“Eötvös found the ratio to be one, to within approximately one part in a million.”
“Fischbach and his collaborators reanalyzed1 Eötvös' data and found a composition dependent effect, which they interpreted as evidence for a Fifth Force.”
Quotes from http://plato.stanford.edu/entries/physics-experiment/notes.html#A4-1.
1. E. Fischbach et al., Phys. Rev. Letters, 56, 2424, 2426, 1986; E. Fischbach, D. Sudarsky, A. Szafer, C. Talmadge, S. H. Aronson, 57, 1959, 1986.
What do you think of this?Remember: right now we believe there are only four fundamental forces: strong, weak, E&M, and gravitational.
Here’s some of the data supporting the fifth force. I’ll discuss this in class.
“Fischbach and his collaborators reanalyzed1 Eötvös' data and found a composition dependent effect, which they interpreted as evidence for a Fifth Force.”
If this is true, two 1-kilogram masses of copper attract each other with a different force than two 1-kilogram masses of aluminum!
Revolutionary! Instant Nobel Prize (on confirmation)! Your name goes down in the annals of physics next to Newton, Maxwell, and Einstein!
Two experiments carried out in 1987 gave conflicting results: one for the fifth force, one against it.
What’s a responsible physicist to do?
Suspend judgment until experiment (supported by theory) offers conclusive evidence.
Ultimately, many experiments were carried out and (eventually) demonstrated conclusively that there is no fifth force.In the reanalysis of the Eötvös experiment, a trend in the data which suggested the fifth force turned out to be a result of statistical fluctuations. The data had a trend, but it was accidental and statistically insignificant.
Physics works!
Interesting discussion: http://plato.stanford.edu/entries/physics-experiment/app4.html.
The Fifth Force affair was great fun and led to a lot of nice research grants, but in the end, there was no fifth force.Most of us believe that four forces are all that are needed, but because we have seen remarkable new discoveries, we are willing to send a few of our colleagues off to search for an experiment which demands a fifth force. If you can show that five forces are needed to explain the universe, I guarantee that you will win a Nobel prize. Please remember me when you cash the check.
If you can show that less than four forces are needed to explain the universe (i.e., that two or more forces are really manifestations of a single force which we do not presently understand in detail) then I also guarantee that you will win a Nobel prize. Good luck, and remember me when you cash that check.
Physical Science:Newton’s Laws
“Classical mechanics” is based on Newton's three laws. These three laws are based on experimental observations which allow us to infer equations which we use to explain and predict how systems respond to forces.We can use these Newton’s laws and their equations to predict observations which have never been made before. That's why we believe them.
You can't "prove" Newton's laws, but you can postulate them and see that they work.
“One reason why mathematics enjoys special esteem, above all other sciences, is that its laws are absolutely certain and indisputable, while those of other sciences are to some extent debatable and in constant danger of being overthrown by newly discovered facts. .”—A. Einstein
Here are the three laws…
On second thought, why don’t you write on a piece of paper, to the best of your knowledge, what you believe Newton’s three laws to be. You can use words or equations. Put your name on the “Newton’s Laws” paper so I can use it to take today’s attendance.
The first law is the law of inertia. It says that if no forces act on an object, its velocity remains constant. A moving body will keep moving and a body at rest will remain at rest.
http://www.glenbrook.k12.il.us/gbssci/phys/mmedia/newtlaws/cci.html
Let me illustrate by pushing a book across the table.
After I stop pushing on it, what happens?
What can you infer?
What was/were the force(s)?
It comes to rest. Its velocity changes.
Forces must have been acting on it.
Friction.
If no forces act on an object, its velocity remains constant.
What did it take to get the book moving?
When a force is applied to an object, the force changes the body's velocity. The bigger the force, the bigger the change in velocity. The force can be applied to make the body speed up (gas pedal) or slow down (brakes).
A force.
What did the force cause?
Acceleration.
This brings us to Newton's second law. This is a non-mathematical course, so I'll first put this law in words.
A force applied to an object changes the object’s velocity.
Acceleration—the physicist’s term for change of velocity—can be either positive or negative, and can lead to greater velocities (“acceleration”) or smaller velocities (“deceleration”).
Only two of the cars are accelerating (after they have started)
It’s not necessarily the “acceleration” that hurts.
…and the red car is not one of them! http://www.glenbrook.k12.il.us/gbssci/phys/mmedia/kinema/acceln.html
Mass is a measure of the resistance of a body to being accelerated. The greater a mass, the more difficult it is to accelerate it.
Mathematically, Newton's second law says F=ma.
My previous Physics 6 class liked the Pig, so it gets an encore.
If you double the force on a given mass, you double its acceleration: 2F=m(2a).
If you double the mass of an object, you double the force needed to produce a given acceleration: 2F=(2m)a.
The units of force are kgm/s2. One newton is one kgm/s2.
F = m a
kilograms meters/second2kgm/s2
If I pushed on you with a force of one newton, what would you feel?
If I tossed you a 1 kg mass, what would it feel like?
The acceleration due to gravity near the surface of the earth is 9.8 km/s2, so a 1 kg mass has a weight of 9.8 newtons, or about 2.2 pounds.
Gravity is a force, right? That means gravity produces an acceleration. We even talk about the acceleration due to gravity. Why don't we feel that acceleration?
The answer is we feel the force of gravity, every day, all the time. We call it "weight." Weight is the force with which gravity is pulling on us.
If forces produce accelerations, why don't we accelerate?
The answer is we do, until we come in contact with the earth. This leads us to Newton's third law, which says "for every action there is an equal and opposite reaction."
In Newton’s writings, “action” and “reaction” refer to forces.
For every action there is an equal and opposite reaction.
http://www.glenbrook.k12.il.us/gbssci/phys/Class/newtlaws/u2l1d.html
For every action there is an equal and opposite reaction.
What does that mean? I have 9-year old twin boys (well, they were 9 the first time* I taught this class). When one gets mad and slugs the other, the other slugs back.
(Don't worry, they know how to slug without hurting. Makes a big noise so their mom yells, but nobody gets hurt.) Is that what Newton meant by "equal and opposite reaction?"
*Helps me keep track of when I originally wrote these notes!
Of course not; you know that. A "reaction" is a force. The figure to the right shows what Newton's third law means.The person is standing on the ground.
These two forces are known as an action-reaction pair.
The person is standing on the ground. The person is pushing down on the ground with a force equal to his weight.
The person is standing on the ground. The person is pushing down on the ground with a force equal to his weight. Newton's third law says the ground is pushing back on the person with a force equal to the person's weight.
These two forces are known as an action-reaction pair. If I push you and you fall down, that is not the action-reaction that Newton's third law talks about. The two forces shown in the original figure are not action-reaction forces.
Think about this: the earth is pulling on the person, right? That's what is giving him his weight. According to Newton's third law, he must be pulling back on the earth with a force equal to his weight. Watch and I'll demonstrate...
When I jump (or drop a book if I don’t have what it takes to jump), you don't see the earth move up to meet me (or the book), because the earth is so much more massive than the book. But the book and I really do pull on the earth.
Newton's second law is extremely powerful. You can solve an incredible number of real-world problems using it and the third law.
According to these laws, if an object is at rest, it has no net forces acting on it. You can use this fact to calculate “everything” you need to know about a system of objects at rest (or moving with a constant velocity).I hope that the engineer who designed your house knows his (her) physics!
A walkway at the Hyatt Regency collapsed because an engineer didn’t double-check a simple physics calculation.
If an object is moving or accelerating, you can use Newton's laws to calculate (in principle) everything you need to know about the motion of the object.
A nonzero acceleration means there is a net nonzero force acting on the object.
Figure taken fromhttp://hyperphysics.phy-astr.gsu.edu/hbase/incpl2.html#c1
.I paid for use of material on this site
A moving but not accelerating object probably has a number of forces acting on it, because in the real world there is almost always friction present. If an object is moving with a constant speed and has friction trying to slow it down, some other force must be applied to counteract the friction. (Where is the “engine force?”)
constantvelocity
forward force of road on tires
weight
upward force of road
airresistance
other friction forces
Here's an interesting problem to consider. You are in a car going around a curve at a constant speed. Are there any net (unbalanced) forces acting on you? Are you being accelerated? If yes, what is causing the acceleration?
The force of the road on the tires causes an acceleration directed towards the “center of curvature.”
road forcehttp://136.142.138.22/CARS.HTM
This is all pretty dry stuff. Let's do a little experiment on forces. Hopefully this experiment will not dampen our spirits...
The little demonstration illustrates a number of important features of physics problem-solving.
If you took a physics class with math and problem-solving, you would first draw a picture of your problem, so let’s do that…
It is critical to focus on the important things in the problem, and ignore those things which are not important (after double-checking that they really are unimportant).What do you believe I think is the important thing in this demonstration?
Skip to this slide if do diagram on board.
First the important thing—the card!
Next, show all the forces acting on the card.
weight of cardweight of water
air pressure
air pressure
Any other forces? If not, looks like I was in trouble!
weight of cardweight of water
air pressure
air pressure
A “sticking force” between wet card and glass jar?
Maybe a tiny one. Not big enough to show in the diagram.“Suction” due to an air bubble in the jar?
No, no, no! Vacuums do not suck! Plus air is not a vacuum.
weight of cardweight of water
air pressure
air pressure
Oh, I almost forgot. The glass jar is touching the top surface of the card. It could be pushing down on the card.That doesn’t help, does it?
Air pressure! There is more “underneath” card surface for the air to push up on than there is “above” card surface for the air to push down on.
weight of cardweight of water
air pressure
air pressure
Come on, Physics, help us out… the card doesn’t accelerate. Newton says the up and down arrows must balance.
Come on, Physics, help us out… the card doesn’t accelerate. Newton says the up and down arrows must balance. Air pressure!
weight of cardweight of water
air pressure
air pressure
In fact, the upward force of the air is very large, and the glass jar pushes down with a big force in order to balance the air force.
If all goes according to plan, the upward and downward forces balance, and I am “happy.”
Some interesting facts: there is enough air pressure to hold up a 34 foot high column of water, or about a 30 inch (960 millimeter) high column of mercury
You couldn't get a straw more than 34 feet long (vertically) to work, because you rely on air pressure to push the liquid up the straw. Nor could you rely on air pressure alone to lift water more than 34 feet up from a well.
Here's something to think about. How do you measure mass? I expect most answers will involve some kind of a balance or scale. Spring scales use a spring to balance your weight. Would a spring scale work on the moon?
I expect most answers will involve some kind of a balance or scale. Spring scales use a spring to balance your weight. Would a spring scale work on the moon? Yes, but not accurately.
Would a spring scale work in outer space?Would a spring scale work in outer space? No.
Please visit howstuffworks.com to see how lots of stuff works.
I need to revisit the subject of mass for a bit…
Some of you may have thought of a balance scale. The simplest kind works like the one you saw in the first lecture:
Two identical pans, identical distances from “hanging” point. Put unknown mass in one pan, knowns in the other pan until balance is achieved.
Triple beam balances are more complex, but still rely on balancing known masses against unknown masses.
Would either of these balances work on the moon?
Yes, and accurately.
Would either of these balances work in outer space?
No! Why not?
How would you measure mass in outer space? You'd have to use Newton's second law and some device that measures the acceleration of an unknown mass as a result of a known force.
A while back we investigated the difference between inertial and gravitational mass, and I concluded there is currently no conclusive evidence to suggest they are different.
This brings up an interesting question: is the mass that responds to gravity the same as the mass that accelerates in response to forces?
Such an instrument is called an inertial balance, and works in space.
Two NASA inertial balance designs (the pictures were “live” links).
All those who are non-science majors, temporarily move to the front three rows of the room, and all those who are science majors, move to the back of the room. Science majors: take a little break.
Non-science majors: after a few minutes of discussion, give me a technical term in your one of your fields that you would like to have the science majors attempt to define.
There are two more subjects I want to cover in this discussion of forces and motion: momentum and density.What does momentum mean to you?
Guess what? Non-science majors get to write your definition of momentum on a piece of paper and turn it in for today’s attendance.
Science majors, please define the term that the non-science majors have chosen. Turn in your definition for today’s attendance.
Expect to get to about here.
Momentum is related to the "mass in motion part" of Newton's first law. Remember, that law says an object in motion remains in motion unless acted on by external forces. Momentum is a measure of a body's "desire" to remain in motion.
In physics, momentum is calculated using the equation p=mv. (I guess since m was already used for mass, "p" is as good a symbol as any for momentum.) The more mass you have, or the faster you are going, the greater your momentum. There is an important law of physics, known as the Law of Conservation of Momentum. This law says that momentum is conserved in any process, as long as no external forces are acting.
If your car and an 18-wheeler collide, what does the Law of Conservation of Momentum have to say?
graphics from http://www.glenbrook.k12.il.us/gbssci/phys/mmedia/
It has been observed experimentally and verified over and over that in the absence of external forces, the total momentum of a system remains constant.
The above is a verbal expression of the Law of Conservation of Momentum.
It sounds like an experimental observation, which it is……which implies maybe we just haven’t done careful enough experiments, and that maybe some day we will find the “law” is not true after all.But the Law of Conservation of Momentum is much more fundamental than just an experimental observation.
For every continuous symmetry of the laws of physics, there must exist a conservation law.
For every conservation law, there must exist a continuous symmetry.
In 1905, mathematician Emmy Noether proved the following theorem:
The laws of physics are invariant under coordinate transformations.
That’s a fancy way of saying you and I can pick different coordinate systems for measurements and experiments, but we will still arrive at equivalent results.
If the laws of physics were not invariant under coordinate transformations, then everybody would have his own version of the laws of physics.
The Law of Conservation of Momentum is a mathematical consequence of the invariance of the laws of physics under coordinate transformations.
Any violation of the Law of Conservation of Momentum would be as revolutionary (if not more so) as Einstein’s relativity. But even if a violation were found, any “new” laws of physics would contain all our “old” ones, which would still work under “normal” circumstances.
Not much point in doing physics, if that’s true!
Conservation laws are fundamental, powerful, and beautiful.
We will see another one soon.
In contrast, Newton’s laws work only in the macroscopic world, and are only an approximate description of nature.
Before I leave momentum, have you ever thought of racing a train across the crossing?
What is a train’s mass? How far does a train traveling at 60 miles per hour go before the friction forces from its brakes cause it to stop?
What is your car’s mass? How far does it take you to stop your car when you are going 60 miles per hour?
Some of the distance traveled is due to your reaction time, and the rest is due to the time needed from the frictional forces applied by your brakes to "use up" your car's momentum.
