by Rod P 41 Replies latest jw friends
why are things reversed right to left in a mirror, but not up and down?
Um, they don't reverse left/right any more than they reverse top/bottom! Face a mirror and point left or right. You are still pointing that same direction in the mirror.
Now mirrors DO reverse in/out. If you point towards a mirror, the mirror will make it appear that you're pointing back towards yourself.
THE ARROW CAN'T REACH POINT B??? LIKE HELL IT CAN'T.
When I was about 15 me and my nephew were playing with a bow and arrow.
For some idiotic reason he shot off an arrow about 50 ft and towards me. The damned thing hit me almost right in the ass. POINT B
Luckily the bow didn't have too much power.
Outoftheorg
On the Mirror question, Yes, there is a reverse image.
Try this experiment. Take a piece of paper and a pen. Now, sit at a table, and place the paper on the underside of the table. Now write your name on the paper while it is on the underneath side. Now look at the paper, and what do you see? Your name will be written backwards. Now go to the mirror and hold up your paper to the mirror. What do you see?
Now, take a piece of paper or a small card and write the following letters:
b q
d p
Look at this in the mirror. You would see in the mirror the same thing that you would see if you could hold the paper up to a light, and then looked at the letters from the other side of the paper.
What if we could turn this paper (card) over (i.e. rotated it about the vertical axis)? Here's what we would see:
p d
q b
In other words, the image would be reversed left-right.
What if we rotated this card or paper 180 degrees (i.e. rotate it about the horizontal axis), so that it is now upside down. Now look at this upside down paper in the mirror, and what do you see? It would look lke this:
p d
q b
In other words, it will be reversed up-down. This is true, regardless of whether we look at it in the mirror, or if we looked at the letters thru the backof the paper (card).
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On the question of Protagorus versus Euathlus:
Where Euathlus went wrong was he was a Defendant in his own lawsuit. (He had a fool for a client). What if Euathlus had hired another lawyer to handle his case. Then if his lawyer won the case for him, then Euathlus would still not have won his first case. That way, according to the terms of the original contract, Euathlus would have to pay Protagorus only on the condition that Euathlus won his first case, and under these circumstance, it could still be said that Euathlus was not yet practicing law, and so had yet to win his first case. Therefor, Euathlus should not have to pay.
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Outoftheorg,
If you go back to Page 1 of this thread, you will see the explanation of Zeno's Paradox that explains the fact that the arrow does move from Point A to Point B, which is mathematically consistent with both space and time.
Terry,
On Page 1 of this thread, in my reply to Dr. Mike, I stated why I avoided taking this paradox analogy down to the minute or sub-atomic particle level.
The Heisenberg Principle of Uncertainty is primarily about the consciousness of the observer affecting the thing being observed, so that one can never know with certainty its exact location/position or speed. I see how the Heisenberg Principle is materially relevant when it comes to the tiniest particles and sub-stomic particles of the universe. But I hardly think this would be all that relevant at the scale of the arrow in the "normal" observable day-to-day world we live in. When we get to the point of measuring the infinitely small units of time and distance remaining for the arrow, I doubt that our observation of the arrow in flight would materially influence its position in space and time. In other words, Heisenberg does not influence, or interfere with, the problematic nature of this paradox.
Rod P.
A few yrs ago this paradox was answered in some science mag. Can't remember the answer, though.
S
Satanus,
I presume you are referring to the Zeno's Paradox.
As stated previously, just go back to Page 1 of this thread, and you will see the explanation.
Rod P.
Why do need to expend energy and my desk does not, when we're both doing exactly the same job? If anyone tells me I'm doing work against gravity, I'll send you to the back of the class with the pointy cap on your head! Work = force x distance, and if I'm holding the briefcase still, I should be doing no work, right? Yet I expend energy to do this no work...
to hold something at arms length and hold it still requires your two sets of opposing muscles to constantly adjust in relationship to each other and that wastes a lot of energy.
Iggy,
On your problem with the briefcase, I quote information from the following websites:
http://www.glenbrook.k12.il.us
Work is defined as Force X Distance (Displacement). More precisely, "Work is defined as a force acting upon an object to cause a displacement. There are three key words in this definition- force, displacement, and cause. In order for a force to qualify as having done work on an object, there must be a displacement and the force must cause the displacement. There are several good examples of work which can be observed in everyday life - a horse pulling a plow through the field, a father pushing a grocery cart down the aisle of a grocery store, a freshman lifting a backpack full of books upon her shoulder, a weightlifter lifting a barbell above her head....In each case described here is a force exerted upon an object to cause that object to be displaced.
So basically, for there to be Work, there must be movement of an object thru a distance. If there is no movement, there is no work, which is the case for your briefcase example.
To understand what is going on here, we must first understand the difference between Kinetic Energy and Potential Energy.
Kinetic Energy is the energy of motion. Kinetic energy is measured by how much is work don to put an object in motion or to rest.
Kinetic Energy = 1/2 (Mass) times (Velocity squared)
On the other hand, Potential Energy is energy due to position or, in other words, "stored energy:. Potential energy is also called gravitational potential energy.
Potential Energy = (Weight) times (Height)
An example of Potential Energy: By stretching a rubber band, you give it potential energy. A book on a shelf has stored potential energy. A briefcase on a desk has stored potential energy. Holding a briefcase up with one's arm muscles also has stored potential energy. A baseball in a glove has potential energy until it is thrown, and it turns into kinetic energy when thrown.
Kinetic and Potential Energy: A waterfall has both kinetic and potential energy. The water at the top of the waterfall has stored potential energy. Once the water leaves the top of the waterfall, the potential energy is changed into kinetic energy.
Gravitational Potential Energy: An object can store energyas the result of its position. For example, the heavy ball of a demolition machine is storing energy when it is held at an elevated position (i.e. potential energy). Gravitational Potential Energy is the energy stored in an object as the result of its verticla position (i.e. height). The energy is stored as the result of the gravitational attraction of the Earth for the object. The greater the mass of the object (eg the demolition ball) the greater the Gravitational Potential Energy. The higher the object is off the ground, the greater the Gravitational Potential Energy. The relationships of these factors are expressed with the following formula:
Gravitational Potential Energy (GPE) = Mass X "g" X Height
("g" represents the acceleration of gravity, which is approximately 10 meters per second per second on Earth.)
To determine the GPE of an object, a zero height position must first be arbitrarily assigned. Typically, the ground is considered to be a position of Zero height. But this is merely an arbitrarily assigned position which most people agree upon. Since many of our labs are done on tabletops, it is often customary to assign the tabletop to be the zero height postion; again, this is merely arbitrary.
In the case of the briefcase in your example, I would use the ground as zero height, and the distance the desktop is off the ground is the height for the briefcase. The distance your arm holding up the briefcase is off the ground is the height for the briefcase.
I hope that helps you to explain the situation with the briefcase. There is no work until it is dropped off the desk to the ground, or you let go of it with your hand and it drops to the ground. The potential energy is changed to kinetic energy, when work is then accomplished.
Rod P.
Hi Rod,
My problem with the briefcase lies in the fact that I have to continue to expend energy to keep the briefcase perfectly still. I'm justified in taking the table top as a local zero for my PE because I could have an assistant lift the briefcase up for me and put it in my hand which was already at table height. In that case, I would have done no work lifting the briefcase, but I would have to continue to expend energy in my muscles to keep the briefcase up at table top height. It really does seem to me that I have to continue to expend energy to change the PE not one jot.
I think (from doing a little reading around) that the answer lies in what goes on in muscle cells to produce muscular contraction. There's a chemical reaction involving long chainlike molecules, which in the presence of ATP (it always seems to come down to ATP in the end in biochemistry!) start to move along each other in a pretty amazing way, causing a contraction in the cell and the conversion of ATP to ADP as usual. In the absence of ATP though the cell relaxes back, hence the need for a constant supply of ATP to maintain muscle tension, hence the constant supply of energy required by me to hold the briefcase vertically still.
Looks like muscle cells are a bit like leaky beach balls, needing a constant supply of energy to maintain their inflation.
http://health.howstuffworks.com/muscle4.htm gives a reasonable explanation I think (sorry, not got time to read it in depth at the mo, but it looks ok on first glance)
ig.
Iggy,
Try this website:
This one will give you a good explanation of what happens with muscles and energy changes.
Force is a Push or a Pull that causes a change in the motion or shape of an object. (eg. Pull of Gravity)
Force = Pushing, Pulling, Stretching, Squeezing, Bending and Falling
Forces can be Equal- eg. Holding a Briefcase up with your hand/arm. Your hand/arm pushes up (using muscle energy) while the Briefcase pushes down (under force of gravity).
Energy is the ability to do work- to cause something to move.
The following has been quoted and extrapolated from the above website:
Food has Chemical Energy which gets converted into Muscle Energy. Actually, muscle fibers, full of muscle cells, are constantly converting the chemical energy stored in glucose into work and heat. The work is used to carry on cell processes like growing, reproducing, moving molecules around, and getting rid of waste. The heat is a byproduct of the fuel "burning" process, and flows from the body to the surrounding air. The muscles meanwhile, contract, converting chemical energy into mechanical energy and heat flow. The energy expended by the muscles holding up the briefcase is offset by the force of gravity, and so there the briefcase sits, being held there in the air.
Rod P.
Rod,
I disagree that the mirror reverses left/right. Yes, when you write something backwards on a piece of paper and hold it up to the mirror it now appears correct. But the left/right reversal was done when you *turned around* the paper to have it face the mirror. The mirror itself didn't do anything to reverse the image.
Write something backwards on a piece of tracing paper, turn the paper around (as if you were going to have it face a mirror), but this time just look at the back of the paper. You will see what you wrote now appears correct... and no mirror was involved. :)
