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#251 saudoso

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Posted 08 May 2012 - 21:38

Yep, but I was hoping that somebody will come up with formula to explain it... (the problem is that when using 'force x displacement' formula I got the wrong result and can't come to terms why I did, but, appropriately for this thread, got the correct result when I considered the power consumption)

As I wrote this reply it occurred to me where the error was, so we'll pretend I was posting it to test everybody else... :lol:



I guess you didn't factor the aircraft displacement?

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#252 Greg Locock

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Posted 08 May 2012 - 22:12

The whole point about inertial reference frames is intersting. The reason, intuitively, you know that school physics experiments with billiard balls work when applied to aircraft is because the surface of the earth is moving rather fast compared to the centre of the earth, which is moving quickly relative to the sun, which is moving very fast relative tot he rest of the universe.

If you couldn't ignore these enormous speeds, billiards would be impossible for our monkey brains.

Incidentally this leads to Greg's depressing thought for the day. KE is not an absolute, usually. It depends what your frame of reference is. Somebody might like to work on the usual collision example attaching the ref frame to one of the bodies, or the ground. It should still work out.



#253 Wolf

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Posted 08 May 2012 - 22:13

GSpeedR, excellent work, esp. considering the restrictzions of text editor. :up:

Saudoso, on the contrary- my result was 105kJ, because I used total distance traveled instead of the distance traveled by the airplane. I was calculating the work done by the aircraft, but obtained total work (plane and flight attendant). :lol: Just for curiositiy, here is it... (the 'power consumption' calculation was OK because it disregarded the distance traveled by the flight attendant)

Posted Image


Edited by Wolf, 08 May 2012 - 22:18.


#254 Kelpiecross

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Posted 09 May 2012 - 11:24

Thinking of another example, if you mount a cannon to the front of the train, and fire it while moving. Clearly the cannon's recoil would either slow down the train, or require it to put some energy out to maintain its speed, in which case the locomotive would work together with the powder charge to put the energy in the cannon ball.


It's been about 40 years since I did a physics problem of this type - so I may be a little rusty.

It seems to me that with problems like the hostie with the drinks cart or the train with the cannon that it is appropriate to work with the principle of conservation of momentum - not kinetic energy. In fact I am not sure if it is valid at all to use kinetic enrgy in problem calculations like these.

I would also make the problem simpler - consider a spaceship stationary in space (or moving - it doesn't matter - as Einy said to me one day "It's all bloody relative, you know Mate").
The spaceship has a mass of 1000kg - it fires a cannonball (mass 10kg) out the front at 100m/s. Using conservation of momentum principle: The starting momentum was zero (in that frame of reference) - the ship imparts a momentum of 1000kgm/s to the cannonball - thus it "recoils" with a momentum of the same value in the opposite direction. The ship is 1000kg - its momentum is 1000kgm/s - its velocity in the opposite direction to the cannonball is 1m/s.

I don't think that there can be a argument with this conclusion.

What happens if you apply kinetic energy principles to the question?

The 10kg ball travels off at 100m/s - this is a KE of 50,000 joules. Using the logic which people on this forum seem to have been using - the ship recoils with a KE of 50,000 joules. If the ship's mass is 1000kg - using the "KE" calculation its velocity works out to be 10m/s - which is incorrect. You can't use KE in calculations like this.

If you change the situation slightly: A spaceship of 1000kg travelling slowly at 1m/s, a 10kg cannonball approaches (in a straight line in the opposite direction) at 100m/s and zooms straight down the barrel and stays there - equating to an inelastic collision. Clearly the momentums cancel and the combined mass is stationary.

But if you apply KE to the calculation - the ball has 50000 joules KE, the ship 500 joules. Where did 49500 joules of KE go? You know the combined mass is stationary (which actually means zero KE) and not travelling off with a KE of 49500 joules. The KE has gone where it always goes in inelastic collisions - Christ only knows - but it's gone.

If I have got this completely arse-up remember I am only a bloody dog and it's been 40 years.

















#255 manolis

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Posted 09 May 2012 - 14:12

“The 10kg ball travels off at 100m/s - this is a KE of 50,000 joules. Using the logic which people on this forum seem to have been using - the ship recoils with a KE of 50,000 joules. If the ship's mass is 1000kg - using the "KE" calculation its velocity works out to be 10m/s - which is incorrect. You can't use KE in calculations like this.”: Kelpiecross

Kelpiecross,
What makes you think that the KE of the spaceship needs to be the same with that of the ball?

The conservation of energy simply says that the total energy of the system before and after the boom cannot change.

This means that since, after the boom, the kinetic energy of the ball is not zero, someone has provided this energy to the ball. For instance the “fuel” burned inside a cannon.
Or, for simplicity, a zero-mass compressed spring between the ball and the spaceship.

Think this case of the compressed spring of zero-mass:

It expands pushing the ball at one direction and the spaceship at the opposite direction.

For each 100mm the ball moves at one direction, the spaceship moves at the opposite direction for only 1mm (because the spaceship mass to the ball mass ratio is 100:1), otherwise the center of gravity of the system “ball and spaceship” cannot stay immovable (as it was at the beginning).

The force the one end of the spring applies to the ball is equal and opposite to the force the other end of the spring applies to the spaceship.

So, we have two equal and opposite forces, with the one moving 100 times longer as it pushes the ball than the other as it pushes the spaceship.

This means that for each 100 Joule of kinetic energy provided by the spring to the ball, the spring provides only 1 Joule of energy to the spaceship.

This means that, with the ball having 50000 Joules KE after the boom (caused by the expansion of the spring, or by the explosives in a cannon etc), the KE of the spaceship after the boom will be 500 Joules, which means its velocity is only 1m/sec.

As for the total energy, the 50500 Joules of energy stored into the compressed spring changed to 50000 Joules KE of the ball and another 500 Joules KE on the spaceship.

Thanks
Manolis Pattakos

#256 gruntguru

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Posted 09 May 2012 - 23:10

If you change the situation slightly: A spaceship of 1000kg travelling slowly at 1m/s, a 10kg cannonball approaches (in a straight line in the opposite direction) at 100m/s and zooms straight down the barrel and stays there - equating to an inelastic collision. Clearly the momentums cancel and the combined mass is stationary.

But if you apply KE to the calculation - the ball has 50000 joules KE, the ship 500 joules. Where did 49500 joules of KE go? You know the combined mass is stationary (which actually means zero KE) and not travelling off with a KE of 49500 joules. The KE has gone where it always goes in inelastic collisions - Christ only knows - but it's gone.

Heat mate - it always ends up as heat. Heat at ever decreasing temperature, ever decreasing quality, ever increasing Entropy. Its quite depressing actually.

#257 Kelpiecross

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Posted 10 May 2012 - 04:50

[quote name='manolis' date='May 10 2012, 01:12' post='5703585']
“The 10kg ball travels off at 100m/s - this is a KE of 50,000 joules. Using the logic which people on this forum seem to have been using - the ship recoils with a KE of 50,000 joules. If the ship's mass is 1000kg - using the "KE" calculation its velocity works out to be 10m/s - which is incorrect. You can't use KE in calculations like this.”: Kelpiecross

Kelpiecross,
What makes you think that the KE of the spaceship needs to be the same with that of the ball?

I don't think that - I got the impression other people were using KE like this in a fashion similar to momentum.

What you say appears to be fair enough.

Basically what I was trying to say was that people seemed to attempting to solve an "inelastic collision" problem (or the reverse situation - which amounts to the same thing) by using an erroneous "conservation of KE principle".

I notice that you used (in a roundabout sort of way) the conservation of momentum principle as a means to working out KEs etc. - which is the correct way to do things.




#258 Kelpiecross

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Posted 10 May 2012 - 05:07

Heat mate - it always ends up as heat. Heat at ever decreasing temperature, ever decreasing quality, ever increasing Entropy. Its quite depressing actually.


I know how it feels. Yes it does all end up as heat ultimately. But as I said to Young Manny - that is not what I was writing about primarily - it was that people were using the wrong approach to the question.

You could also have a situation where, in the example of the ball going down the barrel, that it compressed Manny's spring (without any heat losses etc.) and locked in this position. At some later time it could then release and push the ball out thus creating a delayed reaction conversion of an inelastic collision to an elastic collision.

Another point I was trying to make was that you should try and keep the examples simple. That is; not having a situation where the plane or train is constantly using energy to maintain speed, or return to its initial speed again after the hostie goes out the front. These things are only distractions that complicate the underlying situation.

#259 Greg Locock

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Posted 10 May 2012 - 10:44

Another point I was trying to make was that you should try and keep the examples simple. That is; not having a situation where the plane or train is constantly using energy to maintain speed, or return to its initial speed again after the hostie goes out the front. These things are only distractions that complicate the underlying situation.

I feel a bit miffed about that, since the hostie with the drink trolley in the 747 one worked perfectly well. There was a bit of accounting at the end I missed out to show where the aircraft's contribution came in, but that's pretty much book-keeping, not physics.

Incidentally if you really want to wrap your brain around this stuff I recommend two books. The first is the first volume of Feynman's lectures. The second is a book (any book) on the principle of least action, and hamiltonians. The first is readable but tough, the second is boggling.




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#260 GSpeedR

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Posted 10 May 2012 - 12:26

I know how it feels. Yes it does all end up as heat ultimately. But as I said to Young Manny - that is not what I was writing about primarily - it was that people were using the wrong approach to the question.


Total energy and momentum are conserved within a system, and both can be used to solve classical dynamic problems (and relativistic or quantum problems with a few corrections). Momentum and energy are related quantities (see the formula for total energy in special relativity).


#261 Kelpiecross

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Posted 10 May 2012 - 13:11

Kinetic energy never did make sense to me either. Why should it be squared with speed, from intuition POV? Absent friction, why should it take more energy to speed up the object by 1 mph when it's traveling at 30 mph as opposed to when it's traveling at 20 mph? I've seen mathematical proof, so I know that it's true within the boundaries of classical mechanics, but that doesn't help me understand intuitively why it's true.


Very good question. Any equally good answers of the "non-mathematical/well that makes sense" variety?

#262 gruntguru

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Posted 10 May 2012 - 23:24

It is because more distance is covered during the acceleration process at the higher speed. So accelerating from 30 to 31 mph in one second requires the same force and time (impulse) as accelerating from 20 to 21 mph in one second, but the force must be exerted over a 50% (approx) greater distance, so the work (and therefore delta KE) will be 50% greater.

Edited by gruntguru, 10 May 2012 - 23:24.


#263 Kelpiecross

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Posted 11 May 2012 - 03:16

It is because more distance is covered during the acceleration process at the higher speed. So accelerating from 30 to 31 mph in one second requires the same force and time (impulse) as accelerating from 20 to 21 mph in one second, but the force must be exerted over a 50% (approx) greater distance, so the work (and therefore delta KE) will be 50% greater.


Which makes sense - but does this mean that (to use an example) a rider on a pushbike (no mechanical losses, no air resistance etc.) actually has to supply more energy from his/her/its muscles to accelerate from 30 to 31 than 20 to 21 mph?

I would think the logical answer would have be "yes" - the greater amount of KE has to come from somewhere. Is this correct - is it actually physically more difficult to accelerate at the higher speed?

#264 bigleagueslider

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Posted 11 May 2012 - 03:55

This discussion reminds me of the old joke- "It's not the fall from a great height that kills you. It's the sudden stop at the end."

#265 gruntguru

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Posted 11 May 2012 - 11:29

Which makes sense - but does this mean that (to use an example) a rider on a pushbike (no mechanical losses, no air resistance etc.) actually has to supply more energy from his/her/its muscles to accelerate from 30 to 31 than 20 to 21 mph?

I would think the logical answer would have be "yes" - the greater amount of KE has to come from somewhere. Is this correct - is it actually physically more difficult to accelerate at the higher speed?

If you have ever tried it you must know that it is.

If you do the experiment in any geared vehicle and select a gear that gives the same rpm at both road-speeds, the higher speed requires a higher gear with less torque multiplication, so to achieve the same acceleration the engine will need to produce more torque, more effort - yes it is more difficult.

#266 Magoo

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Posted 11 May 2012 - 11:55

We could put the units in a more human scale and note that 21 mph is 31 feet every second and 31 mph is 46 feet every second. Does that help?

Why the increase in kinetic energy with velocity is quadratic (sq to the v) rather than linear is lots more simple than it may look -- and will become more intuitive when, instead of visualizing a body in motion, we imagine we are standing in front of it trying to stop it. The mechanics are the same, merely reversed.

So let's say we have a runaway truck A with a velocity v, and a second runaway truck B, identical except that its velocity is 2v.

Thus truck B has twice the quantitus motus ("quantity of motion," Newton called it) aka momentum. Truck B travels twice as far as truck A in the identical time, or an identical distance in half the time.

If a constant and equal force F is applied to either truck to slow it and bring it to a stop, the time required to stop truck B will be twice as great. And since truck B is going twice the velocity over twice the period of time, it's going to cover four times the distance in the process of getting stopped. There is no more to it than that.

All of which is only another way of noting that the work required to stop Truck A or Truck B (or to make them go -- same difference) is the dot product of force and distance, which for Truck B is four times as great as Truck A.




#267 Dmitriy_Guller

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Posted 11 May 2012 - 13:53

It is because more distance is covered during the acceleration process at the higher speed. So accelerating from 30 to 31 mph in one second requires the same force and time (impulse) as accelerating from 20 to 21 mph in one second, but the force must be exerted over a 50% (approx) greater distance, so the work (and therefore delta KE) will be 50% greater.

I thought of that, but that's really just a way of stating the W = F * d equation in English. You're going to be applying the same force for a longer distance at higher speed, but you will still be applying it for the same amount of time, because the object you're pushing is going faster. I guess the fundamental lack of intuitive understanding comes from seek why work has to be a distance multiple of force being applied, and not time multiple of force being applied. Maybe fundamentally, it's not the KE = .5 * m * v^2 that I'm having trouble with, but rather W = F * d.

#268 saudoso

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Posted 11 May 2012 - 14:47

For me looking at units used to sort these messes out.

Like this:

1 J - 1kg (m/s)^2

Force X distance: (1kg*m/s^2)*(m) = 1kg (m/s)^2
So I'd look at it and think: Well, I can't find a way to throw time there and get proper units, so time is a non issue.

Force X speed: (1kg*m/s^2)*(m/s) = 1kg*(m^2*s^3) = (1kg (m/s)^2)/s
Now you see power is the rate of change in energy, what's true indeed.



#269 Dmitriy_Guller

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Posted 11 May 2012 - 17:21

For me looking at units used to sort these messes out.

Like this:

1 J - 1kg (m/s)^2

Force X distance: (1kg*m/s^2)*(m) = 1kg (m/s)^2
So I'd look at it and think: Well, I can't find a way to throw time there and get proper units, so time is a non issue.

Force X speed: (1kg*m/s^2)*(m/s) = 1kg*(m^2*s^3) = (1kg (m/s)^2)/s
Now you see power is the rate of change in energy, what's true indeed.

That's mathematics, mathematics I can easily grasp, I have a degree in that. Why work is defined to be force times distance (in a very simplified case) is not something that you can answer with unit algebra.

#270 Magoo

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Posted 11 May 2012 - 17:46

Intuition might be overrated. As Locock shrewdly alluded earlier, "intuition" is another word for monkey brain.

One can easily prove experimentally that W = f * d with a kitchen floor, a toy car, and a couple of rubber bands. Armed with an hour or two of this enriching experience, one can then rejoin the world with a new and improved power of intuition, better equipped for the next scientific inquiry.

#271 saudoso

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Posted 11 May 2012 - 19:30

I've just writen a huge BS on the rally car thread based on intuition.

And yes, what you call unit algebra can answer a lot of questions. It even has a fancy name: Dimentional Analysis.

#272 Dmitriy_Guller

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Posted 11 May 2012 - 23:28

Intuition might be overrated. As Locock shrewdly alluded earlier, "intuition" is another word for monkey brain.

I think it's vastly under-rated. In my line of work, which admittedly is not related to engineering or physics at all, going with the calculations without a doing a whole bunch of sanity checks on them is professional malpractice. It's also a very good skill to be able to approximate what answer running the numbers will give you before you actually run them. It's a lot easier to do that if you have an idea of how things fit together that is independent of the fancy mathematics.

#273 gruntguru

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Posted 12 May 2012 - 06:52

I thought of that, but that's really just a way of stating the W = F * d equation in English. You're going to be applying the same force for a longer distance at higher speed, but you will still be applying it for the same amount of time, because the object you're pushing is going faster. I guess the fundamental lack of intuitive understanding comes from seek why work has to be a distance multiple of force being applied, and not time multiple of force being applied. Maybe fundamentally, it's not the KE = .5 * m * v^2 that I'm having trouble with, but rather W = F * d.

Try this. F x t feels quite easy if the force is just your body weight leaning on a wall, applying say 10 kg (22 lb) - you could probably sustain this for an hour if you had to. Now try it with the wall moving away at 5 m/s. Now you are doing work on the wall (500 W) and you will become exhausted much sooner doing this than you would if the wall was stationary, or even moving at say 2.5 m/s.

#274 Kelpiecross

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Posted 12 May 2012 - 11:48

We could put the units in a more human scale and note that 21 mph is 31 feet every second and 31 mph is 46 feet every second. Does that help?

Why the increase in kinetic energy with velocity is quadratic (sq to the v) rather than linear is lots more simple than it may look -- and will become more intuitive when, instead of visualizing a body in motion, we imagine we are standing in front of it trying to stop it. The mechanics are the same, merely reversed.

So let's say we have a runaway truck A with a velocity v, and a second runaway truck B, identical except that its velocity is 2v.

Thus truck B has twice the quantitus motus ("quantity of motion," Newton called it) aka momentum. Truck B travels twice as far as truck A in the identical time, or an identical distance in half the time.

If a constant and equal force F is applied to either truck to slow it and bring it to a stop, the time required to stop truck B will be twice as great. And since truck B is going twice the velocity over twice the period of time, it's going to cover four times the distance in the process of getting stopped. There is no more to it than that.

All of which is only another way of noting that the work required to stop Truck A or Truck B (or to make them go -- same difference) is the dot product of force and distance, which for Truck B is four times as great as Truck A.


Thank you and GG for your explanations. Sadly I have to confess I never realised that it became harder to accelerate as you went faster. I think that I imagined that at any speed if you applied a bit of "F" to your "m" then away you went at "a" - it was not dependent on your speed. (and I suppose, if you use the right frame of reference this is actually true).

So to accelerate/change speed etc. you need to supply energy for the change in KE. The resistance to change the state of motion (or accelerate) is inertia/momentum - thus inertia/momentum requires energy/KE to change it.

As I tend to rely very much on "monkey intuition" (or should that be "canine intuition") I like to always try and relate the world I see around me to these basic properties of physics. For example the "friction toy" - (or a wheeled toy that stores its energy for motion in a flywheel). You push the toy along - it stores internal energy in its flywheel - it then exhibits a an apparent momentum that far exceeds its actual mass and speed. Can this be related somehow to the related properties of momentum and KE? Could the internal rotation of particles be the storage for KE that is then manifested as momentum.

Where am I? What day is it? Who are you? Where's my dinner?










#275 Kelpiecross

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Posted 12 May 2012 - 11:52

I think it's vastly under-rated. In my line of work, which admittedly is not related to engineering or physics at all, going with the calculations without a doing a whole bunch of sanity checks on them is professional malpractice. It's also a very good skill to be able to approximate what answer running the numbers will give you before you actually run them. It's a lot easier to do that if you have an idea of how things fit together that is independent of the fancy mathematics.


I tend to agree. I presume that most (if not all) all of the established and proven theories and equations etc. originated as "intuition". Truly new ideas and concepts rely on "intuition".

#276 GSpeedR

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Posted 12 May 2012 - 15:13

Thank you and GG for your explanations. Sadly I have to confess I never realised that it became harder to accelerate as you went faster. I think that I imagined that at any speed if you applied a bit of "F" to your "m" then away you went at "a" - it was not dependent on your speed. (and I suppose, if you use the right frame of reference this is actually true).


F = m*a still applies. The difference is the work/energy required to apply that F is dependent upon speed. Newton's 2nd law is actually F = d(mv)/dt. Force equals the time derivative of momentum (m*v), which for a constant mass, m, turns into F=m*a. Once you see that all the relations between force, motion, momentum and energy then I think things will become more intuitive. Here's another relation that I use to convince myself that KE = 1/2*m*v^2:

F = m*a = m*(dv/dt) = m*(dx/dt*dv/dx) = m*(v*dv/dx) [This is called the Chain Rule in calculus, so now we have acceleration described in terms of distance rather than time]
Work = integral(F*dx) [Work is Force times distance, though more correctly, it is the summation of Force times (dot-product) infinitesimally small distances (dx)]
Work = integral(m*(v*dv/dx)*dx) [Now the dx's will cancel and we are left with an integral wrt dv]
Work = m*integral(v*dv) [now simply solve the integral]
Work = 1/2*m*v^2

So this tells me that kinetic energy can be thought of as a moving body's 'ability to do work'.

IMO for all great physicists, the mathematics come first. There's a phenomenon that they cannot explain with intuition and they use math to describe it. The great ones are then able to break the complicated math down in a way that can be understood by those you haven't or can't perform the math. Without math, there would be no intuitive reason for the Universe to not rotate around the Earth.

As I tend to rely very much on "monkey intuition" (or should that be "canine intuition") I like to always try and relate the world I see around me to these basic properties of physics. For example the "friction toy" - (or a wheeled toy that stores its energy for motion in a flywheel). You push the toy along - it stores internal energy in its flywheel - it then exhibits a an apparent momentum that far exceeds its actual mass and speed. Can this be related somehow to the related properties of momentum and KE? Could the internal rotation of particles be the storage for KE that is then manifested as momentum.


If I'm thinking of this toy correctly it looks like a vacuum , but you can think of mass/inertia as a kinetic energy storing element. As this toy is pushed along the flywheel is accelerated (storing KE) and the toy is probably difficult to push (requires more work than a similar toy with no flywheel). As the flywheel decelerates, it is releasing kinetic energy back into the toy and the toy continues in motion even without an externally applied force (until friction stops it). If I'm thinking of the wrong toy, then please disregard my post. :D

#277 Dmitriy_Guller

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Posted 12 May 2012 - 18:54

Try this. F x t feels quite easy if the force is just your body weight leaning on a wall, applying say 10 kg (22 lb) - you could probably sustain this for an hour if you had to. Now try it with the wall moving away at 5 m/s. Now you are doing work on the wall (500 W) and you will become exhausted much sooner doing this than you would if the wall was stationary, or even moving at say 2.5 m/s.

There is a limit to intuition based on work done by your body. Your body is not a simple physical object, it does a whole bunch of internal work just to be in the same state. For example, assuming no friction, it doesn't take any physical work to keep moving at 5 m/s without pushing anything. However, my legs would be accelerating and decelerating constantly to actually sustain that pace.

#278 Tony Matthews

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Posted 12 May 2012 - 19:36

It takes 200 muscles in constant fidgit just to stand up and not collapse. Well, it does in my body...

#279 Magoo

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Posted 15 May 2012 - 10:36

I think it's vastly under-rated. In my line of work, which admittedly is not related to engineering or physics at all, going with the calculations without a doing a whole bunch of sanity checks on them is professional malpractice. It's also a very good skill to be able to approximate what answer running the numbers will give you before you actually run them. It's a lot easier to do that if you have an idea of how things fit together that is independent of the fancy mathematics.


That's not inuition. By definition, intuition excludes not only mathematics but induction, deduction, formal and informal logic. Essentially, intuition is ideation without reasoning. Intuition is nothing more than a naked hunch.

Intuition is a perfectly fine place to initiate a decision-making process, but a horrible place to end one. If one's intuition does not accept common mechanical principles like kinetic energy, then obviously the problem is with the intuition, not with the mechanical principles.



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#280 Magoo

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Posted 15 May 2012 - 10:45

The whole point about inertial reference frames is intersting. The reason, intuitively, you know that school physics experiments with billiard balls work when applied to aircraft is because the surface of the earth is moving rather fast compared to the centre of the earth, which is moving quickly relative to the sun, which is moving very fast relative tot he rest of the universe.

If you couldn't ignore these enormous speeds, billiards would be impossible for our monkey brains.

Incidentally this leads to Greg's depressing thought for the day. KE is not an absolute, usually. It depends what your frame of reference is. Somebody might like to work on the usual collision example attaching the ref frame to one of the bodies, or the ground. It should still work out.


You might enjoy this:

http://plato.stanfor...cetime-iframes/

#281 Dmitriy_Guller

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Posted 16 May 2012 - 16:04

That's not inuition. By definition, intuition excludes not only mathematics but induction, deduction, formal and informal logic. Essentially, intuition is ideation without reasoning. Intuition is nothing more than a naked hunch.

Intuition is a perfectly fine place to initiate a decision-making process, but a horrible place to end one. If one's intuition does not accept common mechanical principles like kinetic energy, then obviously the problem is with the intuition, not with the mechanical principles.

Good job refuting an argument that no one made.

#282 Magoo

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Posted 17 May 2012 - 00:49

Good job refuting an argument that no one made.


I'm only saying that if your intuition leads you to believe that the increase in kinetic energy with velocity is linear, then your intuition is not very good. Now, it's hardly a sin. Lots of people have no mechanical aptitude.

#283 Dmitriy_Guller

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Posted 17 May 2012 - 02:55

I'm only saying that if your intuition leads you to believe that the increase in kinetic energy with velocity is linear, then your intuition is not very good. Now, it's hardly a sin. Lots of people have no mechanical aptitude.

You do realize that of the two of us, you're the only one on record actually making that mistake, right? You should probably be careful when making veiled patronizing statements like that.

#284 desmo

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Posted 17 May 2012 - 03:11

Ad hominem arguments are really not welcome here. If that is your style please find another forum.

#285 Kelpiecross

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Posted 17 May 2012 - 03:38

I'm only saying that if your intuition leads you to believe that the increase in kinetic energy with velocity is linear, then your intuition is not very good. Now, it's hardly a sin. Lots of people have no mechanical aptitude.


I don't want to get in the middle of a fight here but I think that the increase in KE with the square of the velocity is distinctly non-intuitive (and, yes, I do know that it is not linear).

If you had no knowledge of the basic equations of physics and didn't habitually brake your car from very high speed - what in real-life experience would lead you intuitively to think KE was not linear in the same way as momentum. Or in indeed, to think that KE and momentum were different phenomena?

I had to Google "ad hominem".

#286 Dmitriy_Guller

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Posted 17 May 2012 - 06:23

Speaking of this topic, just this Sunday I watched the new Mythbusters episode. Adam and Jaimie were firing guns into the pavement at an angle, and noting the speed of the bullet before the ricochet and after the ricochet. In his comments, Adam several times equated the percentage loss of bullet speed with the percentage loss of the energy of the bullet. While neither Adam or Jaimie are physicists or engineers, they're not the people that I would say are lacking mechanical aptitude, and yet their intuition betrayed them.

#287 Magoo

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Posted 17 May 2012 - 11:25

If you had no knowledge of the basic equations of physics and didn't habitually brake your car from very high speed - what in real-life experience would lead you intuitively to think KE was not linear in the same way as momentum. Or in indeed, to think that KE and momentum were different phenomena?


I'm only noting that intuition is overrated. It's extremely limited. If intuition is in conflict with basic physics, the problem is with the intuition, not with the physics, obviously. Intuition is perfectly fine right up to the point where it interferes with curiosity and objectivity. Why limit oneself in that manner? With a toy car and some rubber bands you can experiment for yourself. As a result you gain a new and improved sense of intuition.

#288 Kelpiecross

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Posted 17 May 2012 - 12:02

Speaking of this topic, just this Sunday I watched the new Mythbusters episode. Adam and Jaimie were firing guns into the pavement at an angle, and noting the speed of the bullet before the ricochet and after the ricochet. In his comments, Adam several times equated the percentage loss of bullet speed with the percentage loss of the energy of the bullet. While neither Adam or Jaimie are physicists or engineers, they're not the people that I would say are lacking mechanical aptitude, and yet their intuition betrayed them.


I also seem to recall on Mythbusters that they were firing various types of gun at a kids' park roundabout (merry-go-round) in an attempt to make it spin up. If I remember correctly (and I may not because I didn't keep a recording of the show) they were equating each bullet's KE with its ability to move the roundabout - when they should have been using the bullet's momentum. I think they have confused KE and momentum on other occasions.

I agree with you - you could hardly accuse them of lacking in mechanical talent etc. - some of their apparatus making and scientific conclusions are very sharp.

And Kari is one of the best-looking sheilas on telly.

#289 munks

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Posted 17 May 2012 - 14:20

... didn't habitually brake your car from very high speed ...


And even then, I don't think it's intuitive. Eventually you'll start to feel the effects through brake fade, but it doesn't intuitively tell you that a deceleration of X from high speed is different than the same deceleration at low speed. Said another way: other than the temperature effects on the brake friction and/or boiling up some bubbles in your brake lines, the deceleration is pretty much proportional to your foot's pressure on the brake pedal.

#290 Magoo

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Posted 17 May 2012 - 15:06

And even then, I don't think it's intuitive. Eventually you'll start to feel the effects through brake fade, but it doesn't intuitively tell you that a deceleration of X from high speed is different than the same deceleration at low speed. Said another way: other than the temperature effects on the brake friction and/or boiling up some bubbles in your brake lines, the deceleration is pretty much proportional to your foot's pressure on the brake pedal.


Showing once again that intuition is overrated.

For example, here intuition apparently has indicated to someone, somewhere along the line, that operating the brakes on a motor vehicle should be a useful way to understand velocity, momentum, and kinetic energy. I think it's a bad way. Experientially it's all wrong. You aren't stopping the vehicle, the brakes are. You are merely operating the pedal. Your sense of the experience will depend mainly on things like pedal ratio and master cylinder piston diameter.

Surely we can come up with a better way for humans to experience and understand momentum and KE. However, we won't be able to intuit one. Intuition, by definition, is ideation without reasoning. To come up with an effective demonstration, we should probably apply some conscious thought to the process, hmm.

#291 Dmitriy_Guller

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Posted 17 May 2012 - 16:42

How about we summarize the logical conclusion: no, we should not use intuition to get answers, we should use analytical thinking, but it's a lot easier to avoid mistakes in analytical thinking when it doesn't run counter to our intuition. One of the basic and most common mistakes one can make with analytical thinking is failing to employ it altogether because one doesn't realize that the situation is not as simple as it appears.

#292 Kelpiecross

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Posted 18 May 2012 - 05:18

And even then, I don't think it's intuitive. Eventually you'll start to feel the effects through brake fade, but it doesn't intuitively tell you that a deceleration of X from high speed is different than the same deceleration at low speed. Said another way: other than the temperature effects on the brake friction and/or boiling up some bubbles in your brake lines, the deceleration is pretty much proportional to your foot's pressure on the brake pedal.


I agree - but I couldn't think of any other way that KE manifests itself.

A bit of quick researching on Google reveals that KE was not discovered intuitively - its existence was implied from mathematical analysis of Newton's equations.

On the other hand I suspect the general property of inertia/momentum was probably well known even before Newton. For example - it is fairly obvious when trying to move a boat weighing several tons - hard to get it moving then it wants to keep moving - equally hard to stop.

I also see on Google that the definition of "intuitive" is not quite what I thought. I thought it meant more like "logical deduction with a bit of inspiration" or "lateral thinking" - instead its correct meaning is more "jumping to conclusion without consciously thinking".

#293 Dmitriy_Guller

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Posted 18 May 2012 - 19:04

Hmm, interesting. It's not what I thought either. It may explain some of the miscommunication in this philosophical discussion.

#294 saudoso

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Posted 18 May 2012 - 19:34

Of everything I've learned in engineering school I guess the concept that I took longer to accept was the contant pressure burn in gas turbines. Everytime I'd think hard of it I'd start unlearning the stuff. If the pressures are even why in hell the thing does not backfire.

Edited by saudoso, 18 May 2012 - 19:34.


#295 Magoo

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Posted 28 May 2012 - 12:08

No wonder the general population is confused.


I've spent the last several months in the marketing and advertising biz, and I've found the experience eye-opening to say the least. First, imagine three stories of offices filled with people who make their living on words, and there isn't a single dictionary in the entire building.

It's not that these people are ignorant or apathetic about the language -- or about art, culture, or science for that matter.

It's their open, mocking contempt for all forms of human knowledge. They truly don't care anything about anything, except to whatever extent a particular idea can be borrowed, co-opted, twisted, and perverted to suit their ends.

Words and ideas have no meaning or purpose or value beyond their power to sell crap.

It's a totally nihilistic, valueless world, and they don't even realize it. They think this is normal.

Bill Hicks said it all:



#296 desmo

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Posted 28 May 2012 - 13:30

It's applied psychology without any redeeming academic values. How else does one apply psychology profitably?

#297 Canuck

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Posted 28 May 2012 - 19:57

Why must everything be profit-centric? The vast and staggeringly enormous sums of money being spent on sophisticated neurological studies by marketing firms terrifies the crap out of me. Magoo said it exactly right

Words and ideas have no meaning or purpose or value beyond their power to sell crap.

It's a totally nihilistic, valueless world, and they don't even realize it. They think this is normal.


Not only is it without values, it's without value.

#298 Grumbles

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Posted 28 May 2012 - 21:45

I've spent the last several months in the marketing and advertising biz, and I've found the experience eye-opening to say the least. First, imagine three stories of offices filled with people who make their living on words, and there isn't a single dictionary in the entire building.

It's not that these people are ignorant or apathetic about the language -- or about art, culture, or science for that matter.

It's their open, mocking contempt for all forms of human knowledge. They truly don't care anything about anything, except to whatever extent a particular idea can be borrowed, co-opted, twisted, and perverted to suit their ends.

Words and ideas have no meaning or purpose or value beyond their power to sell crap.

It's a totally nihilistic, valueless world, and they don't even realize it. They think this is normal.


As far as they are concerned people are nothing but gullets that live only to gulp products and crap cash. And as you say they think this is normal. That's bad enough, but what I find just as bad is how many of us now accept the role of consumer without protest. And what I find really repulsive is the focus on young kids, and how effectively it works on them. Many of the current generation will live their entire life as a consumer, not knowing or caring that they are being abused.


#299 Canuck

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Posted 29 May 2012 - 03:48

This is why I don't have television, and my kids media / screen exposure is both highly controlled and informed. Now they'll watch some whiz-bang commercial at the grandparent's house, look over at me and say "they're just trying to get us to give them our money for garbage aren't they dad". Make me proud or what.

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#300 johnny yuma

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Posted 29 May 2012 - 04:18

[quote name='Magoo' date='May 28 2012, 12:08' post='5746477']
I've spent the last several months in the marketing and advertising biz, and I've found the experience eye-opening to say the least. First, imagine three stories of offices filled with people who make their living on words, and there isn't a single dictionary in the entire building.

It's not that these people are ignorant or apathetic about the language -- or about art, culture, or science for that matter.

It's their open, mocking contempt for all forms of human knowledge. They truly don't care anything about anything, except to whatever extent a particular idea can be borrowed, co-opted, twisted, and perverted to suit their ends.

Words and ideas have no meaning or purpose or value beyond their power to sell crap.

It's a totally nihilistic, valueless world, and they don't even realize it. They think this is normal.
-------------------------------------------------------------


There is another last refuge for scoundrels...the Law.Jimbo Jones comment to Homer Simpson after his vigilante group failed
"You let me down,man.I don't believe in nothing no more.I'm going to Law School !"

Edited by johnny yuma, 29 May 2012 - 04:19.