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Author Topic: Understanding kinetic energy  (Read 7038 times)

aleks

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Understanding kinetic energy
« on: January 31, 2008, 08:06:21 PM »
I have a question I have never found answer to.

You all know what a kinetic energy of a physical body is: it depends on the mass and body's velocity vector.

However, what makes me wonder is HOW the kinetic energy is stored in body if we think of the body as a body consisting of particles that have 'state'. So, each particle's average kinetic (and vectorial) energy will depend on the body's energy divided by the number of particles. Hopefully this is correct and reasonable.

Now, if I give this body a spin (this will require a throw of a bit of mass in direction perpendicular to the velocity vector), particles within this body will start to rotate together with the body.

But why then average energy of these particles will still be pointing towards the original direction (when the body was not spinning)? Does this mean that energy of the body IS NOT stored in the particles as their state variable? Otherwise we could easily put any body into zig-zag motion without even throwing away mass from the body.

Something is definitely absent in modern physics.

supersam

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Re: Understanding kinetic energy
« Reply #1 on: January 31, 2008, 09:17:31 PM »
aleks,

you might want tto look at the physics of a curve ball.  i keep tring to understand them for i believe like p-motion, that this might be a clue.  i just can't seem to get out of my three dimentional frame of thinking.  but, is it neccessary?


lol
sam

aleks

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Re: Understanding kinetic energy
« Reply #2 on: January 31, 2008, 09:46:06 PM »
aleks,

you might want tto look at the physics of a curve ball.  i keep tring to understand them for i believe like p-motion, that this might be a clue.  i just can't seem to get out of my three dimentional frame of thinking.  but, is it neccessary?


lol
sam
Thanks for the tip. But what kind of physics is that? I guess it is related to non-linear hydro/aerodynamics? Then fun is here: http://www.grc.nasa.gov/WWW/K-12/airplane/foil2b.html

But this does not give me answers about HOW body's kinetic energy retains its direction while its particles are rotating. The paradox is, if you take a single particle, its velocity vector spins, but if you take a body, single particle's velocity vector stays unchanged.

The idea I'm currently having about this is that kinetic energy can be seen as an area of aether's compression that surrounds body. So, when you are giving a kick to the body you are compressing aether: the stronger you kick, the higher the compression of aether is.

If aether is considered, a paradox I see can be resolved while body's spinning motion contributes to a more complex aether compression setup building up around the body.

fletcher

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Re: Understanding kinetic energy
« Reply #3 on: January 31, 2008, 10:36:14 PM »
A body in motion has Total Kinetic Energy - it is made up of two components - Rotational KE & Translational KE - they sum to Total KE - this means that whatever the mix [spinning or not] & relative velocity of the body, the body's Momentum is the same - Momentum is expressed as a vector quantity unit [magnitude & direction] but is really a measure of a body's scalar Total KE.

Have a hunt around the index for Energy in the web site below

http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html

aleks

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Re: Understanding kinetic energy
« Reply #4 on: January 31, 2008, 11:07:32 PM »
A body in motion has Total Kinetic Energy - it is made up of two components - Rotational KE & Translational KE - they sum to Total KE - this means that whatever the mix [spinning or not] & relative velocity of the body, the body's Momentum is the same - Momentum is expressed as a vector quantity unit [magnitude & direction] but is really a measure of a body's scalar Total KE.

Have a hunt around the index for Energy in the web site below

http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html
OK, but if we go down to atoms, how this translates to atoms?

pequaide

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Re: Understanding kinetic energy
« Reply #5 on: February 02, 2008, 09:05:52 PM »
aleks: suppose you are sitting on a portion of the Earths surface that is moving 360 m/sec to the east. You have a cannon that can shoot a 1 kg projectile 360 m/sec. If you fire the cannon east the velocity change is from 360 m/sec to 720 m/sec for a momentum change of 360 units of momentum. If you fire the cannon west the velocity change is from 360 m/sec to 0 m/sec for a momentum change of 360 units of momentum. So you can shot deer to the east or to the west, there is no difference in the ballistics of the shell.

If you fire the cannon east the velocity change is from 360 m/sec to 720 m/sec for an energy change of 194,400 joules. This is 259,200 ? 64,800 joules; ? mv?

If you fire the cannon west the velocity change is from 360 m/sec to 0 m/sec for an energy change of 64,800 joules.

So does this mean that you should place your deer blind facing west or does it mean that the Law of Conservation of Energy is false? How can the same powder cause two different changes in energy?

This year our family shot one deer to the east, one to the north, and two to the southeast from our blinds. We made no ballistic corrections for the Law of Conservation of Energy. I guess we shot our deer with momentum.   

Yes; I know there are other planetary motions, but the point is the same.  You should be able to detect a difference if there is one.

aleks

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Re: Understanding kinetic energy
« Reply #6 on: February 03, 2008, 11:04:57 AM »
Yes; I know there are other planetary motions, but the point is the same.  You should be able to detect a difference if there is one.
That's another paradox with kinetic energy I suppose. 1/2*mv^2 does not cover impulse acceleration: when you are accelerating via small bursts, each burst increases speed of an object (relative to some base) linearly and energy spending is linear as well. On the other hand this is not a violation of laws of energy conservation since the law is built around this 1/2mv^2 (meaning you can multiply anything by another anything, and divide it, and build a system of equations around it, etc). It is only a violation of common sense: while 1/2mv^2 might be working well in closed system of equations, it violates a common sense of understanding of energy spending: if you are doing equal bursts (be it small explosions, ion streams or anything else), each such burst will increase speed of an object by equal value. While e=1/2mv^2 suggests each burst will be adding a decreasing value to velocity (do not forget that each burst takes an equal amount of energy).

But this is not related to my original question.
« Last Edit: February 03, 2008, 05:26:27 PM by aleks »