| Topic: |
Science > Physics |
| User: |
"Brablo" |
| Date: |
11 Sep 2006 03:59:01 PM |
| Object: |
Why does it require energy for levitation? |
I was thinking that it should not take energy for a 1 kg. book to
float, given all the energy-related equations out there, and once the
book is already lifted off the ground. According to these equations,
no work is being done to levitate/support a book, since work is F*D*COS
THETA. THETA is 90 degrees, and there is no kinetic energy. I realize
that it takes energy to position the book off of the ground, and this
energy is MgY, where Y is the height off of the earth. However,
suppose that the object is now levitated in that spot. How much energy
would this take to levitate the object?
I realize that this very elementary question may seem silly to you, but
I'm being deadly serious.
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| User: "CWatters" |
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| Title: Re: Why does it require energy for levitation? |
12 Sep 2006 02:56:53 PM |
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"Brablo" <gestureofrespect@yahoo.com> wrote in message
news:1158008341.787564.216960@i42g2000cwa.googlegroups.com...
I was thinking that it should not take energy for a 1 kg. book to
float, given all the energy-related equations out there, and once the
book is already lifted off the ground. According to these equations,
no work is being done to levitate/support a book, since work is F*D*COS
THETA.
That's correct no work is done on the book. That does not mean no work needs
to be done elsewhere in the system.
For an object to remain stationary all the forces acting on it must sum to
zero. So in the vertical direction you have to provide a force to counter
the effect of gravity. Human muscles and or helicopters are inefficient -
they require energy to produce a force even without movement. This waste
energy is typically released as heat. A pit prop is more efficient.
For info: In the hover the efficiency of a helicopter is ZERO because it's
burning power but not moving.
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| User: "Sam Wormley" |
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| Title: Re: Why does it require energy for levitation? |
11 Sep 2006 04:05:55 PM |
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Brablo wrote:
I was thinking that it should not take energy for a 1 kg. book to
float, given all the energy-related equations out there, and once the
book is already lifted off the ground. According to these equations,
no work is being done to levitate/support a book, since work is F*D*COS
THETA. THETA is 90 degrees, and there is no kinetic energy. I realize
that it takes energy to position the book off of the ground, and this
energy is MgY, where Y is the height off of the earth. However,
suppose that the object is now levitated in that spot. How much energy
would this take to levitate the object?
I realize that this very elementary question may seem silly to you, but
I'm being deadly serious.
All that's required (to first order) is counter the weak force
of gravity.
For a person hovering without motion, no work is done.
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| User: "" |
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| Title: Re: Why does it require energy for levitation? |
11 Sep 2006 04:35:59 PM |
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Brablo wrote:
I was thinking that it should not take energy for a 1 kg. book to
float, given all the energy-related equations out there, and once the
book is already lifted off the ground. According to these equations,
no work is being done to levitate/support a book, since work is F*D*COS
THETA. THETA is 90 degrees, and there is no kinetic energy. I realize
that it takes energy to position the book off of the ground, and this
energy is MgY, where Y is the height off of the earth. However,
suppose that the object is now levitated in that spot. How much energy
would this take to levitate the object?
I realize that this very elementary question may seem silly to you, but
I'm being deadly serious.
I think that without knowing what mechanism you are proposing for
"levitation" it's kind of hard to say. For example, to "levitate" a
helicopter takes a constant supply of energy, whereas to "levitate" a
book by placing it on a table takes no energy. If you have a proposal
for some novel method of "levitation", then it either may or may not
require energy, depending on how it's supposed to work.
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| User: "tadchem" |
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| Title: Re: Why does it require energy for levitation? |
12 Sep 2006 04:42:10 PM |
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Brablo wrote:
I was thinking that it should not take energy for a 1 kg. book to
float, given all the energy-related equations out there, and once the
book is already lifted off the ground. According to these equations,
no work is being done to levitate/support a book, since work is F*D*COS
THETA. THETA is 90 degrees, and there is no kinetic energy. I realize
that it takes energy to position the book off of the ground, and this
energy is MgY, where Y is the height off of the earth. However,
suppose that the object is now levitated in that spot. How much energy
would this take to levitate the object?
I realize that this very elementary question may seem silly to you, but
I'm being deadly serious.
There are many kinds of levitation (in the strictest sense - no 'books
on tables' sort of thing here):
http://en.wikipedia.org/wiki/Magnetic_levitation
These two require only the energy necessary to set up the arrangement.
Superconducting levitation requires 'magnetic mirrors' made of
superconductors that can generate and oppsing magnetic field to a
magnet without electrical loss due to resistance (currently this
requires cryogenic conditions):
http://www.chem.ryukoku.ac.jp/~wada/HomePage/English/Topics(english).html
Magnetic levitation - gyroscopically stabilized. In principle, if this
configuration is established in a vacuum, it can be maintained
indefinitely - the only energy loss is drag from air on the spinning
top:
http://www.geocities.com/dsligar.geo/levitron.html
Other forms of magnetic levitation (maglev trains) require a continuous
supply of electricity to maintain the magnetic fields.
There is also electrostatic levitation, but Earnshaw's Theorem requires
dynamic electric fields, which use energy:
http://en.wikipedia.org/wiki/Electrostatic_levitation
Tom Davidson
Richmond, VA
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| User: "PD" |
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| Title: Re: Why does it require energy for levitation? |
12 Sep 2006 06:28:00 AM |
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Brablo wrote:
I was thinking that it should not take energy for a 1 kg. book to
float, given all the energy-related equations out there, and once the
book is already lifted off the ground. According to these equations,
no work is being done to levitate/support a book, since work is F*D*COS
THETA. THETA is 90 degrees, and there is no kinetic energy. I realize
that it takes energy to position the book off of the ground, and this
energy is MgY, where Y is the height off of the earth. However,
suppose that the object is now levitated in that spot. How much energy
would this take to levitate the object?
None at all. The problem is keeping the Earth from doing work on it to
accelerate it to the ground. Unless there is something in place to
prevent that, the Earth will be more than happy to supply energy to
make it fall downwards.
Stick a table under the book, though, and the table does zero work to
keep the book from falling.
PD
I realize that this very elementary question may seem silly to you, but
I'm being deadly serious.
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| User: "" |
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| Title: Re: Why does it require energy for levitation? |
11 Sep 2006 04:13:11 PM |
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In article <1158008341.787564.216960@i42g2000cwa.googlegroups.com>, "Brablo" <gestureofrespect@yahoo.com> writes:
I was thinking that it should not take energy for a 1 kg. book to
float, given all the energy-related equations out there, and once the
book is already lifted off the ground. According to these equations,
no work is being done to levitate/support a book, since work is F*D*COS
THETA. THETA is 90 degrees, and there is no kinetic energy. I realize
that it takes energy to position the book off of the ground, and this
energy is MgY, where Y is the height off of the earth. However,
suppose that the object is now levitated in that spot. How much energy
would this take to levitate the object?
When the book is on its shelf, how much energy you think the bookshelf
is using to support it?
Mati Meron | "When you argue with a fool,
meron@cars.uchicago.edu | chances are he is doing just the same"
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| User: "Jan Panteltje" |
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| Title: Re: Why does it require energy for levitation? |
11 Sep 2006 04:37:54 PM |
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On a sunny day (11 Sep 2006 13:59:01 -0700) it happened "Brablo"
<gestureofrespect@yahoo.com> wrote in
<1158008341.787564.216960@i42g2000cwa.googlegroups.com>:
I was thinking that it should not take energy for a 1 kg. book to
float, given all the energy-related equations out there, and once the
book is already lifted off the ground. According to these equations,
no work is being done to levitate/support a book, since work is F*D*COS
THETA. THETA is 90 degrees, and there is no kinetic energy. I realize
that it takes energy to position the book off of the ground, and this
energy is MgY, where Y is the height off of the earth. However,
suppose that the object is now levitated in that spot. How much energy
would this take to levitate the object?
I realize that this very elementary question may seem silly to you, but
I'm being deadly serious.
Once your school trip takes you to planet earth, you wil find there is
a force called 'gravity' there, you will have to counter it to stay where
you are, or else be glued to earth for a long time.
And that [countering] takes energy, and for that energy you will have to pay
Universal Tokens.
Them space credits.. what have you got to trade?
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| User: "John C. Polasek" |
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| Title: Re: Why does it require energy for levitation? |
12 Sep 2006 08:57:18 AM |
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On Mon, 11 Sep 2006 21:37:54 GMT, Jan Panteltje
<pNaonStpealmtje@yahoo.com> wrote:
On a sunny day (11 Sep 2006 13:59:01 -0700) it happened "Brablo"
<gestureofrespect@yahoo.com> wrote in
<1158008341.787564.216960@i42g2000cwa.googlegroups.com>:
I was thinking that it should not take energy for a 1 kg. book to
float, given all the energy-related equations out there, and once the
book is already lifted off the ground. According to these equations,
no work is being done to levitate/support a book, since work is F*D*COS
THETA. THETA is 90 degrees, and there is no kinetic energy. I realize
that it takes energy to position the book off of the ground, and this
energy is MgY, where Y is the height off of the earth. However,
suppose that the object is now levitated in that spot. How much energy
would this take to levitate the object?
I realize that this very elementary question may seem silly to you, but
I'm being deadly serious.
Once your school trip takes you to planet earth, you wil find there is
a force called 'gravity' there, you will have to counter it to stay where
you are, or else be glued to earth for a long time.
And that [countering] takes energy, and for that energy you will have to pay
Universal Tokens.
Them space credits.. what have you got to trade?
For a book on a shelf, the shelf will compress by dx so the work done
is P*A*dx/2 = W*dx/2
and for a Youngs modulus Y and thickness T we get the strain dx such
that:
Ydx/T = P = W/A so
dx = WT/YA
and Energy = W^2*T/2YA
That's the only way you can fight gravity, and both air and vacuum
effectively have Y = 0.
John Polasek
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| User: "Bruce Scott TOK" |
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| Title: Re: Why does it require energy for levitation? |
16 Sep 2006 02:55:59 PM |
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|> Why does it require energy for levitation?
The short answer is that in four-D spacetime, the world line of a
stationary object in a gravitational field is not a geodesic. It costs
energy to depart from a geodesic, just as it costs energy to accelerate.
--
ciao,
Bruce
drift wave turbulence: http://www.rzg.mpg.de/~bds/
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| User: "Sorcerer" |
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| Title: Re: Why does it require energy for levitation? |
16 Sep 2006 04:26:02 PM |
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"Bruce Scott TOK" <Use-Author-Supplied-Address-Header@[127.1]> wrote in
message news:200609161955.k8GJtxsn025048@ipp.mpg.de...
| |> Why does it require energy for levitation?
It doesn't. I've got books levitated on shelves all over the place.
Androcles
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| User: "G=EMC^2 Glazier" |
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| Title: Re: Why does it require energy for levitation? |
19 Sep 2006 01:01:14 PM |
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Bruce It takes energy to create a magnetic field. Bert
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