| Topic: |
Science > Physics |
| User: |
"Damon B." |
| Date: |
02 Oct 2003 03:57:22 AM |
| Object: |
resultant force and moment.... |
My intuition is clashing with a simple concept. Here is a general example to
express the concept. Consider an unbounded uniform bar of mass which is
initially at rest and is being acted on only by a force couple. Both forces
of the couple act on the same side with respect to the bars center of mass.
|
| ---->
|
| <----
|
|
|
|
|
|
Basically, the laws of physics say that, because the resultant force is zero
( due to the force being a couple), the bar's center of mass will not move.
However, the force couple creates a moment (torque) about the bar's center
of mass that causes the bar to rotate around its center of mass.
I understand completely what physics says should happen. But my intuition
tells me that the bar would rotate around a point located between the
forces. I discussed this with my statics teacher, and his comment was
basically that systems like the one I used as an example are uncommon in
nature. Forces in the mechanical world rarely act along straight lines and
systems are affected by all kinds of external forces like gravity and
friction.
I prefer to understand physics conceptually rather than just mathematically,
so I am wondering if anyone knows of any experiments which have proved that
a force couple creates no displacement to a rigid body's center of mass, or
possibly a real world example that might help make this concept feel more
intuitive.
I attempted to conceive an experiment which might prove this concept.
However, I could not imagine a mechanical method that is without flaws. The
closest I could get would be to float a magnetic bar on a magnetic field and
in a vacuum. Then introduce a horizontal force couple by applying maser
beams to the object. I do not claim to be well versed regarding magnetism so
forgive me if that proposition is flawed.
Damon
.
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| User: "Mathew Orman" |
|
| Title: Re: resultant force and moment.... |
02 Oct 2003 06:29:28 AM |
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"Damon B." <damon_bohlsSPAM_SUCKS@yahoo.com> wrote in message
news:SLReb.2054$0V1.251@newssvr24.news.prodigy.com...
My intuition is clashing with a simple concept. Here is a general example
to
express the concept. Consider an unbounded uniform bar of mass which is
initially at rest and is being acted on only by a force couple. Both
forces
of the couple act on the same side with respect to the bars center of
mass.
|
| ---->
|
| <----
|
|
|
|
|
|
Basically, the laws of physics say that, because the resultant force is
zero
( due to the force being a couple), the bar's center of mass will not
move.
However, the force couple creates a moment (torque) about the bar's center
of mass that causes the bar to rotate around its center of mass.
I understand completely what physics says should happen. But my intuition
tells me that the bar would rotate around a point located between the
forces. I discussed this with my statics teacher, and his comment was
basically that systems like the one I used as an example are uncommon in
nature. Forces in the mechanical world rarely act along straight lines and
systems are affected by all kinds of external forces like gravity and
friction.
I prefer to understand physics conceptually rather than just
mathematically,
so I am wondering if anyone knows of any experiments which have proved
that
a force couple creates no displacement to a rigid body's center of mass,
or
possibly a real world example that might help make this concept feel more
intuitive.
I attempted to conceive an experiment which might prove this concept.
However, I could not imagine a mechanical method that is without flaws.
The
closest I could get would be to float a magnetic bar on a magnetic field
and
in a vacuum. Then introduce a horizontal force couple by applying maser
beams to the object. I do not claim to be well versed regarding magnetism
so
forgive me if that proposition is flawed.
Damon
Simple!
Your experiment setup is incomplete.
You need to specify the source point of your forces.
Right now your setup consists of two reaction less forces.
Why you would ask.
Because any force is always defined by two points that must be part of
massive body.
The only exception is radiation pressure of electromagnetic wave.
Remember that force is only measurable when there is a motion
Refine your experiment and have a bite of "Newton's Apple".
Sincerely,
Mathew Orman
www.ultra-faster-than-light.com
www.radio-faster-than-light.com
.
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| User: "Danny Deger" |
|
| Title: Re: resultant force and moment.... |
02 Oct 2003 08:43:10 PM |
|
|
"Damon B." <damon_bohlsSPAM_SUCKS@yahoo.com> wrote in message
news:SLReb.2054$0V1.251@newssvr24.news.prodigy.com...
My intuition is clashing with a simple concept. Here is a general example
to
express the concept. Consider an unbounded uniform bar of mass which is
initially at rest and is being acted on only by a force couple. Both
forces
of the couple act on the same side with respect to the bars center of
mass.
|
| ---->
|
| <----
|
|
|
|
|
|
Basically, the laws of physics say that, because the resultant force is
zero
( due to the force being a couple), the bar's center of mass will not
move.
However, the force couple creates a moment (torque) about the bar's center
of mass that causes the bar to rotate around its center of mass.
I am a very experienced aerospace engineer and my intuition was (is?)
confused also. I think it leads from the mind "assuming" something to apply
the force that is fixed. Obviously if the push and pull forces are applied
by something fixed in space, the bar will rotate about a point between the
two forces (but the forces would not stay equal). It helps me to picture
the force applicators as small rockets and have no mass.
Perhaps this will help you out.
Danny Deger
.
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| User: "Edward Green" |
|
| Title: Re: resultant force and moment.... |
04 Oct 2003 01:54:19 AM |
|
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"Danny Deger" <junk.deger@earthlink.net> wrote in message news:<blik7h$ctbqc$1@ID-195254.news.uni-berlin.de>...
I am a very experienced aerospace engineer and my intuition was (is?)
confused also. I think it leads from the mind "assuming" something to apply
the force that is fixed. Obviously if the push and pull forces are applied
by something fixed in space, the bar will rotate about a point between the
two forces (but the forces would not stay equal). It helps me to picture
the force applicators as small rockets and have no mass.
Good answer. Maybe intuition is thinking of our two hands wielding a bat.
.
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| User: "OC" |
|
| Title: Re: resultant force and moment.... |
02 Oct 2003 10:30:56 AM |
|
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"Damon B." <damon_bohlsSPAM_SUCKS@yahoo.com> wrote in message news:<SLReb.2054$0V1.251@newssvr24.news.prodigy.com>...
My intuition is clashing with a simple concept. Here is a general example to
express the concept. Consider an unbounded uniform bar of mass which is
initially at rest and is being acted on only by a force couple. Both forces
of the couple act on the same side with respect to the bars center of mass.
|
| ---->
|
| <----
|
|
|
|
|
|
Basically, the laws of physics say that, because the resultant force is zero
( due to the force being a couple), the bar's center of mass will not move.
How do you sum two forces that are applied to two separate points?
Can you sum forces applied to different points?
OC
.
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| User: "Laszlo Lemhenyi" |
|
| Title: Re: resultant force and moment.... |
02 Oct 2003 09:34:27 PM |
|
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"Damon B." <damon_bohlsSPAM_SUCKS@yahoo.com> wrote in message news:<SLReb.2054$0V1.251@newssvr24.news.prodigy.com>...
My intuition is clashing with a simple concept. Here is a general example to
express the concept. Consider an unbounded uniform bar of mass which is
initially at rest and is being acted on only by a force couple. Both forces
of the couple act on the same side with respect to the bars center of mass.
|
| ---->
|
| <----
|
|
|
|
|
|
Basically, the laws of physics say that, because the resultant force is zero
( due to the force being a couple), the bar's center of mass will not move.
However, the force couple creates a moment (torque) about the bar's center
of mass that causes the bar to rotate around its center of mass.
I understand completely what physics says should happen. But my intuition
tells me that the bar would rotate around a point located between the
forces. I discussed this with my statics teacher, and his comment was
basically that systems like the one I used as an example are uncommon in
nature. Forces in the mechanical world rarely act along straight lines and
systems are affected by all kinds of external forces like gravity and
friction.
I prefer to understand physics conceptually rather than just mathematically,
so I am wondering if anyone knows of any experiments which have proved that
a force couple creates no displacement to a rigid body's center of mass, or
possibly a real world example that might help make this concept feel more
intuitive.
I attempted to conceive an experiment which might prove this concept.
However, I could not imagine a mechanical method that is without flaws. The
closest I could get would be to float a magnetic bar on a magnetic field and
in a vacuum. Then introduce a horizontal force couple by applying maser
beams to the object. I do not claim to be well versed regarding magnetism so
forgive me if that proposition is flawed.
Damon
-------------
Hy
You said that>
" Basically, the laws of physics say that, because the resultant force is zero
( due to the force being a couple), the bar's center of mass will not move."
I do not think that the laws of physics say that.
You misinterpretted them but it is OK while you realised that something is missing.
Systems of particles ( bodies too) have two basic kinds of movement :
translational and rotational.
And they tend to keep both unaltered!
This is where from we have 2 of the basic conservation laws (liniar
momentum and angular momentum conservation).
To keep the bar still is not enough to have the summ of the
forces = zero , because there are 2 equilibrium requirements not one.
That is what the laws of physics say!
But there is a GREAT IDEA hidden in this kind of situations !!!
Namely ,that ANGULAR MOMENTUM IS AN INTRINSIC PROPERTY OF SYSTEMS of particles!
And any time when a "thing" has a non zero angular momentum THAT "THING"
IS A SYSTEM !. At least this is my understanding of the issue : angular
momentum is the key for telling apart elementary particles from composite
particles.
This could be a useful idea in particle-physics, isn't it?
Best wishes
Laszlo Lemhenyi , Toronto,Canada
.
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| User: "OC" |
|
| Title: Re: resultant force and moment.... |
03 Oct 2003 03:49:10 AM |
|
|
But there is a GREAT IDEA hidden in this kind of situations !!!
Namely ,that ANGULAR MOMENTUM IS AN INTRINSIC PROPERTY OF SYSTEMS of particles!
And any time when a "thing" has a non zero angular momentum THAT "THING"
IS A SYSTEM !. At least this is my understanding of the issue : angular
momentum is the key for telling apart elementary particles from composite
particles.
This could be a useful idea in particle-physics, isn't it?
Best wishes
Laszlo Lemhenyi , Toronto,Canada
The electron has an intrinsic angular momentum, but so far it has not
shown signs of being a system. Theories that consider the electron as
a point-like particle work fine.
In any case, your idea has been used (proton and neutron have
intrinsic angular momentum and are both systems of particles).
OC
.
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| User: "Timo Nieminen" |
|
| Title: Re: resultant force and moment.... |
02 Oct 2003 07:19:05 PM |
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On Thu, 2 Oct 2003, Damon B. wrote:
My intuition is clashing with a simple concept. Here is a general example to
express the concept. Consider an unbounded uniform bar of mass which is
initially at rest and is being acted on only by a force couple. Both forces
of the couple act on the same side with respect to the bars center of mass.
|
| ---->
|
| <----
|
|
|
|
|
|
Basically, the laws of physics say that, because the resultant force is zero
( due to the force being a couple), the bar's center of mass will not move.
However, the force couple creates a moment (torque) about the bar's center
of mass that causes the bar to rotate around its center of mass.
I understand completely what physics says should happen. But my intuition
tells me that the bar would rotate around a point located between the
forces.
OK, consider your two forces:
| ----> A
|
| <---- B
|
| CoM
|
|
|
|
What if only force A is applied? Then there will both a torque and a
non-zero total force. Might be a bit hard to see what happens. So, add
another force C, acting through the CoM, to make the total force = 0.
If only force C is acting, then there will be no rotation. With both force
A and C acting:
| ----> A
|
|
|
CoM | <---- C
|
|
|
|
it looks exactly like a bar balanced on a pivot (force C), with a force A
acting on one end (but sideways). For a balanced bar, the force acting on
the end causes rotation about the pivot point (the CoM). So force A only
causes rotation about the CoM (and, in the absence of other forces, linear
acceleration of the CoM).
Exactly the same applies for force B. Since A and B each only cause
rotation about the CoM, their combination only causes rotations about the
CoM.
I prefer to understand physics conceptually rather than just mathematically,
so I am wondering if anyone knows of any experiments which have proved that
a force couple creates no displacement to a rigid body's center of mass, or
possibly a real world example that might help make this concept feel more
intuitive.
Balanced bar + an extra force. Note that if the bar doesn't start
balanced, then the reaction force due to the pivot is not equal to the
applied extra force, and the CoM will move.
--
Timo Nieminen - Home page: http://www.physics.uq.edu.au/people/nieminen/
Shrine to Spirits: http://www.users.bigpond.com/timo_nieminen/spirits.html
.
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