Re: Action-Reaction: Newton's most misunderstood law
- From: "Y.Porat" <y.y.porat@xxxxxxxxx>
- Date: Thu, 20 Aug 2009 08:17:12 -0700 (PDT)
On Aug 20, 1:15 pm, Helmut Wabnig <hwabnig@ .- --- -. dotat> wrote:
On Thu, 20 Aug 2009 02:44:17 -0700 (PDT), Uncle Ben <b...@xxxxxxxxxxx>
wrote:
Action-Reaction
Everybody knows Newton's Second Law: F=ma.
But hardly anybody knows Newton's Third Law:
For every action, there is an equal and opposite reaction.
What about you? Try these problems:
1. A horse pulls a wagon up a hill at an increasing rate of speed.
Neglecting the mass of their connection, which is greater?
1) The force the horse exerts on the wagon
2) The force the wagon exerts on the horse
3) Neither of the above
2. At the country fair, one of the rides is a set of swing seats being
whirled around from a center post. The seats are connected to the
post by a massless rope. The post pulls on each seat hard enough
to keep it going in a circle. What is the reaction force to this
force
the post exerts on seat No. 1?
1) a fictitious force pulling outward on the seat
2) a real force the seat exerts on the post
3) neither of the above
Uncle Ben
Actio = Reactio means that you cannot move anything.
Any force excerted on an object will be counteracted
by a counterforce of exactly the same magnitude.
(Clearly there is something wrong, but what precisely is it)?
In "Conceptual Physics" by Baliff & Dibble they give a nice example.
THE MOOSE AND THE LOG
We tie a moose to a fallen trunk
and politely ask him to pull it for us.
The educated elk refuses and cites Isaak Newton:
http://ia310836.us.archive.org/2/items/newtonspmathema00newtrich/newt...
(page 89 of the pdf)
cit.
If a horse draws a stone tied to a rope, the horse (if I may so say)
will be equally drawn back towards the stone: for the distended rope,
by the same endeavour to relax or unbend itself, will draw the horse
as much towards the stone, as it does the stone towards the horse,
and will obstruct the progress of the one as much as it advances that
of the other.
end cit.
So it is useless to pull on that rope, because as much we try to pull,
the load will pull back with exactly the same force, says the elk.
That moose is not just lazy, he is well educated and has a degree
in physics. Therefore we trust him and will follow his procedure.
Want a beer?
Don't even try to move!
Have somebody else bring the stuff.
Because you know, after learning from the elk,
that your massive body will counteract your impetus
to move towards the refrigerator.
STAY WHERE YOU ARE !
YOU CANNOT MOVE !
w.
--------------------
there is a difference between
a 'static situation'
in which there is a mutual force
of action and reasction
and 2
a dynamic situation!!
in which the force between actor and reactor causes motion
you have to see the more complete
picture:
he horse and the stone are not alone there as if in vacuum
he horse is acting not only on the stone
heis acting as well on he ground!
the force he is acting on the ground
is bigger than that one that he is acting
on the stone
and the difference becomes ma
(another fact that enters that situation
is friction of the stone on the ground
and friction between horses laggs and groung)
so it is not as in Vacuum!!
in that case
if it is in vacuum than indeed thj horse cannot move the stone
(unless he can do a big ....
i am ashamed to say whaht big ...(:-)
and then it works as a rocket in space ....
ATB
Y.Porat
--------------------
.
- References:
- Action-Reaction: Newton's most misunderstood law
- From: Uncle Ben
- Re: Action-Reaction: Newton's most misunderstood law
- From: Helmut Wabnig
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