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Subject: "Newton Smewton"     Previous Topic | Next Topic
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Davey Gravy
guest
Dec-18-01, 08:13 PM (EST)
 
"Newton Smewton"
 
   Hi I've been told that Newton's 3rd law says that every action has an equal and opposite reaction. How then does anything move? I mean that if say something pushed to the right with such a force then an equal force shall push back to the left and cancel things out, including the movement or displacement. hmm. The situation is identical from any angle. Is this a paradox? Probably not because i'm typing this! (or am I? ha ha)
PS My engineering teacher just laughs when I try to explain it to him. Is he mad or am I?


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alexb
Charter Member
672 posts
Dec-18-01, 09:00 PM (EST)
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1. "RE: Newton Smewton"
In response to message #0
 
  


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Desert Nomad
guest
Jan-10-02, 09:43 PM (EST)
 
2. "RE: Newton Smewton"
In response to message #1
 
   How can you say that the motion of the second billiard ball is
an OPPOSITE reaction? It's in the same direction.


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alexb
Charter Member
672 posts
Jan-10-02, 09:48 PM (EST)
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3. "RE: Newton Smewton"
In response to message #2
 
   >How can you say

I can't.

>that the motion of
>the second billiard ball
>is
>an OPPOSITE reaction? It's in the same direction.

That's correct.


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Amanda
guest
Jan-11-02, 08:20 AM (EST)
 
4. "RE: Newton Smewton"
In response to message #0
 
   >Hi I've been told that Newton's 3rd law says that every
>action has an equal and opposite reaction. How then does
>anything move? I mean that if say something pushed to the
>right with such a force then an equal force shall push back
>to the left and cancel things out, including the movement or
>displacement.

Ahhhh, but when we start talking about movement and displacement, Newton's 1st and 2nd laws come into play and the important thing about them is that they apply to the *external* forces exerted *on* a single object. The classic example of the N3 (Newton's 3rd) dilemma is the horse and cart - when the horse pulls the cart, the cart exerts an equal and opposite force on the horse, so how do they move?

Now if we look at the horse (that is define our single object to be "horse", the external forces acting on it include the force exerted on the horse by the cart and frictional forces between the horse and the ground, but it *does not* include the force of the horse pulling the cart because that is a force exerted *by* the horse and not *on* the horse.

But if we define our single object to be "horse + cart", the force exerted on the cart by the horse and on the horse by the cart are ignored when looking at the motion of the system because they are now *internal* forces. The only forces we worry about now are external ones such as friction which are acting on the "horse + cart" object.

Hope that helps

Amanda


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