# The Third Law

1. In the definition of the Laws of Motion Newton states:

Whenever a first body exerts a force F on a second body, the second body exerts a force −F on the first body. F and −F are equal in magnitude and opposite in direction.

Later someone attempts to clarify with:

Whenever a particle A exerts a force on another particle B, B simultaneously exerts a force on A with the same magnitude in the opposite direction. The strong form of the law further postulates that these two forces act along the same line. This law is often simplified into the sentence, "To every action there is an equal and opposite reaction."

While I agree that the first statement is partially true, it is also very easy to jump to a secondary belief, that is not correct, as illustrated in the latter clarification.

Whenever a first body exerts a force F on a second body, the second body exerts a force −F on the first body. TRUE, but only with the addition of, for a split second. Then one of two things occur:
If the first body has a greater force(F) than the second body has resistance(-F), the second body will move. If the secondary body has greater resistance(-F) than the force(F) of the first body the secondary body will remain at rest.

In other words, If I am (F) the first body, and a mountain is (-F) , the second body, and I push on the mountain... it will not move. But if I(F) push against a pebble(-f), it will move. because the resistance(-f) , [the amount of force] that the pebble has is less than,(not equal to) the amount of force I am exerting on it. So the forces are opposite, but not equal, hence motion. In the time it takes for the forces to interact there is a split second where the forces are equal and opposite, but after that split second the forces are UN-equal and may or may not be opposite.

If the bodies (the two forces) act along the same line. In the case of (-F) being the greater force, the force(F) of the first body is redirected in the opposite direction, the direction of origin, and causes the first body to "bounce".

In the case of deflection, the forces are not opposite and one or both "bodies" will alter course(trajectory) based on the amounts of force applied, mass of the bodies, and velocity.

Am I Wrong?

1. I can't quite follow what you are trying to say, or what your question is, but the forces exerted on each object will be equal, even though the result of the force to the naked eye looks different.  Just think if a Ford Fiesta ran head on into a Mac truck.  The Force at impact will be equal, but the reaction of the two vehicles will be disturbingly different. I think what you are trying to understand is the Conservation of Momentum.  What you described, from what I can gather is an elastic case unless you are saying it loses its kinetic energy.  Then it would be inelastic.

Check it out along with Newton's 3rd Law here

http://230nsc1.phy-astr.gsu.edu/hbasees/hframe.html
Click Mechanics/Conservation of Momentum

1. NO it won't the force of the Mac truck will be greater and the Mac truck will continue along it's trajectory. Equal and Oppostie would mean they (both objects) would stop instantly at the precise point of impact. Of course if the third law was true, they couldn't have gotten past the intial effect to acheive motion in the first place...

1. Yes, it would.  What you are describing is the conservation of momentum.  The Force would even be the same if a mosquito flew into the windshield of the same Mac Truck.  Maybe this will do a better job of explaining it for you.  Remember, equal and opposite.

http://www.physicsclassroom.com/Class/N … /U2L4a.cfm

1. Whenever a first body exerts a force F on a second body, the second body exerts a force −F on the first body. TRUE, but only with the addition of, for a split second.

Newton was wrong about the rest of it, because then one of two things occur:
If the first body has a greater force(F) than the second body has resistance(-F), the second body will move. If the secondary body has greater resistance(-F) than the force(F) of the first body the secondary body will remain at rest.

But the forces are NOT equal and opposite for anything more than the split second it takes for the greater force to overpower the lesser force

2. LOL, ok man whatever you say.  Did you even review the conservation of momentum?

The Force= mass x acceleration (delta V/delta T)

The Force wont even become a factor until the collision, or whatever it is being exerted on.  One object isn't going to randomly produce any more Force than the other.  That is why you need to review the Conservation of Momentum.  No, an object remains at rest unless acted upon by an outside Force.  That is the First law not the third.  lastly, there is no greater.  If there was airplanes wouldn't fly, and cars wouldn't drive down the street.  Sorry, but Newton got that right.  You are mixing up the laws.

http://hyperphysics.phy-astr.gsu.edu/HB … kc.html#c2

1. Your trying to add a secondary theory, that is not relevant to the discussion we're having. Though it is in a related field, and plays a seperate part.

My point is Newton and you are saying: if Every force is equal and opposite, then force=force and mass x acceleration is irrelivant... which obviously isn't correct, so you and Newton are wrong.

1. I'm not adding any secondary theory.
F=ma is Newton's second Law.

We wouldn't even have a 3rd if the second wasn't there.  Like I said, you are getting them mixed up.  You are confusing impulse Force and the Conservation of Momentum.  Lastly, it's not irrelevant if you want to know how much Force there was at the time of impact.  It's very useful, and helps determine safety restraints etc..

1. And that is my point, the second makes the latter part of the third incorrect, There IS NOT an equal and opposite reaction, there are consequences that may or may not be equal and/or opposite.

1. No, the second LAW proves there is a Force, and the third just explains what that Force does.

The Force is always equal and opposite.

1. if F=MA then MA=F and if F=ma; MA=f=ma right?
then MA=ma still with me?
M=10
F=10
m=1
a=1
then your saying 10 times 10= 100 and 1 times1 =1 so 100=1?
the forces are not always equal. nor are they always opposite, what if the mosquito t-bones the Mac truck...

the third law states F=f always every single time...and is therefore WRONG.

1. LOL, sorry but you are really confused. That is correct what you said.  In that case the Force cancels out. Think of it like this.  There is a book, resting on a table, producing a downward Force.  In turn that table is producing the same exact force upward, and they cancel out.  Reading the Law and being able to apply it are two different things.  What you described earlier is an inelastic collision.  In addition, the force is used to calculate many things including Work.  If the Mosquito T-bones the truck the Forces would still be equal in opposite directions.  The truck is only going to put back as much Force as the Mosquito exerts onto it...always....and forever.  It's not like the Mosquito hits the truck, and the truck says, "you son of a B!@#ch" and then can magically conjure up some super Force to propel back at the Mosquito.  It's basically an eye for an eye every time.

1. I don't think so...It is true that for a split second the force that the baseball applies to the bat is equal and opposite, but only for that split second. Then the greater force, mass, velocity of the bat UN-equals the action of the ball and sends it flying. The action of the ball flying is an action unto itself, the stopping of the ball is too an action. The equal and opposite reaction of the "ball stopping" action, is that the ball never stop moving. Since the ball does stop, again the third "law" is proved inaccurate...

Your example of the table isn't a very good example becasue the force of gravity and the forces of the objects are all interacting...

1. LOL, that is my whole point.  That is what the third law says.  My book example is two stationary objects exerting a Force.  You are confusing that with two objects in motion.  You can't do that! THERE IS NO ACCELERATION OF A BOOK RESTING ON A TABLE! I don't know how I can say it any more clearly.  Read the CONSERVATION OF MOMENTUM!

http://library.thinkquest.org/11902/phy … entum.html

Lastly, if you want to believe that Newton's Laws are incorrect, because you are unwilling to actually put forth the effort to understand them, then be my guest.

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Denno66posted 12 years ago

Is that like the third law of robotics?

1. We are in way over our heads here....we gotta split, man, split like a fat man's pants.

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Denno66posted 12 years agoin reply to this

I thinks yer right....

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Denno66posted 12 years agoin reply to this

OMG! You rock!!!! 1. Fn-A....sorry, that's MY favorite phrase.  But I looked yours up.

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Denno66posted 12 years agoin reply to this

I say it so often , I may have to retire it....

2. Thank You Thank You 4. Only if it doesn't conflict with the first or second law.

5. Here is a really good example of Newton's third law.

http://www.batesville.k12.in.us/physics … dCart.html

If you really want to understand it, then you will read the whole thing.  Additionally, I also realized that you don't understand how to sum vector Forces.  I highly recommend reviewing that also to fully understand Newton's laws.

http://www.physicsclassroom.com/Class/n … /u2l2d.cfm

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