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Homework Help: Newton's third law is absurd: argue why this isn't true.

  1. Mar 18, 2010 #1
    1. The problem statement, all variables and given/known data

    If the forces in Newton's third law are equal and opposite, they cancel, and thus nothing can ever accelerate, so the law is absurd.

    2. Relevant equations

    Argue why this isn't true.

    This is from my textbook, a very strong argument.

    3. The attempt at a solution

    Hey I'm in intro-physics, so my knowledge is very limited.
    But I would argue that if an object was moving at constant velocity, and impacted a wall with equal opposing force, then object would stop accelerating immediately. If this law is absurd, then we would not be able to identify the counter force coming from the wall. Thus, we would conclude the object just stops moving.

    anyone got a great argument? Please feel free to add/correct anything.
  2. jcsd
  3. Mar 18, 2010 #2
    This is not true. My explanatory abilities are limited, but think about it this way. If for example you jump in the air, you are applying a force on the earth, and the earth is applying a force on you.
    Lets assign this force an arbitrary value..30 N. F=ma. Lets say you have a mass 60kg. when you jump with a 30 N force you will have an acceleration of 0.5 m/s^2.
    Now look at the force you apply to the earth. 30N/5.98E24kg gives it an acceleration so insignificant it is unnoticable.

    Apply this to other bodies..differing masses allow for accelerations to exist when forces are equal and opposite.
  4. Mar 18, 2010 #3
    Of course, if you dont believe Newton's second law either, then that argument is no good.
  5. Mar 18, 2010 #4
    This is veering off in the wrong direction...

    A significant aspect of Newton's Third Law is that while the forces are opposite in direction and are equal in magnitude, they act on different objects, so there is no "cancelling out".
  6. Mar 18, 2010 #5
    This would be better off as an example of Newton's First Law. In any case, "impacted a wall with equal opposing force" does not make any sense. Rather, the object exerts a force on the wall when it hits the wall, and the wall exerts a force on equal magnitude on the object as well.
    Also, the object was not accelerating in the first place. It is the impact/ the force exerted on the object by the wall that causes the object to decelerate to rest.
  7. Mar 18, 2010 #6
    Netwon's third law has 3 characteristics,

    1. Equal magnitude.
    2. Different direction.
    3. Acts on different bodies.

    So if I push you, I exerted a force on your body. By Netwon's third law, your body exerted a force on my hand. You still fly away because no opposite forece exerted on your body.
  8. Mar 19, 2010 #7

    So isn't it when equal opposing force? If you push me, is there an equal opposing force? I think your force is greater, so therefore, I move forward. But the law says the object does not accelerate, with opposing and equal forces.

    Thanks to all for your help.

    If any one else can contribute, it will be appreciated.
  9. Mar 19, 2010 #8
    you've got what you need to understand this concept here. fightfish has explained it well. there is an equal and opposite force, but the opposite force is applied on a different object than the equal one. There is no greater force.

    when you say the law says that the object does not accelerate with "equal and opposing force". that doesn't make a whole lot of sense. lets say i punch the side of my house. when i hit the wall, the wall stops my hand almost immediately by applying a force opposite the direction of my punch. but because the wall is applying that force, my hand is also applying the same force to the wall. my hand ends up decelerating from whatever speed i was punching with, to 0. but since the wall is so large, and connected to all my other walls, it doesn't move at all. the force dissipates through the molecular structure of the house. Imagine you and I are skating on ice. if you stand right in front of me, and you push me, im going to go backwards, but so are you. that is because whatever force you have applied to my chest, my chest has applied back to you. keep asking questions if you are still unclear. it will help us pinpoint your issue.
  10. Mar 19, 2010 #9
    I think everyone has covered this pretty good. Yes the forces at a boundary are equal and opposite, but they act on different objects so they don't cancel. An interesting way to look at Newtons 3rd law is to apply Newtons 2nd law at the boundary or surface between two objects to get:


    Since this surface is essentially a plane for flat boundarys it has no thickness. Likewise if it has no thickness then it has no mass. So it follows that:

    F1 = -F2

    So you can see that indeed our two forces are in fact equal and opposite from the application of Newtons second law. It is also interesting that even if the system has some acceleration the forces will still be equal and opposite because the mass is zero.
  11. Mar 24, 2010 #10
    thank you everyone!
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