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Newton's Third law

  1. Mar 5, 2010 #1
    I can't understand the significance of Newton's third law.
    What would happen if there were no reaction forces?
     
  2. jcsd
  3. Mar 5, 2010 #2
    Then Newton's Third Law would probably be "every force has no equal or opposite reaction force". but what is your question really. The significance to me of Newton's third law is that force is a kind of conservative value. It kind of governs motion. When you apply force to a wall, why doesn't the wall have an acceleration according to F=ma? Well that is because the wall is also applying the same force in the opposite direction, counter-acting your force. If there was no reaction force, the wall would have to move will acceleration F/m. I feel like I may not be answering your question though, would you like to be more specific?
     
  4. Mar 5, 2010 #3
    Nothing would happen because there would be absolutely NO forces.
     
  5. Mar 5, 2010 #4
    Is it not true that the wall is not moving because of frictional force between wall and the ground?
     
  6. Mar 5, 2010 #5
    Why no forces? I mean , what would happen if there is only action force,no reaction?
     
  7. Mar 5, 2010 #6
    no, the wall is not moving because of internal bonds in the brick or whatever it is. its cemented into the ground and so on, so it is not moving for those reasons. If you put the wall on skates and then pushed it, it would move. But think of what happens when you punch a wall. your hand hurts. why does it hurt? Of course its because of that reaction force. And I don't even think i can tell you what happens when there are no reaction forces. Bouncing a ball would be impossible, because the ground wouldn't push back up, you could compress things infinitely small with literally infinitesimal amounts of forces. Its a waste of time to think of what the world would be like with no reaction forces.
     
  8. Mar 5, 2010 #7
    An action force without reaction means no force.When you are acted upon by a force, you feel the force, because inertia is a property of masses
     
  9. Mar 5, 2010 #8
    Newtons three laws represent among other things a definition of force Couples. In the interaction of objects what arises is a force couple which is a thing in itself that is composed by 2 force vectors each acting on one of the 2 objects in the interaction.When doing problems it is sometimes useful to ignore one of the members of the couple but usually neglecting the other member is the mistake made most often.When thinking about the nature of the laws, thinking about only one member of the force is not useful and ultimately meaningless. What the interaction has created is the couple. If the interaction had not happened neither of the components would exist.And to this point no interaction has been observed that only created half a force couple. So the conclusion if the third law was not true then no forces would exist. This is my logic and there are probably more convincing and rigorous arguments out there.
     
  10. Mar 5, 2010 #9
    Your question has no answer.

    Imagine two stationary objects (A and B) with like charges and equal inertial mass. Since they have like charges they will repel each other. We'll call A the action mass and B the reaction mass. But it really doesn't matter which is the action and which is the reaction. The point is that the third law says that A will move exactly the same as B, just in the opposite direction.

    Now suppose, as you are proposing, that there is no reaction. How will you determine which object moves and which one remains stationary?
     
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