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Doubt with Newton's 3rd law

  1. Nov 25, 2014 #1
    I understand that if for an instance one object pushes another with a determinate force, the other object acelerates with that force, and the pushing object acelerates in oposite direction with the oposite force as well.

    However if I punch a piece of paper I don't feel the same oposite force from my punch than with a heavier object like a furniture, shouldn´t it be the same?

    I've google this and I saw this response:

    It considers a1=a2

    According to Newton's law F2,1=-F, right? then the block 1 wouldn´t move at all (f-f=0) and the object 2 would accelerate with an F force. I'm so confused.. Where does this F1,2 , F2,1 and F come in the first place, shouldn't F transform into F2,1 and F1,2? How come are they added in F=ma if they don't coexist.

    thanks in advance
  2. jcsd
  3. Nov 25, 2014 #2


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    Staff: Mentor

    Try googling for "horse cart problem newton's third law" and you'll find a number of good explanations.

    The key here is that the two forces that make up a third-law equal-and-opposite pair are acting on different objects. When you push on the block you are applying a force to the block and accelerating. The block is exerting an equal and opposite force on you. You would slide backwards if you were standing on a slippery floor; more often the friction between the soles of your shoes and the floor acts against the force the block exerts on you so the net force on you is zero and you stay put.
  4. Nov 25, 2014 #3

    Doc Al

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    You can easily punch a heavy object with more force than you can punch a piece of paper. But either way, whatever force you exert on the object (whether paper or furniture) will be equal and opposite to the force that that object exerts back on you.

    No, not right. F is an applied force. The correct third law pair here is F2,1 & F1,2: They are equal and opposite.
  5. Nov 25, 2014 #4


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    It takes less force to give the same acceleration to a piece of paper than to a sofa. Therefore the reaction force from the piece of paper is also less.
  6. Nov 25, 2014 #5

    Andrew Mason

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    The force of a punch depends on what it is hitting. If you punch the air, you don't apply much force to the air and the air does not apply much force to your fist. But if you punch a cement wall, the wall will apply a very large force to your fist. Your fist will stop in a very short time so its momentum change per unit of time (F = dp/dt) is very large. The air will just slow your fist down by a very small amount in a very long time, so the force = dp/dt is very small.

  7. Dec 1, 2014 #6
    One think is there

    newtons 3rd law needed particular medium(ex.air)to be apply

    so in vaccume( Space) How it apply to rocket,as there is no medium
  8. Dec 1, 2014 #7


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    Gold Member

    The force upon the motor is opposite to the force that the exhaust experiences in its escape from the motor.
  9. Dec 1, 2014 #8


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    The rocket engine pushes against the fuel exhaust to accelerate it out of the nozzle. Therefor there is also a force pushing back against the rocket engine, which is what accelerates the rocket.
  10. Dec 1, 2014 #9
    Ok thanks..

    one more thing , what happend to that exhausts from rocket engine in vaccume ( Space )
  11. Dec 1, 2014 #10


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    Staff: Mentor

    It just keeps going.
  12. Dec 1, 2014 #11
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