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Newton's third law of motion - help me to explain it to someone

  1. Aug 25, 2009 #1
    So there's this guys argument that newtons 3rd law is wrong:

    "When I say that for every action there CAN'T be an equal an opposite reaction, it is a FACT.

    It's mathematically IMPOSSIBLE to have an EQUAL reaction to every action. Because then NO action would succeed. ALL actions would be right away countered by equal reaction! No action would be possible.

    Newton was 300 years ago. His ideas were revolutionary at that time, but like most great people he missed several points.


    And here you can see videos of him basically saying the same thing:

    I can see his mistake very well, but i'm not that good at English to explain it to him, so i want you guys to help me. Explain him his mistake as good as you can and i'll try to enlighten him :D... It MUST be a really good explanation because he's a complete dumbass - i mean, he thinks Earth is stationary...lmfao
    Scary... O_O

    Explain, why Newton's third law of motion IS correct.
    Last edited: Aug 25, 2009
  2. jcsd
  3. Aug 25, 2009 #2

    Doc Al

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    Frankly, arguing with crackpots is a waste of time. But if you insist, here are a couple of points to mention:

    (1) Forget the archaic terms "action" and "reaction", which imply that one causes the other. Newton's 3rd law is a description of how forces work: If body A exerts a force on body B, then body B exerts an equal and opposite force on body A.

    (2) Note that the "action" and "reaction" forces act on different bodies, thus they never "cancel". Say you punch someone in the face. Your fist and his face exert equal and opposite forces on each other. Do they "cancel out"? Not hardly!
  4. Aug 25, 2009 #3
    Why bother explaining it? People who dont understand the law and know they dont understand it, get people to explain it to them. People who dont understand and make videos claiming that something univerally accepted and that we regulally use every single day fall into 2 groups.

    a: the stupid.
    b: the stubborn.

    Almost any book on motion clearly explains it, if they dont understand it or refuse to see the point then they will never be convinced. It doesnt matter what you do.
  5. Aug 25, 2009 #4
    Newton's third law of motion is specifically describing forces. If I am at rest and a particle hits me with a force, [itex]\mathbf{\vec{F}}[/itex], then it feels a force exerted by me equal to [itex]-\mathbf{\vec{F}}.[/itex]
  6. Aug 26, 2009 #5
    I also had a lot of confusion regarding the concept earlier, but I pondered over it and later figured out (mabey what I figured out was wrong) that the force the bodies will exert will be equal, but the force which will win, will be the force with a higher acceleration. I know it doesn't make a lot of sense, and mabey the concept's completely wrong, but it was enough to clear it up for me so I could study higher level concepts that use the law.
    Also, I read that 'the action and reaction forces are equal and opposite, but they are unbalanced', but that didn't quite get through my head.
  7. Aug 26, 2009 #6
    For every action, there is an equal and opposite reaction. For example, a block is in the surface or floor and it is not moving. You know why it is not moving? The block exerted a force equals to its weight. In addition, the surface exerts a force equal to the force exerted by the block. So, if they are equal, the result is zero.
  8. Aug 26, 2009 #7

    Doc Al

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    Careful! The reason the block doesn't accelerate is not due to Newton's 3rd law, but Newton's 2nd law: The net force on the block happens to be zero.

    One must properly identify the 3rd law force pairs (the so-called "action" and "reaction"). The following are correct 3rd law pairs:
    (a) The floor exerts an upward contact force on the block; the block exerts a downward contact force on the floor.
    (b) The earth exerts a downward gravitational force on the block; the block exerts an upward gravitational force on the earth.

    The following is not a correct example of a 3rd law pair:
    (c) The floor exerts an upward contact force on the block; the earth exerts a downward gravitational force on the block. FALSE! These forces are not examples of "action/reaction".

    Nothing in Newton's 3rd law says anything about the block being in equilibrium. Nothing in Newton's 3rd law says that the two forces in example (c) have to be equal and opposite, since they are not 3rd law pairs.
  9. Aug 27, 2009 #8
    Sorry. I think this will be correct now. If you slap the face of someone with 50N, then the face of whom you slapped will also give 50N.
  10. Aug 27, 2009 #9


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    Think of a force as the rate by which a packet of "momentum" is transferred from object A to object B.

    The loss of momentum A experiences is exactly equal to the gain of momentum that B experiences, like a ball thrown from A and received by B.

    Furthermore, this concept yields a ready depiction of the distinction between "contact forces" and "forces working over distance":

    For contact forces, the momentum transfer occurs instantaneously, so at the SAME moment, A experiences a momentum transfer rate (i.e, force) that is equal to the force felt by B, but in the opposite direction.

    For forces action over distances (say, magnetic forces), the situation is trickier:

    The momentum package, as depicted going from A, will use some time to reach B.

    Thus, it is NOT true that at the same moment, the sum of A's and B's momenta will equal what that sum was PRIOR to the release of the momentum package!

    Instead, that package of momentum will for some time lie "in-between" A and B, i.e, within the electro-magnetic field.

    Momentum is in this case conserved as long as we sum the momenta of A and B AND whatever momentum is carried by the E-M field.
  11. Aug 27, 2009 #10
    how can someone not understand newton's third law, it is very practical
    and almost half of it we see everyday.
  12. Aug 27, 2009 #11
    This someone who doesn't even understand newton's third law also has claimed to refute the Copernican heliocentric system..lol..what a joke..
  13. Aug 27, 2009 #12
    i wud say it is only (a)
  14. Sep 6, 2009 #13
    well u are wrong with this explanation
  15. Sep 6, 2009 #14
    newtons 3rd law tells that for every action there is equal an d opposite rection
    but action and reaction pair do not act on same body
    suppose you press the earth with some force. the earth will give u same amount of reaction. only the reaction will act on u and u will move. whereas the action causes negligible accelaration of earth
  16. Sep 6, 2009 #15


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    I concur with Doc Al.

    Some examples:

    Let's say you're driving a terrain vehicle, in the woods, and you got yourself bogged down. You're not going anywhere, because you have no traction. You unreel the winch, attach the cable to a nearby tree, and you start reeling in the cable.

    On the tree two equal and opposite forces are exerted. The cable is pulling the tree, the tree is firmly rooted, and the grip of the tree roots in the ground is easily enough to prevent motion of the tree. That is cancelation of two forces, that are exerted upon the same object.

    Back to the terrain vehicle:
    You're reeling in the cable, but your wheels are deep in the mud and the mud grips the wheels. Something has to give, and if the winch, cable and tree are strong enough it's the grip of the mud that will give: your terrain vehicle sets into motion. That was again two forces exerted on the same object, but unbalanced, the force exerted by the cable was bigger than the grip of the mud. The tree provided the leverage.

    Third example: your terrain vehicle is so multi-purpose that it will even serve as a spacecraft. You're out in space, tens of meters from a big space station. You need to get to the space station. The problem is: you're in space, you have no traction! But there's your trusty winch, you clamp on magnetically or so, and you reel yourself in.

    Or hang on, the space station is much more massive than your vehicle, was in fact the space station reeling you in? Actually, it was mutual. There is a common center of mass of your vehicle and the space station. When the cable started pulling it exerted the same force on both objects, and both of you were set into motion, towards the common center of mass. Each craft was causing the other craft to move.

    Most of the motion was on your part. The space station has much more inertial mass than your vehicle, so it has much more leverage.

  17. Sep 24, 2009 #16
    While on this subject, I wonder, if walking on a planet causes the earth to be pushed back by a certain amount, does the rotational speed of the earth not change on say, yearbasis, depending on what directions have been walked? Or does all the combined walking cancel each other out? What if everyone would be walking in the same direction? Similarly, if two planets attract each other, why do they not pull each other out of orbit? Does a planet with a greater orbital period than the second balance out the attrection of the first? I realize the force must be extremely small, but it is still greater than 0. Should this not create some acceleration somewhere?
  18. Sep 24, 2009 #17


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    Of course this is a pure thought experiment, the actual change in Earth angular momentum would be far below detectable treshold, even with the most accurent current equipment.

    To bring the dimensions down somewhat, imagine a small asteriod, say several tens of kilometers in diameter, roughly spherical, and slowly spinning. Can a team of astronauts, on the surface of the asteroid, slow it down?

    They can if they can dissipate some rotational kinetic energy with friction. The astronauts position themselves at one of the poles, and then they jump right to the equator. Touching down on the equator they are initially not co-moving with they asteroid. Friction between the astronaut and the asteroid will bring the astronaut up to speed. Once the astronaut is co-moving with the asteroid he walks back to the pole, making sure all the time he does not slide.

    Going through many cycles of that must slow down the asteroid, for kinetic energy is converted to heat during the friction phase.

    The planets do cause perturbation of each other's orbits, but in the case of our solar system the perturbation is too small to cause any buildup of effect.

    In their orbit around the Sun for each planet there is also interaction between a planet's own spinning and its orbit. It's a very weak effect, but it does accumulate, and over time it tends to make planetary orbits more circular. That tends to stabilize the solar system against random perturbations.

    Also, look up information about the celestial body 'Oterma'. Oterma is not ejected from the solar system, but it alternates between periods of orbiting inside the Jupiter's orbit and orbiting outside Jupiter's orbit. When Oterma comes close to Jupiter it is pulled to another orbit.

  19. Sep 25, 2009 #18
    Wow very clear, thanks!
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