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Newton's Third Law and Acceleartion

  1. Mar 6, 2010 #1
    Right now I am taking a classical physics course and find myself struggling with Newton's Third Law, which states that for every action there is an equal and opposite reaction. I am struggling to interpret this in a way such that I can still accept movement of objects.

    If I attempt to slide a block across the table, according to Newton's 3rd Law no matter how hard I push on that block there will be an equal and opposite force to cancel out the force I exert.

    Obviously, this is not true, since frictional force is completely dependent upon normal force. Once the maximum value of the frictional force is exceeded, the block moves. However, this seems to contradict that there would be an equal and opposite reaction for every action.

    In other words, there exists a very large chasm in my understanding of the most fundamental laws of physics, and it requires filling.
  2. jcsd
  3. Mar 6, 2010 #2
    The opposite force when you push a block, acts on your hand, not the block.
  4. Mar 7, 2010 #3
    You are right, those laws are terribly worded and 3rd law is superfluous having the 2nd law, i.e. explanation of the 3rd law is in the 2nd law: F=m*a. Because to have whatever force we need to have at least TWO objects interacting, lets take two most elemental particles. Take large mass proton and small mass electron, plug the numbers in equation, solve for both particles, then note the force between them will indeed be equal in magnitude and opposite in direction.

    Yes, but it is not really cancellation of the forces, but cancellation of ACCELERATION. However, if table breaks or two objects bounce or anyhow move, then object(s) with larger mass will have less velocity than the object with smaller mass, but individual forces themselves will again be equal in magnitude and opposite in direction.

    Just start using "F=m*a" on some practical examples, like try to model solar system, and you will see 3rd law is simple mathematical consequence of the 2nd law.

    Actually there is, whether friction is exceeded or not, whatever amount of whatever force remains, in whatever direction - there will always be force equal in magnitude and opposite in direction. Again, to have some force in a first place there need to be at least TWO objects interacting, that is "in range" of their gravity or electromagnetic fields.

    Real world interactions where objects come in contact are essentially electromagnetic, the forces involved are many and superimposed between all the atoms, electrons and protons, so the 3rd law might not be very obvious in complex collisions.
    Last edited: Mar 7, 2010
  5. Mar 7, 2010 #4


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    Try pushing it on a frictionless surface. You will find it doesn't push back on your hand as much. This is because N3 works. It even works on friction.
  6. Mar 7, 2010 #5
    The third law is not about acceleration. Although acceleration can be involved, that is not what the third law is about. Acceleration is covered in the second law. The third law tells us that whenever an object is caused to move through the application of a force, then another object also moves with equal momentum in the opposite direction.
  7. Mar 7, 2010 #6


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    No, the force you are exerting is not being "cancelled out" by anything. What Newton's third law says is that the force with which your hand pushes on the block is equal to the force with which the block pushes back on your hand. Notice that these two forces each act on two DIFFERENT objects. The block itself has only one force acting on it (ignoring friction with the table). Hence, by Newton's 2nd law, the block will accelerate.
  8. Mar 8, 2010 #7
    The normal force for the block is different then the normal force for you.

    Lets say your weight is 100 pounds, and you push on a 1 pound block with 2 pounds of force.

    2 pounds of force is exerted on you as well, but since you are 100 pounds and friction for a heavier object is much greater... then you are not going to move.

    Now do the opposite experiment. You a 100 pound guy... go push on a 10000 pound object... you will be the one that moves, if that object does not have wheels :)

    So equal and opposite forces are applied between you and the object, but your frictional force with the ground is greater since the object in question is small, so you do not move... but the lesser frictional force of the object allows you to overcome this, and turn the rest of your force into motion.
  9. Mar 8, 2010 #8
    This is the simple concept to understand really... since you know you do not move, something is keeping you stationary. What is bizarre is when you are NOT on earth, or in a freefall senario like the vomet comet airplane.

    If you toss that same pound ball now... you move backward at a slower rate, but you do move backward.
  10. Mar 8, 2010 #9

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    Newton's second law says absolutely nothing about needing two objects interacting to produce a force. In fact, Newton's second law doesn't say very much about physics, period. It instead defines net force in terms of acceleration (better yet, in terms of change in momentum. Newton's second law is [itex]F=dp/dt[/itex], which simplifies to F=ma in the case of constant mass.) Implicit in that law is that forces are subject to the superposition principle.

    The second law just says something about the mathematics we use to describe the universe. In comparison, Newton's third law is a true physical law. The latter law says something about the universe (e.g., to have a force at least two objects have to interact). This is a scientifically testable statement. Newton's second law, in and of itself, is not testable. It is definitional.

    Newton's third law says a lot about the universe. The weak form of Newton's third law is equivalent to conservation of linear momentum; the strong form adds conservation of angular momentum and conservation of energy to the mix. That is saying a lot. Newton's second law says nothing regarding those conservation laws. Lift oneself by ones bootstraps? No problem per Newton's second law. Big problem per Newton's third law.

    You are not using Newton's second law here. Newton's second law says absolutely nothing about specific forces such as gravity or electrical attraction. You need Newton's law of gravitation and Coulomb's law to calculate those forces -- and those two laws were explicitly formulated with Newton's third law in mind.
  11. Mar 8, 2010 #10
    what happen when the object moving at the universe ?what law we can used?
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