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Newtons Third Law: Cause/Effect or Simultaneous?

  1. Jul 10, 2004 #1
    When object A and object B collide, where A is moving and B is stationary and where B seems to compress:

    Does B expand in response because a pre-existing internal force exists in B and exerts an equal and opposite reaction, or

    Is the entity motion transferred from A to B redirected back at A from B the equal and opposite force in return?

    If A and B were the exact mass and had a collinear collision and both went in equal and opposite directions, is this proof of internal forces that change the direction or a transfer of the entity motion?

    If A causes B to accelerate after the collision, is:
    1. the force of B exactly the same as before
    2. the force of B the same as before, but more force because of the new velocity, or
    3. the same force as before, but the new velocity is a new shape of the same medium in which the same value of force is expressed?

    I'll take any response.
    Last edited: Jul 10, 2004
  2. jcsd
  3. Jul 11, 2004 #2
    I have no idea what you are saying, but I'll state the third law in simple terms.

    Whenever a force is acted on B by A, a force equal in magnitude and opposite in direction is acted on A by B simultaneously.

    1. Forces must work in pairs. It's impossible for any force to exist on it's own.
    2. The action-reaction pairs must act on different objects, in different directions.

    Did it help?
  4. Jul 11, 2004 #3
    Let me try again.

    Assumption #1 I assume when an object accelerates, the object that it contacts is imparted with some 'physical entity' to it to make it move faster.

    Assumption #2 I also assume 'no physical entity' is imparted, and that no object has has anything extra because it's moving faster or has anything less because it's moving slower.

    After a baseball is hit, the baseball flys away from the bat because of energy given to it by the bat or the base ball accelerates because it's own equal and opposite energy is repsonding to itself?
  5. Jul 11, 2004 #4

    What you are asking may seem simple but it is, in reality, extremely complicated.
    The following is not an answer to your question but a general comment of the subject matter.

    You question(s) bring to mind the concept of non-linear force propagation, meaning that a shock wave does not behave the same as a linear "push"
    A gentle "push" of one object to another obey's classical linear laws of force transfer. However, a sudden impact does not.
    The force propagation of a sudden impact is not linear, it is spherically expressed, and it's effects are not well understood.
  6. Jul 11, 2004 #5
    First, we must remember that in most of the experiments that we do, we assume the objects to be perfectly rigid, which is not the case in this one. The ball is compressed when being hit.
    Okay, back on topic. This is only a gues though, so feel free to correct it if you find anything wrong. The bat hits the ball. A lot of momentum is transferred to the ball. However, since only part of the ball is in contact of the bat, the other part has not yet felt a force to move forward(away from the bat). So, the part that touches the bat collides into the part that doesn't touch, and this causes the ball to be compressed. The momentum is then quickly distributed over the ball by the intermolecular forces within the ball, and soon every part of the ball is moving forward with the same velocity. The ball is hence expanded back into it's original shape and zooms away.
    So it's a transfer of energy from the bat, to "some of the ball's particles", and then "all of the ball's particles". The ball itself cannot "create energy", but it helps transfer momentum to "un-moving" particles to help create motion. Otherwise, the ball would tear apart.
  7. Jul 12, 2004 #6
    If I accelerate something and the mass of that thing is constant before and after the collision creating the acceleration, is an increase in velocity equivalent to a positive thing I am adding to the accelerated object's mass?

    Displacement of mass is obvious to everyone. Everyone has seen things move. But if the mass is constant before and after a collision and the velocity increase is proportionately to a decrease betweeen two colliding objects, isn't this displacement of velocity? Isn't velocity being displaced? All things of the physical world I sense are positive. Isn't then velocity a positive entity? What postive thing does velocity represent?
  8. Jul 13, 2004 #7
    I'm not sure what you're talking about. Perhaps you should as someone else...
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