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Is Newton's 3rd Law valid here?

  1. Jan 22, 2010 #1
    Let us assume a bird to be moving in a straight line with a velocity 'u' and a man to be chasing it with a velocity 'v' in the same direction. If v>u in magnitude, then the man will observe the bird to be moving towards him with a velocity 'v-u' in magnitude. However, the bird will still be flapping it's wings towards him.
    Now as he is moving with a constant velocity (or in other words, as he is in an inertial frame of reference), he should find Newton's laws of motion to be valid. But from his point of view, the action of the wings of the bird and the reaction of the air on the bird appear to be in the same direction. Isn't this against Newton's 3rd law of motion?
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  3. Jan 22, 2010 #2


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    How do you perceive the 3rd law? I'm confused as to your scenario. And your interpretation of 'equal force in opposite direction' to the force being applied..
  4. Jan 22, 2010 #3


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    Ok let me try to understand this..

    If the man runs towards the bird at a higher speed, he will outrun the bird. So now the bird is chasing after the man.

    If the bird is flying through the air, the force of the muscles flapping exerts the same force against the air through which the bird is flying. The air exerts same force on the bird, in opposite direction to the wings, thus creating lift, which is perpendicular to the bird.

    The man exerts force on the air as he is running against it, while the air exerts the same amount of force against the man creating drag, which is parallel to the man

    The man and the bird have nothing to do with each other, other than the fact that this air is exerting same force, of different magnitudes and directions, on a man and the bird
    Last edited: Jan 22, 2010
  5. Jan 22, 2010 #4
    The air reacting to the wings of the bird will be moving toward the 'chaser' at a faster velocity than the surrounding air which has not been disturbed. This indicates a force has been applied to it (more technically a very quick transfer of momentum, as velocity is constant) by the bird's wings.
  6. Jan 24, 2010 #5


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    Air resistance is proportional to the relative velocity difference between the bird's motion and the surrounding air.

    This is an invariant, irrespective of the velocity V the bird happens to have in some inertial observer system.
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