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Effect of force on motion

  1. Nov 16, 2006 #1
    Hi all
    In newtonian mechanics we say that the 'force acting on an object dont affect the velocity of that object which is perpendicular to the force'
    Consequeces of this fact are like if I fire a bullet from my gun, and drop the same kind of bullet at the same time I fire , both the bullets will reach the ground simultaneously. Because vertical motion is not affected by horrizontal motion.
    There is one more example - If a ball is heading towards a wall with 2mt/sec.
    As it is 2 meters away from the wall, wind blows from sideways and deflects the direction of the ball; But still the ball would reach the wall with in the same time it would have reached without wind. This is all okay.
    Question: Doesnt this rule get violated if the speed of the wind is so high that it takes the ball along and ball never touches the wall

    Please help me understand this....thanks in anticipation

    Abhishek Jain
  2. jcsd
  3. Nov 16, 2006 #2


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    How fast does the wind have to be going to do that?
  4. Nov 16, 2006 #3
    Won't happen.
    The ball will always touch the wall.
    Greater wind speed just means that the ball travels a greater distance in the direction of the wind before it touches the wall.

    Imagine running down an endless corridor, and the walls on either side of you are closing in, they are accelerating towards each other, and will touch in twenty minutes. It doesn't matter how hard you run, how fast you run, how far you get - the walls will still crush you in exactly twenty minutes. I think I have issues...
    Last edited: Nov 16, 2006
  5. Nov 16, 2006 #4


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    What am I missing? The wind exerts a sideways force on the ball. You have to consider ALL forces acting you can't pick and choose.
  6. Nov 16, 2006 #5
    Integral, the sideforce by the wind is the only force on the ball. What is your point? In any case, the ball can be travelling at an infinite sped sideways, but it's speed in the original direction of motion is unchanged, and it will thus hit the wall in the same amount of time as if the wind was not there.
    Unless you try and consider relativistic effects...
  7. Nov 16, 2006 #6
    hmmm... just a side thought, doesn't a rotating bullet generate lift? I don't know too much about aerodynamics so does anyone know the answer?
  8. Nov 16, 2006 #7
    Velocity is made up of two components perpendicular to each other. Lets say something is moving north-east. The velocity is made up of a North component and an East component. If a force starts acting on the object toward the west then the object's east velocity will reduce, and then go negative (so the velocity would be positive in the west direction) and eventually the object would be going north-west. But it's northward velocity component will not change at all (in a frictionless, ideal system).

    But of course that is in a frictionless system. In reality it would be somewhat different.

    As for the ball missing the wall entirely the rule isn't violated, it just missed the wall. The forward velocity component of the ball is uneffected. (Again, ideal).
  9. Nov 17, 2006 #8
    I would have thought that the lift could be up or down on a rotating bullet because it depends on the wind speed direction across the bullet. Anyway, here is a link that should explain most things:

  10. Nov 17, 2006 #9


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    If the wind exerts a sideways force, then you must be considering air resistance. There is then also a force oposing the forward motion of the ball. So there is no guarentee that it will reach the wall in the real world.
  11. Nov 17, 2006 #10
    Reaching the wall is just a way of signifying forward velocity. But other then that Integral is 100% correct. But note he says in the real world. In most of the situations presented in highschool physics (frictionless) a force acting on the ball from the side would not affect the front at all.
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