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Newton's third law and air resitance?

  1. Oct 2, 2009 #1
    Hello,

    Few questions about these things.This is not homework!!

    1. Is Newton's 3rd law caused by conservation of momentum?
    2. If I drop a stone into a pond. The stone would exert a force on the water and the water would exert a force on the stone. So why doesn't the stone move up!!
    3. Is air resistance caused by collisions with air particles. How come it takes time neutralize the force. I mean if I hit a air molecule with 10N force it would exert 10N back. So If I threw something at 10N why wouldn't it immediately stop after hitting an air molecule.

    I know these questions demonstrate a very poor understanding but please help me so I can improve!!Thank you!! :smile:
     
  2. jcsd
  3. Oct 2, 2009 #2

    Doc Al

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    Think of them as two sides of the same coin. (Conservation of momentum is the more fundamental.)
    The force of the water is not the only force acting on the stone, and that force does cause it to slow down.
    When you say you "threw something at 10N", I presume you mean you exerted a force of 10N on something. (And while you were exerting that force on it, it was exerting the same force back on you.) But that does not mean that it exerts a force of 10N on the air.
     
  4. Oct 2, 2009 #3
    Thanks a lot Doc Al I'm a bit more clear now :smile: In the third question if I throw a ball at 10 N why wouldn't it hit the air molecule at 10 N. What causes it to be less.

    When I hit a ball onto a concrete floor why does the ball move back. Is it due to absense of resistance forces and the ball moves back as the reaction force.Is this a clear example of newton's third law.

    Also in the stone question if a stone falls at 10N (due to gravity). Does the earth under the water move towards the stone as the reaction force. Why does it only move a bit and how come the ball doesn't move up because of this. Thanks a lot for help so far :smile:
     
  5. Oct 2, 2009 #4

    Doc Al

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    The force that your hand exerts on the ball while you're throwing it has nothing to do with the force that the ball exerts on an air molecule.

    The ball bounces back because of the force that the floor exerts on it. I don't see this as having anything special to do with Newton's 3rd law.

    The earth pulls on the stone with a gravitational force of 10N, thus the stone pulls up on the earth with an equal force. The acceleration of the stone is about 9.8 m/s^2, but since the earth is so incredibly massive, its acceleration due to the stone's upward pull would not be noticable. For all practical purposes, the earth doesn't move.

    What determines the motion of the stone is the net force on the stone, which is a combination of gravity and the force from the water.
     
  6. Oct 2, 2009 #5
    Thanks a lot Doc Al. I only have one question left :smile:


    if I throw a ball at 10 N and someone stands on the way he will get hit with 10 N

    Why not same with air molecule?
     
  7. Oct 2, 2009 #6

    A.T.

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    Not really. The force you use to accelerate the ball can be very different from the force that guy in the way uses to deaccelerate it again. The two acceleration times are not the same in general.
     
  8. Oct 2, 2009 #7
    Please explain further I would really appreciate. Thanks :smile:
     
  9. Oct 2, 2009 #8

    A.T.

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    You can accelerate the ball slowly with little force or quickly with more force, achieving the same ball speed and force on impact. The force on impact is independent of the force used to accelerate the ball.
     
  10. Oct 2, 2009 #9

    Doc Al

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    That's not true.

    Let's say you exert some force F on the ball for some time, thus giving the ball some speed as it leaves your hand. When that ball hits something (whether a person or an air molecule) the force exerted depends on the nature of the collision. Let's say the ball hits a giant soft pillow versus a hard steel wall. Which do you think will involve the greater force being exerted on the ball? (And note that the force exerted during the collision has nothing to do with whatever force was used to accelerate the ball to its initial speed.)
     
  11. Oct 2, 2009 #10
    Thank you all but I'm sorry I still don't understand. Can you provide a basic example through maths and formulas.

    If I throw a ball at 10 N inside a vaccum would the person on the way get hit with 10 N force.

    Why would larger mass recieve force differently.

    My question is really how air resistance is formed.

    This is my understanding is this wrong.

    Air resistance is the reaction force in an collison with an air molecule. If I hit an air molecule with 10 N it would hit me back at 10 N and that is air resistance!!
     
  12. Oct 2, 2009 #11

    arildno

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    sameera:

    Force is the amount of momentum transfer per time.

    Thus, if the same amount of momentum is transferred to objects A and B, but the process goes more slowly in the case of B than in A, the average force acting on B is less than the average force acting on A.

    Thus, you shouldn't think of a "force amount" attached to some object independent of interactions, "momentum amount" is the appropriate quantity!

    If I throw a ball at a hard metal wall, then the time by which the ball decelerates to the speed of the wall (or, the wall accelerates to the same speed as the ball), that time is way shorter than if I threw my ball on a stretched fabric of linen.

    Therefore, the forces involved in the first momentum exchange is a lot larger than the forces involved in the linen example.
     
  13. Oct 2, 2009 #12

    Doc Al

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    No. Why do you think that?
    IF you could hit an air molecule with 10 N of force, then yes it would hit you back with 10 N of force. But that's a big IF! :wink: The force that the ball/air molecule exert on each other is tiny.

    Try this experiment: Punch a heavy bag as hard as you can. See how it feels. Then hang a piece of tissue paper in the air from a string. Punch it as hard as you can. Feel any difference?
     
  14. Oct 2, 2009 #13
    Thank you Aldrino but I don't get it yet. Sorry I know all of you have been trying.

    I think if you can teach me with an example it would be good. I'll make one

    A ball is thrown at 20 N force. It's mass is 2 Kg and acceleration is 10 ms/s

    Then it hits an air molecule 1 Kg(just to make it easier) with a speed of 10m/s . The collision takes 2 seconds. What would be the force exerted by the air on the ball. Can you teach me this concept with this question. I just made this up. Thank you!!
     
    Last edited: Oct 2, 2009
  15. Oct 2, 2009 #14
    Air resistance is not only caused by the fact that an object hits air molecules. It is also caused by the viscocity, the tail tubulence etc..
    OK, I agree that if you throw an object with 10N force, a man on the way can be hit at nearly 10N, because the mass of him is much larger than the object and he is considered unmoved. But the object you throw should be in the macro scale, say , about 100 - 200 gr or so. With the time you exert the force, you can calculate the average velocity of the object. And now with that velociity, don't you expect that an air molecule will be hit by 10N? . No, because the mass of a molecule is too small compared to the object.
     
  16. Oct 2, 2009 #15

    russ_watters

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    Lets say the ball is in contact with your hand for 0.5 seconds when you throw it and the ball weighs 0.1 kg. When it hits something, it decelerates in 0.01 seconds.

    a=f/m=10/.1=100m/s/s
    100m/s/s*.5s = 50m/s
    The ball leaves your hand at 50m/s (impressive, yes).

    50/.01=5000 m/s/s
    5000m/s/s*.1kg=500N

    Note that one of your big misunderstandings here is that force doesn't cause velocity, it causes acceleration. Accelration is a time rate of change in velocity. Regarding the water example, dropping a ball in water does accelerate the ball upwards.
    Sure, but air molecules are small and so you can't hit an air molecule with a 10N force. Consider a cluster of billiards balls. Sweeping your hand through the air is like sweeping your hand through that cluster of billiards balls. You push them out of the way at about whatever speed your hand is moving. But the air molecules are much lighter than billiards balls so the force you apply to the air molecules is much smaller. Your arm has a limited range of speeds it can move at, so the limiting factor in the force of the interaction isn't the speed or acceleration of your arm, it is the mass of the air molecules: you can't move your arm fast enough to apply 10 N of force to an air molecule.
     
  17. Oct 2, 2009 #16

    A.T.

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    That "throw a ball at 10 N" means nothing, if you don't say for how long or over which distance you apply 10N to the ball, and what the mass of the ball is. Even then, to know the impact force you would also need to know how deformable the ball is, which affects how long it needs to decelerate.
    While in general you don't have a constant force during a collision, for a single time point this is right. But it doesn't mean that that molecule was previously accelerated by the same force.
     
  18. Oct 2, 2009 #17
    I think this is sameeralord's biggest misunderstanding. When you speak of moving objects, force is applied over space and over time, and it is crucial to mention the interval over which a force is applied, not just the amount of force applied.

    For example, if you are in space and you tell me you applied a force of 100,000 N to an object, I would be impressed. Until I find out that you did so over a length of only 0.00001 meters! That ends up giving it 1 Joule of kinetic energy.

    I would tell you that I could apply merely 1 N of force to that same object but instead do so over 2 meters, and that object would move with twice as much kinetic energy!

    The interval matters!
     
  19. Oct 2, 2009 #18
    1. If I apply a force of 10 N to a ball inside a vaccum my understanding is ball would keep travelling at 10 N unless acted upon by an external force. Isn't this newton's first law? If I throw 10 N force for 1 m where does the force dissapear in the absence of resistance forces. Where does it go? At this point does the object reach terminal velocity?

    2. When I hit an air molecule is it less than 10 N force? Why is Russ Waters equation show 500 N. Thanks for your example and response though it was helpful.
     
    Last edited: Oct 2, 2009
  20. Oct 2, 2009 #19

    russ_watters

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    Force isn't speed, force is force. Newtons are a unit of force. Newton's first law is about speed.
     
  21. Oct 2, 2009 #20
    Ok I understand the answer for first question.

    2. When I hit an air molecule is it less than 10 N force? Why is Russ Waters equation show 500 N. Thanks for your example and response though it was helpful.

    This is what I have trouble with. How did it become 500 N. How long does it take a ball to decelerate when hit by an air molecule. Wait it is impossible for the ball to decelerate when hit by one air molecule right it must be hit by heaps of air molecules. Did I get it. Did you get a decelerating time for the 500N question because it was a person who got hit. An air molecule would move away with any speed but a cluster of air molecules would take more. Or this is simply pushing a large object and small object right. A large object needs more force. Am I right?
     
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