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When we r considering a ball that is thrown at 45 angle, with the air

  1. Dec 22, 2011 #1
    when we r considering a ball that is thrown at 45 angle, with the air drag present, why does it take longer to fall back after reaching its highest point???
     
  2. jcsd
  3. Dec 23, 2011 #2
    Re: projectile

    The air exerts a sort of friction force on the ball, taking away some of its mechanical energy and converting it into thermal energy.

    So as the ball travels along its trajectory, some of its kinetic energy is being converted into thermal energy. Hence the ball's velocity is decreasing, which means it takes longer to reach the ground than it did to reach its highest point.
     
  4. Dec 24, 2011 #3

    Drakkith

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    Re: projectile

    Consider a more extreme example. I throw a feather straight up. It takes about 1 second for it to reach maximum height, but several seconds to fall back down to the height I released it at.
     
  5. Dec 25, 2011 #4
    Re: projectile

    an explanation stated that when its going up, the air resistance is in the same direction as the weight(mg) acting, and when its going down, the air resistance is in the opposite direction as that of the of the weight(mg)..thus in the second half of the motion (that is when its coming down) the speed is lesser..
    what i dont get is that when its going up shouldnt the speed be lesser since the resistance and weight BOTH r acting to oppose the motion.........?
     
  6. Dec 25, 2011 #5

    Danger

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    Re: projectile

    I think that the operative factor here is that upon descent the only forces acting upon the ball or bullet or whatever are gravity and air resistance. That same projectile typically undergoes several g's upon launch.
     
  7. Dec 25, 2011 #6

    phinds

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    Re: projectile

    So you reckon that if it was happening in a vacuum it would take longer to come down than to go up? And that it would git the earth will less G force than that with which it was launched? It certainly SOUNDS like you believe that.
     
  8. Dec 25, 2011 #7

    Danger

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    'No' and 'yes', in that order. (I assume that the last sentence is supposed to have the words 'hit' and 'with' in it.)
    As I pointed out, one of the factors is air resistance. The vacuum state would eliminate that. In normality, though, the object can be accelerated at any rate that doesn't achieve escape speed, but will always cease accelerating upon return when it hits terminal speed. Of course, the initial acceleration ends upon release from the launcher whereas gravity is constant, but the launcher power is ungoverned within engineering limits.
     
  9. Dec 25, 2011 #8

    phinds

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    Re: projectile

    I'm clearly missing something since it appears that you are contradicting yourself. You say that air resistance / terminal velocity is why it would not hit the ground (in an atmosphere) with the same G force with which it was ejected, and then you also say (rightly) that the vacuum state would eliminate that. Them implication to me of all that is that, clearly, it would hit the ground with exactly the same G force as that with which it was ejected, but you say that's not true. I don't follow your logic
     
  10. Dec 25, 2011 #9

    Danger

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    Okay... I can see where there might be some misunderstanding due to my choice of words.
    Try this:
    If you fire a rifle straight up, the bullet leaves the muzzle at several thousand g's. When it reaches the apex of it's trajectory, it essentially stops and begins to fall (not exactly, of course, but close enough). On the way back down, it accelerates at 1 g. The terminal speed that it reaches will be nowhere near the muzzle velocity of the firearm.
     
  11. Dec 25, 2011 #10

    phinds

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    Re: projectile

    NO, by your definition, we are talking here about a VACUUM. There is no terminal velocity Why would the bullet not have exactly the same velocity when it comes back as when it left?
     
  12. Dec 25, 2011 #11

    Danger

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    I very specifically pointed out that I am not referring to a vacuum. You, in fact, were the one who introduced that concept. I'm dealing with real-life situations, in which a vacuum plays no part. I have no idea of how you got so turned around on this.
     
  13. Dec 25, 2011 #12

    phinds

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    Re: projectile

    OOPS. You're right. Don't know how I got it in my mind that YOU brought up vacuum when clearly I did that.
     
  14. Dec 25, 2011 #13

    Danger

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    Re: projectile

    No sweat, pal. :smile:
     
  15. Dec 26, 2011 #14
    Re: projectile

    why several 1000 g's in the beginning.....................??
     
  16. Dec 26, 2011 #15

    Drakkith

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    Because it accelerates in the barrel from stationary to around 2000 ft per second or so in milliseconds.
     
  17. Dec 26, 2011 #16
    Re: projectile

    Don't forget that air resistance varies with velocity so it is much greater when the object is traveling very fast so it will slow the object very quickly at the beginning.
     
  18. Dec 26, 2011 #17

    phinds

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    Yes, you are correct, but it has no effect on the vertical component of velocity so really doesn't matter to this discussion.
     
  19. Dec 26, 2011 #18
    Re: projectile

    and a second quote Yes, you are correct, but it has no effect on the vertical component of velocity so really doesn't matter to this discussion.

    I'm posting information regarding the first question of the thread which concerned an object being thrown through the atmosphere rather than through a vacuum. I can only assume the second quote refers to a vacuum because most certainly air friction slows both components of the velocity as the ball moves through air as it slows the actual velocity. So my response is definitely relevant.

    the faster the object moves through the air, the more molecules it will hit and the more it will be slowed, lose energy to friction and so on. It will reach it's lowest upward velocity at the maximum height and from there, even though gravity is acting on it in the downward direction, air friction is acting on it in both the horizontal and vertical directions slowing it more and more in the horizontal direction and reducing the acceleration in the vertical direction. You can take video of the ball in the air and frame by frame it to see exactly what happens and to determine the velocity for any interval so you can see this is true. The horizontal velocity does not limit the time in the air though so we have to consider exactly what happens to the vertical velocity and assume the object falls the same distance vertically as it rose. ON the up trip both friction and gravity are acting downward and friction is the largest when the object is first thrown and you have friction in the horizontal direction but again this doesn't affect the time in the air, only how far horizontally the ball will travel. So greatest deceleration is at the beginning of the up trip but also greatest velocity. On the down trip gravity speeds the ball up and friction slows it. So to decide if the ball really does take longer to come down you have to decide if the velocity is totally symmetric in the vertical direction. The path is certainly not symmetric but what about the y velocity?
     
    Last edited: Dec 26, 2011
  20. Dec 27, 2011 #19

    phinds

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    Re: projectile

    netgypsy, you are obviously correct. I got sidetracked and was thinking that the entire conversation had been about horizontally fired bullets ... my bad.
     
  21. Jan 2, 2012 #20
    Re: projectile

    This question had not yet been satisfactorily been discussed.

    Consider two possible situations - one in which the launch velocity exceeds terminal velocity and one in which it does not.
     
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