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Help end an argument in regards to perfect release mechanisms

  1. Oct 2, 2007 #1
    A friend and i have been arguing for weeks initially starting when we were driving in hisi car and i had a small american football, i contended that if i were to throw it out the car window, at the point of release it would be doing the speed of the car + the speed of my throw relative to the ground and would travel forward a few meters due to the previous force applied through the car/hand into the ball,

    Is this the case? My friend replied that in a "perfect release mechanism" the moment the ball is released from a moving yet perfect release mechanism all forces on it are nullified instantly and it would drop perfectly straight down from the exact point of perfect release, my contension is that due to the release mechanism be it perfect or imperfect being in motion at the time of release that force will be transfered into the ball relative to the ground?

    I accept his argument that the imperfect release will affect thing such as trajectory, but he also seems to be arguing that this perfect release nullifies the speed the ball had been traveling at in the instant of release?

    Can someone please settle this once and for all?

    I am i completely wrong?
  2. jcsd
  3. Oct 2, 2007 #2
    The ball will still be moving when it is released from the car window, it will have a velocity equal to the velocity applied by the car+the velocity applied by you.

    When it is released it will be immediately struck by air resistance, which will have a decellerative force which is related too the shape of the ball and the speed at which it is moving. EDIT: However the decelleration will not cause a sudden "stopping" of the ball, it will just decellerate it.

    If you want a counter argument to your friends suggestion, suggest to him that during a plane crash, instead of using a parachute to decelerate you, you keep on the plane, a moment before the plane hits the ground, jump off the plane, from his theory, the momentum would be "Nullified" immediately (by some sort of pseudo-super-force since it would be acting in an infitely small time gap) and he would land safely, or alternatively, tell him to jump off anything that is giving him motion (such as his car, or a train) and he should, according to his theory, stop and be able to probably land on his feet.

    I can produce some of the maths if you want...

    EDIT (Maths):

    Acceleration and decelleration are usually described in alot of ways, but one of the common ones is:
    [tex] F=ma [/tex]

    Where: F is force, m is mass and a is acceleration or decelleration, rearranging gives

    [tex] a = \frac{F}{m} [/tex]

    This implies that a force "F" is required to decellerate a mass "m "by decelleration "a".

    Force is equal too the rate of change of momentum, momentum change divided by time.

    [tex] F = \frac{mv}{t} [/tex]

    it follows that, if your friends theory of a ball losing all its forward velocity is correct, the velocity of the ball must go from:

    [tex] V_{total} = V_{car}+V_{throw} [/tex]

    to zero, this would require a decelleration of infinity, dont be confused in thinking that if the decceleration is equal to the speed the speed will negate immediately, it will still take a second, an infinite decelleration would, in turn, requre an infinite force, which would require (using the rate of change of momentum) and infitely small amount of time.
    Last edited: Oct 2, 2007
  4. Oct 2, 2007 #3
    Yes please, my friend is using the mehtod of argument that He has done A-level physics and i have not so essentially whatever i say is rubbish, i suppose the only way for me to come back to th is argument is with hard facts, and yes your example of the plane is essentially my argument summed up.

    If a "Perfect release mechanism" was possible in the real world surely the ball even at the moment of a perfect release (when the whole mechanism ie mounted in a plane or car and moving it'self) still has a momentum, yes it will decelerate rapidly but not instantly... I was saying where does this force go, from what i understand forces cannot be stopped in that manor, only diverted or transfered elsewhere...
  5. Oct 2, 2007 #4
    The decellerative force must be applied, by air, by carbrakes, the energy is usually changed into thermal energy in most of those cases. As the ball flys through the air after release, air will strike the ball, decellerating it, the decelleration of the ball implied that its kinetic energy, shown by:

    [tex] E=\frac{1}{2}mv^{2} [/tex]

    Will have to go somewhere, some of it might be transferred as kinetic energy to the particles in the air, if the particles in the air have more kinetic energy, their temperature will go up (but this temperature increase will dissipate through the atmosphere causing a completely unnoticable temperature change) the energy can also turn into sound, when the ball hits the ground its strike can cause compression waves in the air (which contain kinetic energy) causing "sound" and the energy the sound contains is carried away very quickly.

    When the ball hits the ground, it will probably bounce also, due to the compression and decompression of gases within it, this bouncing isnt a harmonic occilation though, the ball will eventually stop bouncing as its remaining kinetic energy is transferred to the air, or as sound.
  6. Oct 3, 2007 #5
    Thank you for taking the time to help me AbedeuS, as a result my friend conceded and we can finally put this long running argument to rest.

    I bid you a good day sir.
  7. Oct 4, 2007 #6


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    Well, you could have just whapped him with a stick, but AbedeuS' explanation is more elegant.
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