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Quick stupid question.

  1. May 17, 2008 #1
    if something is moving at a wall with a given velocity. and is in contact with the wall for say 0.2 seconds.

    the chance in velocity for those 0.2 seconds would be (-) the velocity it hit it win?
  2. jcsd
  3. May 17, 2008 #2

    Could you perhaps try again?
  4. May 17, 2008 #3

    ignore resistances of air and things

    if something is moving at a wall with a given velocity. and is in contact with the wall for say 0.2 seconds.

    the change in velocity for those the time it is in contact will the wall, would be (-) the velocity it hit it with?

    there's no point in being so picky when you know exactly what i mean
    Last edited: May 17, 2008
  5. May 17, 2008 #4


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    Hey … DeanBH … I didn't understand it either! :frown:

    Still not sure I do … the 0.2 seconds seems irrelevant … :confused:

    If the ball just squished against the walll and stopped dead, then the change in velocity would be (-) the velocity it hit it with.

    If the collision was perfectly elastic, the change in velocity would be twice that.
  6. May 17, 2008 #5


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    In reality, almost nothing (aside from maybe a sandbag or a lead-filled snowshoe) will just stop dead with no rebound. Also, even those things would gain an immediate downward velocity due to gravity unless the wall is out in space somewhere.
  7. May 19, 2008 #6
    The ball is thrown into the wall a specific angle, leaving the thrower's hands with a specific amount of force, traveling in an arc-like path to hit a wall, thus causing some of the energy to dissipate as sound and heat, and bouncing off the wall at a downward angle.

    You gave a single number: 0.2 s. which defines how long the ball was in contact with the wall.

    That number has absolutely no significance, whatsoever. All you've done here is give the image of a ball hitting a wall, here.
  8. May 22, 2008 #7
    The change in velocity is called the acceleration. Say your object moves from left to right, then contracts the wall. The change in velocity, or acceleration is (-), as you say is in the other direction, from right to left. I think this is what you've been asking about.
  9. May 25, 2008 #8
    Just think about it this way. [tex]\Delta V=V_{final}-V_{initial}[/tex] so, if the initial velocity is 5 m/s and it comes to a dead stop, [tex]\Delta V=-5m/s[/tex]. Assuming no mass loss, [tex]\Delta \vec{P}=m\Delta \vec{V}=\vec{F}\cdot t[/tex]

    When doing impulse or the acceleration, that is when the time matters.
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