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Basic kinematic physics equations do not make sense to me.

  1. Feb 21, 2010 #1
    If I drop a ball, how far will it fall in 2 seconds?
    ΔX = ViT + 1/2AT^2
    ΔX = (0)(2) + 1/2(9.8)(2)^2
    ΔX = 19.6 m

    I tested this, and there is no way an object falls 19.6 m in only 2 seconds. Think about it. Drop something right now and it will only fall like 1 meter in one second. I don't understand. These equations don't fit reality to me. Can someone show me what I am doing wrong?
     
  2. jcsd
  3. Feb 21, 2010 #2
    Yea make your test more accurate.
     
  4. Feb 21, 2010 #3
    The equations are fine.
    One answer is misperception. When the object lands it takes the brain at least 200ms before you register that the object hit the ground. And some more time before you hit the stop watch again.
     
  5. Feb 22, 2010 #4
    Don't "think about it", just go up 20 meters and drop something. See how long it takes, then come back with the results.

    You have to be willing to set aside what you "think" and be open to other possibilities.
     
  6. Feb 22, 2010 #5
    SUVAT equations don't take into account air resistance, so strictly an object will take 2 seconds to fall that distance in a vacuum.

    Also, given that the object accelerates at 9.8ms-2, after one second of falling it will have a velocity of 9.8ms-1, so it will be travelling quite quickly after two seconds and hence of covered quite a distance. I believe it makes more sense if you consider it that way.
     
  7. Feb 22, 2010 #6
    What is a SUVAT equation?
     
  8. Feb 22, 2010 #7

    QuantumPion

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    I certainly wouldn't want to free fall for 2 seconds. That's like a fall from a 5 story building...splat. 2 seconds is longer than you think. Try throwing a ball straight up in the air as high as you can and count how long it takes to fall back down.
     
  9. Feb 24, 2010 #8
    SUVAT equations are equations like you mentioned at the beginning of the thread, such as
    s = ut + 1/2at^2 , or v = u + at. They are named SUVAT equations because they all contain either the terms s, u, v, a or t.
     
  10. Feb 24, 2010 #9
    The equations are straight forward
    and very self explanatory, i don't see
    a problem with it...
     
  11. Feb 24, 2010 #10
    There is no problem with those equations. They reflect reality on a classical level just fine.

    Trust me an object dropped will travel much farther than a meter in one second. If you really want to test this out, try doing something compared to what Galileo did by having a ball roll down a ramp. You can much more easily time how far the ball has moved because the ball is moving with an acceleration smaller than g. You will find that the times you get will be in perfect harmony with those kinematic equations
     
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