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Can you explain this? Hunting physics

  1. Feb 6, 2010 #1
    I have heard about people bow hunting whitetail deer where a perfectly placed shot can turn into a complete miss due to the deer's reaction. When I heard this, I thought it was impossible. But I found a clip on the internet showing a step by step shot missing a deer. Here's my basic thinking- a bow may shoot over 360 ft/s, but just say 350 ft/s. Say a shot is placed from 25 yds. away. Take away time for sound to travel from when the arrow is released, minus the deer's reaction time, I don't see how it's possible. The deer will drop enough so that the arrow will shoot over the deer completely. Look at the link and explain


    just curious....

  2. jcsd
  3. Feb 6, 2010 #2

    Doug Huffman

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    I don't believe that sound is the only cue. There is a reason that prey/grazers' eyes are placed as they are. We enjoy binocular vision and a relatively narrow field of peripheral vision and we are not prey depending on subliminal cues for survival.
  4. Feb 6, 2010 #3


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    Reading that article is like learning a new language...:bugeye:

    So you're suggesting that the deer saw movement, either the arrow coming or the bow release, giving it time to react.

    At 25yds, a visual cue has a good 1/5th of a second over a sound cue. I can see that.
  5. Feb 6, 2010 #4
    22.86 m / 106.7 ms^-1 = 0.214 s. But the article claims it reacts after just 0.020 s. Taking the speed of sound as 343.4 m/s, I get 0.066 s for the sound to reach the deer.

    Possible explanations: the estimate of 0.020 s (how was it made, could it have been off by a little over a factor of 3?), the deer reacted to the shot by sight, the deer reacted to some suspicious noise just before the shot or whatever triggered the timer, speed of sound (how much could air conditions have caused it to differ from this estimate? I'm thinking not enough, but I don't know...), rounding off (I used Wolfram Alpha for the unit conversion).
  6. Feb 6, 2010 #5

    Doug Huffman

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    Sorry, got distracted. Mammalian reaction times are in the 100 msec range depending on the nerve length and processing time. Reflex arcs are shortest.

    I just had a nerve conduction test suite on my right shoulder and had numbers for velocity but ... Wikipedia says 30 - 120 m s^-1 in a cat spinal motor neuron.
  7. Feb 6, 2010 #6
    It must be some other trigger than sound. It said the deer dropped 8"s in less than 1/8 second. That is faster than gravity, so it would seem impossible.
  8. Feb 6, 2010 #7


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    First of all, gravity is an acceleration, not a speed, so it's technically meaningless to say "faster than gravity." And besides, just the fact that the deer drops by more distance than a freefalling object released from rest would drop in the same time (which is what I think you meant) doesn't make it impossible. Things can definitely drop faster than they would under the influence of gravity alone, like a rocket pointed downward.

    But it is pretty fast.
  9. Feb 6, 2010 #8
    Clearly objects can be accelerated faster than gravity. However in order for that deer to get to the ground faster than a free falling object it would need to somehow pull itself down. I'll admit I don't know much about deer, but it seems unlikely to me that they would be able to pull themselves to the ground.

    Speed of arrow = 350 ft/sec = 106.68 m/s
    Speed of sound = 340.29 m/s
    Distance to deer = 25 yards = 22.86 m
    Distance deer dropped = 8 inches = 0.2032 m
    Time to free fall 8 inches = 0.2036 seconds

    0.067 seconds for sound.
    0.214 seconds for arrow.

    If the deer were to wait for the sound to reach it, then free fall it would take 0.27 seconds, so the arrow would hit it. Keep in mind this includes no time for a reaction time. Even using a visual cue it leaves only 0.01 seconds for reaction time to start dropping. Also I'd argue that the deer not only can not drop faster than free fall but likely drops at least slightly slower. The arrow does not have instant acceleration, but I don't know how to account for it, and I'm guessing it is quite fast.

    This means either some of the inputs are wrong (less than 8 inches, slower than 350 ft/sec), the deer reacted to something slightly before the actual arrow release, deer can react in 10 ms to a visual cue, or deer can somehow pull themselves down faster than simply dropping would allow.

    Personally I'd guess something alerted the deer just before the actual release.
  10. Feb 6, 2010 #9
    The hunter might not have been perfectly still right before the shot also. A lot of variables can come into play whilst hunting
  11. Feb 6, 2010 #10
    Animals usually have a very sensitive peripheral vision, so they can detect slightest of movements in the environment. (Their color vision may not be as high-tech as humans, although humans dont have that good a peripheral vision). I suspect that if that deer noticed any other thing that moved and seemingly coming towards it, it would have reacted the same way and ducked in preparation to run away.
  12. Feb 7, 2010 #11
    Thanks for the help. I just didn't think the deer could hear, react, and drop enough to make an arrow miss. But I hear different hunters saying this is the case. It just did not add up to me, but I thought I may be missing something obvious. Obviously there is something else at play.

    On a different/similiar note, I saw a show recently about a horse that could add. His owner would give him simple additions like 2 + 3, and the horse would stomp the ground with his front hoof 5 times. The crowd was always amazed, yet further research explained how this happened. The horse was actually able to perceive the slightest movement of muscles in the crowd. The horse simple pawed at the ground until the crowd reacted, then he simply stopped.

    Thanks for the replies. First time poster.
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