This is an interesting article that might help us all out at some point and time. Shoot straight! Deer String Jumping Analysis String jumping is a race of physics between the speed of the arrow versus the speed of sound with the finish line being the deer in your sights combined with their alertness and reactionary times before point of impact. To break down and analyze this race and reactions, there several conditions and contributing elements to consider. The largest factors are the speed of the arrow, speed of sound and deers reaction timing. Breaking down the two factors of arrow speed and sound will predict which will hit first. Most people think of and consider the speed of sound to be a constant when using it for calculations. Here I consider the dynamic elements of the speed of sound which may increase or decrease the speed at which the sound travels. The two main dynamic elements which contribute to the speed of sound are temperature and humidity; I chose to include these as dynamic since they may vary greatly depending on location and season of the hunt. Both temperature and humidity can effect the density of the air, the density is what the arrow/sound needs to overcome to travel. Density is the ratio of mass to volume, most commonly people refer to this as thickness such as caramel is thicker than water. Imagine now shooting an arrow through a pool of water versus a pool of caramel and you can imagine how density will affect the speed of an object. The effects of temperature are easy enough to incorporate into the equation by use of the ideal gas law (PV=nRT) where T is temperature. Humidity on the other hand is a bit challenging to calculate. Most people when they cite humidity are actually referring to the relative humidity in the atmosphere (RH). Relative humidity is a ratio of the actual humidity in the air to the humidity required to saturate the air. Humidity, despite making you feel like you can cut the air with a knife, actually lowers the density of air. Any baseball player or golfer will tell you that the ball flies better on hot humid days than any other. This is because the heat and humidity have lowered the density of the air, the ball has less resistance and thus flies faster and carries farther. The air is comprised of mostly Nitrogen with a molecular weight of 28, oxygen is next with a weight of 32 and lastly argon which weighs 40. Humidity is nothing more than water in the air, water has a molecular weight of 18 therefore when water is in the air in the form of humidity it brings down the average molecular weight of the air. Remembering that density is ratio of mass to volume, and volume remains the same, the effect of lowering the average weight is a decrease in the density. Relative humidity as described earlier is the most common term that humidity is expressed in, however this does not tell us the fraction of the air that is actually water. To figure out the net effect of humidity on the speed of sound we need to know the actual fraction of the air that is water and to do this we use what is known as vapor pressure. Vapor pressure is an indication of the evaporation rate of a liquid. Water will continue to evaporate into the air until the vapor pressure threshold is surpassed. Once the vapor pressure is exceeded water cannot be evaporated any longer and will form as condensation or dew. Vapor pressure is a complicated function that is dependent on the temperature of the air, different temperature climates will have different capacity for absorbing water. Have you ever heard someone say wow, its so cold and sticky out there out there, no, you hear its cold and sharp, the air cuts you like a knife or its so hot and sticky like a sauna! Once the vapor pressure is determined for the temperature of interest the next step of the analysis is determining the water fraction in the air from the relative humidity, atmospheric pressure and the vapor pressure. Now armed with how much water is actually in the air and knowing that the molecular weight of water is 18 we can determine the new weight of the air which results in a lower density and thus an increase in our speed of sound. These graphs (see attached photos) will show the increase in speed due to temperature alone holding the humidity to be constant and then due to the humidity holding the temperature constant. Now with the ability to see the effects of our environmental parameters on the speed at which the sound will travel we can plug in the yardage from the shot to the target and determine just how sooner the sound of the shot will reach the deer prior to the arrow reaching the deer. Dividing our distance by the speed of travel we will determine how long after the shot the deer will hear the shot; doing likewise for the speed of the arrow will tell us the time at which the arrow will reach the deer. Subtracting these two numbers will provides us with the means of all this work, what is the offset between the deer hearing your shot versus feeling your arrow! For a 25 yard shot on a 70 degree day with 45% relative humidity and a bow shooting arrows at 300 ft/s. The speed of sound in dry air is 1129 ft/s versus 1131 ft/s considering the humidity. This results in the sounds of your shot reaching the deer approximately 0.18 seconds faster than the arrow. THE MILLION DOLLAR QUESTION: is 0.18 seconds significant enough that a deer would be able to react and jump enough to ruin your perfectly placed shot? See the link below where a gentlemen determined by video taping the deer jump and was able to extract the time from release to the movement of the deer. The deer shown was already dropping at 0.075 seconds and by 0.125 seconds had moved 8 inches from the original position! In conclusion I would say that string jumping is a valid concern at any yardage and deer do have the ability to string jump. Deer will drop as much as 1 1/2 feet and in some cases, chest cavity hitting the ground prior to eventually taking off running. An ultra quiet bow, aiming relatively lower than normal and reading the deers body language are key factors in putting all these puzzle pieces together and being successful in avoiding a miss due to a deer string jumping your shot. bowsite/BOWSITE/features/articles/deer/stringjumping/
Posted on: Fri, 04 Oct 2013 15:44:50 +0000
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