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Effect of gravity on an object

  1. Oct 13, 2007 #1
    Hello,
    I am a chemist with a little physics background. I have a question that relates to bowhunting. I have sighted in my bow at ground level. However, I hunt from a treestand. So will the drop of the arrow be the same when shooting from a tree as it is when shooting from the ground. In other words, will the bow shoot higher or lower when shooting from a tree or will should it shoot the same as if shooting from the ground?
     
  2. jcsd
  3. Oct 14, 2007 #2
    The gravitational constant (9.8m/s/s) increases the velocity at which the arrow will drop over a time interval. It only affects the vertical component of the velocity and not the horizontal velocity of the projectile. Therefore, shooting a bow at an angle below the horizon (to aim at the prey) will have a vertical component that is not zero. In contrast, aiming with an angle of zero will posses a vertical initial velocity of zero. Therefore, the arrowing being shot with an initial vertical component (from a tree downward) will have a bigger drop than one shot from ground level (obviously because you intent to hit the prey below you).

    But your question is troubling. You must also compensate for shooting from ground level so even then the arrow will have a vertical component at launch. The arrow's path will form a parabola from ground level. In contrast the arrow would have a half parabola shape (like rolling a ball off a cliff). Although from the hunter's sights, it will appear to also have a parabolic shape. (from a tree).

    In this respect (from tree), gravity will actually accelerate the projectile not perpendicular to the horizon of your sights but forward and downward. Logically, I would compensate less from a tree.

    I have no experience in hunting and this is only my theory.
     
  4. Oct 14, 2007 #3
    my bow is sighted in to hit the target from the ground. So I have sights on the bow that allow me to shoot at 10, 20, and 30 yds at ground level. I have adjusted the bow to compensate for the drop of the arrow so that it hits in the center of the target at each of these distances.

    Now if I take this bow that is sighted in at ground level and shoot from 15 ft high in a tree at target that is 10 yds away from the base of the tree will it hit the same place as if I shot it from the ground? What if the target was 20 or 30 yds away? What if I was shooting from 20 or 30 ft high in a tree, will any of these variables change the results?
     
  5. Oct 15, 2007 #4
    I'm not 100%, but I think you'd need to change your sights a little. The longer it's in the air, the faster it will ultimately fall. Acceleration is a change in velocity with time, so the more time, the bigger change in velocity. I think the point is that when you're on the ground, you're shooting horizontally and are at the same height as the deer pretty much. The potential energy due to gravity is m*g*h, or mass*acceleration due to gravity*height. If you take the ground to be "zero height", then the potential energy will be smaller when you're on the ground than if you're an additional 15 feet in the air. If the deer were also 15 ft in the air, it wouldn't make a difference, but since it's on the ground, your arrow will have additional energy due to the change in potential from 15 ft to 0 ft. This doesn't even take into account the fact that you'll also be aiming downward, presumably, which will give it a vertical velocity aside from that which gravity leads to. I think your only saving grace might be that this arrow has a fast horizontal velocity and the difference in time that it's in the air is negligible. However, if you have different sights for 20 and 30 yards, this seems to say that this extra time is not negligible. Only one way to find out: get in a tree and shoot at a target and see if you still hit it!
     
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