Back in mid-November, I posted to my 180 blog about the classic monkey and hunter demonstration. In that post I referenced a conceptual explanation as to why the hunter should aim directly at the monkey. Andy asked me to share the conceptual explanation and I’m finally taking time to do so.

For many years the best conceptual explanation I could offer was based on that of a student who came up with the following after seeing the demonstration. Imagine there is no gravity. The angle is such that in the time it takes the projectile to move horizontally, it will move the necessary vertical distance to hit the monkey. The effect of adding back gravity just adds the part of the equation which is the same for the monkey and the projectile.

This year, I developed an alternative conceptual explanation. Put yourself in the frame of reference of the monkey. The difference in the vertical component of the velocity between the monkey and the projectile is the same and will remain the same due to the acceleration of gravity.
Therefore, the projectile has a constant velocity and, if aimed directly at the monkey, will move in a straight line toward the monkey.

I received a GoPro for Christmas and plan to use it to film this demonstration from the perspective of the monkey.

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Andy FitzThanks for the follow up. I get the constant relative velocity idea (especially if you use a v-t graph). Perhaps the GoPro viewpoint will help me wrap my mind around the straight line idea. I wonder if there are people out there who could animate this, or even make it an interactive simulation.