The Physics of Hitting the Bulls-eye: Lessons from a Hollywood Archery Master
A diagram of a traditional bow.
Courtesy of the Office of the State Archeologist, University of Iowa
(Inside Science) -- Archery is back in style with a little help from “The Hunger Games,” “Snow White and the Huntsman,” and “Brave” movies as well as the upcoming summer Olympic Games. I had an opportunity to interview Steve Ralphs, an archery master who trained Kevin Costner for his role in “Robin Hood: Prince of Thieves” and Keira Knightley for her role in “King Arthur,” who taught me about the archer’s paradox and what it means to do a “Robin Hood.”
How does a bow and arrow work?
“The curved inside of the bow [known as the belly] compresses, while the outside (back) of the bow stretches in order to keeping the bow in a neutral, stable position. The bow stores energy as the string is drawn. Upon releasing the string, the energy that the bow stored is imparted to the arrow and sends the missile on its journey.”
What is the archer’s paradox and how does it work?
“All arrows are measured [in terms of a quality called] 'spine,' which is how stiff an arrow is before it bends away from its original shape to a given amount over a given length, normally twenty-six inches. A soft arrow bends more than a stiff arrow. On release of the string, the end (nock) of the arrow starts to move a [fraction of a second] before the front (point) of the arrow. So we have the nock of the arrow starting to overtake the point of the arrow for an infinitesimal fraction of a second. Then, the arrow bends and begins to oscillate. We call this the archer’s paradox. It continues for about 4-5 oscillations and then, the arrow is past the paradox and is flying true.”
I’ve seen the scene in movies where an archer shoots an arrow which splits another arrow already in the center of a target. Is that really possible?
“It is called ‘Doing a Robin Hood' and it is possible. More so with modern equipment and a few archers can manage to replicate this feat on request. If an archer were to use more traditional tackle (archery equipment) it is a time for a big celebration if anyone in your shooting group manages it.”
Ralphs shared with me that one of the things that fascinates him the most about archery is the fact that you can never know it all and just when you think you do, some six-year old makes a shot you have never made yourself.
