14 SERVO 04.2014
axis defined by a shaft protruding
from the opening in the plenum top
— the axis of rotation would be
perpendicular to the top surface of
the plenum top. The spinner would
cause damage with some instruments
of destruction at the outer ends of
the spinning structure. Our proposed
kinetic spinner, however, had to deal
with an issue faced by most weapon
The essential conundrum of a
combat robot weapon is captured by
Newton’s Third Law of Motion. Often
summed up colloquially as “for every
action there is an equal and opposite
reaction,” the law is more precisely
stated as “when one body exerts a
force on a second body, the second
body simultaneously exerts a force
equal in magnitude and opposite in
direction to that of the first body.”
The law is perhaps most efficiently
described by a force diagram.
So, why does this present a
problem for combat robots? Because
if your spinning kinetic weapon or
hammer or flipper imparts a
magnitude of force great enough to
cause grievous bodily harm to your
opponent, guess what? An equal
magnitude of force is coming right
back at your robot.
A classic example of an
exaggerated Hollywood version of
this phenomenon is the oft-repeated
scene where a character shoots a
gun and either the recoil from the
weapon sends the shooter flying
backwards, or the hapless target is
flung about by the impact from the
projectile. Based on the most
simplistic statement of the third law,
this seems consistent. A bullet has a
ton of force, right? Yes, but the real
world is not as simplistic and one-dimensional as a force diagram.
The Hollywood scene is not
replicated in real life because the
force is transferred in myriad ways
instead of solely along the same axis
upon which the projectile was
traveling. To use the less graphic half
of the gun and bullet example,
consider the recoil of the firearm.
The recoil is the example of the third
THE CURRENT STATE OF AFFAIRS.
HOW THE WEIGHT SHOULD LOOK.
THE SCARS OF BATTLE.
THE SPINNER IN PROFILE.