The Physics Behind Bruce Lee’s One-Inch Punch! (Because Science w/ Kyle Hill)


– Today’s episode of Because Science is sponsored by Birth of the the Dragon. Birth of the Dragon hits
theaters everywhere August 25th. (karate yell) What is the most-famous martial arts move? In terms of sheer popularity, I’d argue that it is the one-inch punch. And I think it’s so popular
because it looks impossible. Like it’s generating too much force for how much space it has to do so. But it’s not impossible. It’s not magic. It’s not chi. It’s physics. (karate yell) The story goes that
the West was introduced to the one-inch punch
through a demonstration by the dragon himself at
the Long Beach International Karate Championships in
1964 and as you can see, dramatized in the upcoming
movie Birth of the Dragon, it hits confusingly hard. Here’s one slowed down. It’s a legitimately wow
move, so how does it work? The martial artist was
obviously in fantastic shape. But with only one inch to work with, it looks like all that power we see in the demonstrations has
to come from somewhere other than just arm muscles, and it does. Martial artists call this kinetic linking, but a physicist looking
at the one-inch punch would call it kinematic chains. Wait, wait. That is three inches. Should we tell someone? A kinematic chain is an
assembly of rigid bodies connected by joints in some way. Now think of the entire human body as an assembly of kinematic chains. What the dragon did and what
all good martial artists do is punch with more
than just their fist. It starts at the ground,
translating motion in feet and thighs into
the torso, into the arms and then all the way to the
end point, the final punch. In this way, they punch
with more kinetic energy than the fist would have otherwise. How much kinetic energy
a fist has upon impact will determine the impact force. Energy and force are
often used interchangeably when talking about fight physics, but they are not the same thing. Let’s start with work. Work is some force applied
over some distance. Now let’s add in Newton’s Second Law, that says force is equals
to mass times acceleration. Now let’s add in one of
the kinematic equations of motion, remember those chains? And finally, we expand the expression. Since work is also a
change in kinetic energy, you have to apply a
force over some distance to make something move, we end up with the equation
for kinetic energy. If the object starts at rest. So the faster I move my mass, the more kinetic energy it gets. Now that we know how the equation for kinetic energy works, we can explain where all the power behind
a one-inch punch comes from. Looking back at the
equation for kinetic energy, we can see that an object’s velocity can be much more important than its mass, because it’s a squared value. Secondly, we can see that force and energy are related, but not interchangeable. It depends on how you apply that Wait. It depends on how you apply that energy. I said wait. Rearranging values again in this equation, we can see that the more distance the change in energy occurs over, the less force is imparted. This is exactly why
cars have crumple zones. The further the crumple goes, the less force is applied to you, and that’s good. That’s why, to perform a
successful one-inch punch, you want to do the exact opposite, and bring all the kinetic energy that’s in a fist to a
stop in a short distance. Look at how short of a distance this fist covers before the punch is over. That increases the force. Another way to interpret this relationship is a change in momentum, which is mass times velocity over time. If you bring all of the momentum of a fist to a stop in as short
amount of time as possible, the force goes up. I think this makes more intuitive sense. The faster you hit, the harder you hit, and Lee emphasized incredible
speed in his fighting style. He was so fast, that some
trained martial artists couldn’t even block his punches. What happened? This is how a one-inch punch
can impart so much force by expertly minimizing
either distance covered, or time elapsed. But that’s not everything
behind the punch. (karate yell) Lee wasn’t a big dude. He was my size, but apparently,
he hit like a big dude. That’s because in both momentum and kinetic energy equations,
velocity is critical. And since I can’t change
the mass of my fist, if I hit as quickly as possible, I can hit like a guy twice my size. Form is a factor, too. If you didn’t have the concentration, training, and focus to perfectly align your kinematic chains,
force is gonna be lost. With perfect form, you are
minimizing the distance and time it takes to finish a punch, thereby increasing the force, but if your form is even
just a little bit off, you will increase the
time and the distance it takes to finish the punch, and become like a car’s crumple
zone, lowering the force. The final component of a
one-inch punch is psychology. Look, when you’re being
punched by the most famous martial artist in the world, or who would become the most famous, you’re gonna sell it a bit. There’s no way this guy is actually getting punched across the room. Reputation has something to do with it, and that’s kinda cool. So why is the one-inch punch so powerful? Well, it minimizes impact
distance and impact time, it takes advantage of body
positioning and kinematic chains, and it has a reputation behind it. It’s never gonna be as forceful as a giant haymaker
thrown by a giant fighter because physics, but when
performed by a master, it’s an incredibly efficient way to produce a surprising amount of force across almost no distance
in almost no time, because science. (groans) oh, that was, oh dude, that
was more than one inch. Thank you so much for watching, Joe. If you want even more silliness, check out my new show with
Dan Casey, my colleague, where we get very silly
about a very serious man, called Muskwatch and if
you want even more science, but a little bit more
premium, check out my show the S.P.A.A.C.E. Program
on ProjectAlpha.com if you subscribe to that,
you can get that show and this show two days earlier
than you otherwise would. Thanks for watching, I already said that. Thank you, though. And thanks again to Birth of the Dragon for sponsoring today’s episode. The movie is inspired by true events. To see the one-inch
punch on the big screen, check out Birth of the Dragon
in theaters August 25th, no cameras, no rules, no limits. Come on, oh, oh, might
fly out of your hands and hit you right in the eye. Okay, hold it, come closer. Alright, alright, one-inch punch, using everything that we- – [Woman] Are we rolling? – Yes. Everything that we just learned kinet, kinet, kinetic, kinetic. Come on, come on, ooh. Come on, one more time. Forget it. (laughter) He’s too good, can’t do it. Okay, now hold it. – [Woman] No, this is
gonna hurt your hand. – It’s not gonna hurt. Why would it hurt? It’s styrofoam. Alright, everything we just learned, okay. Okay, see? We got the distance? Hold it! Hold it with both hands, hold it. Put your hands up. Okay, okay, okay. – [Woman] I’m scared your knuckles are gonna hit my knuckles. (karate yell) Yeah. So you extend your fingers, that’s the distance,
you have this distance. – For force? – Yeah, so you. – Just like, like that? – Yeah. Stay away from my styrofoam. You hurt my knuckle. – [Woman] I’m sorry. – Get outta here.

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