How to Get Punched in the Face
Many probably awoke on New Year's Day feeling as if they'd been punched in the face -- hangovers can have that effect. But what's the physics behind actually getting punched in the face?
Stand up and take a T-stance. Now move your hands up to your face: the left should be below your left eye and the right next to your right cheek. If you're a lefty, reverse this configuration. Feel powerful? You should. Your body is brimming with potential energy, just waiting to be converted to kinetic energy. How will you complete the conversion? By coiling your body and leaning onto your back foot, then pushing off and twisting forward while flexing at the shoulder and extending at the elbow. In other words: throwing a punch.
On paper, the energy you're imparting looks a little like this: (1/2)mv^2. M is the mass of your fist, arm, and shoulder. V is the velocity with which that mass is moving. Notice that the velocity (v) is squared. It's this peculiarity of kinetic energy that explains why Davids can be just as powerful as Goliaths. Though a punch from a larger individual brings more mass to bear, it's only boosting the kinetic energy in a linear fashion. On the other hand, smaller individuals -- who are often quicker -- can amp up the energy of their punch exponentially by swinging with greater speed.
The kinetic energy of a punch varies greatly. While beginners might only be able to muster a measly 37.5 joules of energy, experts can deliver over 400 joules, an amount roughly equal to getting shot by a handgun!
A bullet does more damage than a fist because its energy is delivered over a smaller area, but a fist to the face isn't exactly a walk-in-the-park. When a punching fist strikes a motionless head, some of its momentum (equal to mass times velocity) is transferred to the head. This change in momentum results in a force, one that is inversely proportional to time. When delivered quickly, the force can cause the head to jerk back, sending the brain ricocheting around within the skull. But there are ways to reduce this force. Expert boxers know this well:
"When a boxer recognizes that he will be hit in the head by his opponent, the boxer often relaxes his neck and allows his head to move backwards upon impact. In the boxing world, this is known as riding the punch. A boxer rides the punch in order to extend the time of impact of the glove with their head. Extending the time results in decreasing the force and thus minimizing the effect of the force in the collision."
If you're the one doing the punching, the best way to maximize the force is by making contact with your target when your arm is about 80% extended. That's when a punch reaches its maximum velocity.
RealClearScience doesn't recommend punching anyone in the face or getting punched in the face yourself. But if you end up doing either of those things, at least you can now perform them in accordance with physics.