In a kick like a back kick, momentum transfer plays a larger role. In this case if the opponent is coming in, the change in momentum is greater because you add together the momentum of his body and the momentum of your kick.

But what does this all tell us? First of all, from the Kinetic Energy equation we see that the Velocity term is squared. So Velocity has a much bigger effect on Energy transferred that mass. Say you increased the mass three times. The kinetic energy would increase by three times. But if you increased the Velocity by three times, the Kinetic Energy would increase by 9 (32) times!

This helps explain why people with long legs seem to be able to score so much easier. We didn’t state it before, but the Velocity in that equation is the Instantaneous Linear Velocity. That means the speed that the foot is going right at the moment that it hits the chest pad, in the direction going into the chest pad. Your leg can be considered like the radius of a circle that is swept out in your roundhouse kick. Angular Velocity is how fast something goes around in a circle. Your entire leg is moving with the same Angular Velocity, or else it would fall apart. But in order for this to be true, the tip of your leg has to be moving with a faster Linear Velocity than your knee. Thus, you will transfer more energy by hitting someone with the tip of your leg, and the longer your leg, the more energy you will be able to transfer.

Q: How does this apply to attacking with a weapon?

A: You will be able to transfer more energy by hitting with the tip of a weapon or with a longer sword or staff.

So where does the quantity of Force come in? Force is more of a steady state quantity, thus it is of more use to us in grappling. There are two key concepts we need to learn about.

Have you ever seen someone lie on a bed of nails and not get hurt? Why do we slap the ground on our falling techniques? The answer to these questions lies in the concept of Pressure. Pressure is simply the Force applied divided by the Area over which that Force is applied.

Pressure [Pa = N/m2] = Pressure equals Force divided by Area.

Pressure is what really causes impact injuries. When being stabbed by a knife, the Area is extremely small (only the knife point that touches your skin) so the Pressure is large. When a person lies on a bed of nails, the total surface area is the sum of the area of each small nail point. Thus the total pressure is not large enough to cause the nails to puncture the skin. When we do the falling techniques, we lower the Pressure by increasing the Area in contact with the ground.

The next concept is called Torque. Torque is what causes rotation. Torque is extremely useful in trying to flip an opponent or in applying most joint locks. Torque is sometimes referred to as leverage, and it is the product of the magnitude of the Force and the