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About force

Intro

Force is any influence that causes an object to undergo a certain change related its movement, direction, or geometrical construction. A force can cause an object with mass to change its velocity (which includes moving it from a state of rest) or cause a flexible object to deform, or both. Motion only occurs through the application of force. Conversely, the motion can only be reduced, redirected, or stopped by the application of force.

Types of forces used in the martial arts

  • Muscular force. Muscles operate by contraction, so when a muscle pulls a body part in one direction, for the part to return to its original position, another muscle must pull it back. No one muscle in the body may generate the force required to deliver an effective blow; all the muscles of the body must act in concert to generate the required force.
  • Gravity. All objects exert an attraction on all other objects. The force of that attraction is proportional to the mass of the object that attracts. The attractive force of the earth is called gravity. Since gravity always exists and it is predictable, it is useful in combat. You may not be able to move a larger opponent horizontally and you certainly may not move the opponent vertically, but you may use gravity to cause the opponent to fall by causing the opponent to lose balance. This may be accomplished by pushing when the opponent pulls, pulling when the opponent pushes, or by disrupting the opponent’s base, such as by using a foot sweep.
  • Friction. Friction is the resistance a body exerts upon another while moving over it. The friction between two surfaces:
  • Is proportional to the force pressing them together. The heavier you are, the more difficult it is for you for a foot to slip on the floor when punching.
  • Depends upon the nature of their contact surfaces. You are less likely to slip with soft rubber sole shoes than with leather sole shoes.
  • Is independent of the area of the contact surfaces. Contrary to popular belief, there is no change in your friction with the floor whether you are standing on one foot or on both feet. One skidding tire will stop a car just as fast as four skidding tires (as long as the heat generated at the point of contact remains constant, which is nearly impossible).

Muscular force

Force in the body is generated by muscle contraction. The amount of force available for use varies inversely with the speed of muscle movement. The faster a muscle contracts, the less force is available to overcome a resistance. For example, you may be able to pick up a heavy weight by lifting it slowly, but, if you try to lift it quickly, too much muscle tension is used for rapid contraction and too little tension is available to lift the weight. Another example is when trying to open a stuck window sash. If you try to open it with sudden quick pushes, it stays stuck. But if you apply a steady push that gradually increases in power, the sash breaks free and the window open.

When executing a martial arts technique, quick muscle contraction is used initially to increase the speed of movement and the quickness of the technique, but, as the blow nears the target, muscle tension slows and is shifted to overcoming the resistance of the target. If only a light resistance is expected by the target, such as in light-contact free-sparring, more muscle tension may be used for speed. To transfer maximum force to the target, the body is tensed just as contact is made and is maintained for a split second after contact is made.

The math

  • v = s/t Constant or average velocity equals distance divided by time. When people talk about speed (how fast your fist or foot is moving) they are talking about this.
  • M = mv Momentum equals mass times velocity. This is how hard you hit.
  • a = (V2-V1)/t Acceleration is the change in velocity over time. Acceleration equals final velocity minus initial velocity divided by time.
  • F = ma Force equals mass times acceleration.
  • P = Fv Power equals force times the constant velocity.
  • KE = 1/2m v2 Kinetic energy equals one-half of mass times velocity squared. This is a favorite of the proponents of velocity as it places much greater value to an increase in velocity than an increase in mass.

Newton's laws of motion

  • First Law of Motion. A body at rest tends to remain at rest and a body in motion tends to remain in motion, unless acted upon by external forces. This tendency to resist a change in state is called inertia. Since an opponent who is in motion tends to remain in motion, it is easier for a defender to use the inertia of that motion in his or her favor, such as by pulling an opponent who is charging you to add to the inertia and cause him or her to lose balance, than it is to try to stop the motion. In addition, since an opponent who is at rest tends to remain at rest, it makes it difficult for the opponent to overcome the inertia and attack quickly.
  • Second Law of Motion. When a force acts upon a mass, the mass acquires a certain acceleration proportional to, and in the direction of, the force acting upon it, and the acceleration is inverse to the magnitude of the mass. In other words, the heavy an opponent is, the slower he or she moves, but once moving the more difficult he or she is to stop. 
  • Third Law of Motion. For every action, there is an equal and opposite reaction. Thus, when you punch an opponent with a certain force, an equal but opposite force is applied against your fist by the opponent's body. Therefore, you must have a tensed body and a firm, stable stance so you can withstand the force. Hopefully, the opponent will not be prepared and thus must absorb the full force of the punch.

    The third law also applies to technique execution. For example, when one arm is pulled back quickly and twists that side of the body backward, that twisting motion causes the other side of the body to twist forward with an equal but opposite force that can then be applied to the arm on that side of the body. If that arm is executing a punch, the force will combine with other forces in the punch to increase the power of the punch.

Centripetal and centrifugal forces

Two other forces that come into play during the practice of the martial arts are the centripetal and centrifugal forces. Let’s consider a weigh being twirled around at the end of a rope.
  • Centripetal force. This is the force that keeps the weight moving in a circle; it is the tension on the rope. It is what draws objects into a spinning whirlpool and keeps a roller coaster that’s upside down in a loop from falling off the tracks. If you grab an opponent by the lapels and spin him or her around, this is the force that you must exert to keep him or her on a circular path. It is the force you must exert to keep the foot of a spinning hook kick moving in a circle.
  • Centrifugal force. This is is the force that makes the weight move in a straight line. It is what throws objects off a spinning top in a straight line. When the centripetal force used to keep the foot of a spinning hell kick moving in a circle is released, the foot’s stored centrifugal force is released in a straight line to the target.

Momentum

Momentum (force) is produced by an object in motion. Force is equal to the product of the mass of the body and its acceleration (f=mv). During a punch, the mass of the fist does not change, but the velocity of the fist is variable. If the fist constantly accelerates over the distance to the target, the final velocity will increase the striking force of the punch. 

The longer a force acts on an object, the greater the final velocity, so you should maintain the acceleration of the fist until contact is made, don’t snap back before contact or the focus point of the punch is reached. Muscles work in sequence to produce maximum power; you should start contracting each succeeding muscle at the point of greatest velocity achieved from the previous muscle. To reach the maximum force of a punch, all muscle forces must be applied toward movement in one direction.

Impulse

Stopping force is proportional to the product of the mass and the velocity of the object in motion and is inversely proportional to the time required to stop the object. When this is applied to the force equation, the result is: stopping force x time = mass x velocity (ft=mv). Therefore, if the mass and velocity of a punch are constant, then a small stopping force applied to the punch will require more time to stop the punch and a larger stopping force will require less time. The product of the time and the force required to stop the punch is called impulse.

If the time required to stop an advancing object is near zero, then the force required to stop it will nearly equal the momentum of the object. When an object is stopped in an extremely short time, the product of the force and time is called impulsive force. This occurs when a punch strikes the body or a bat strikes a ball.

Potential and kinetic energy

Potential energy is energy at rest; it is stored and available for use. Kinetic energy is energy in motion; it is consumed as it is used. When throwing an opponent, you use your kinetic energy to lift the opponent off the floor against gravity. At the peak of the throw, part of the energy used to lift the opponent is stored within the opponent as potential energy. To complete the throw, you release the opponent and the opponent's potential energy changes to kinetic energy as gravity forces the opponent to the floor.

Deflection of a force

An object in motion tends to remain in motion until acted upon by another force. An opponent’s attack can be either avoided, deflected, or stopped. The attack may come in either a straight line or an arc. Avoiding an attack may not be possible and stopping the attack by meeting it head-on may be painful and you may not always be able to stop it. Therefore, the best strategy is to deflect the attack. While a small force may not be large enough stop a larger force, it may be used to easily deflect the larger force. Deflections should be used in a circular motion in that it moves generally in the same direction as the attack that it is used against

Unskilled attackers will hurl their bodies in the direction of a strike. They are functioning like  fired cannon balls, they are fully committed to moving in one direction, A skilled martial artist who understands the principles energy will not move in this manner. If one part of their body moves forward, another part will move backward (Yin/Yang) balancing the forces. Therefore, pulling this person in the direction of their strike is not an effective defense, because their inertia is balanced and not so focused in one direction.

For a greater blocking effect, you should strike the attacking weapon perpendicular to the direction of its motion. If a strong part of your body strikes a weaker part of the opponent, it can be effective and is usually not a problem to you. However, if your weapon is not strong enough, relative to the target, this method may not have any significant effect on the target and may result in injury to yourself. When you meet force on a line perpendicular to its line of direction, your smaller force can deflect a much larger force.

Reaction force

For every action, there is an equal and opposite reaction. The martial arts have used this law of motion to add power to their techniques.

One way that reaction force is used is the way the pivot foot is used to thrust against the ground when shifting. The reaction force of this thrust against the ground is returned through the leg and hip and used to thrust the body forward.

Another way reaction force is used is through body rotation. Body rotation is created by anchoring one area of the body and using it as a pivot about which the body can rotate. An example of this is the use of hip snap. In a hip snap, the hips rotate around the centerline of the body to add force to a punch or kick. The hip, torso, and leg motion create a pulse of power in one direction on one side of the body, which creates a reactive force in the opposite direction on the other side of the body. This reactive force is applied in a sharp pulse of energy of very short duration, but it can significantly increase the power of an attack executed from this side of the body. Hip recoil also helps maintain stability by helping keep the center of mass centered over the base.

Another way to generate a reaction force is when one hand performs a technique while the other hand is withdrawn in the opposite direction. The speed and scale of the movements of both hands are matched. This makes use of reaction force in two different ways. First, the pullback hand helps rotation around the body’s centerline because the force of the hand and arm being pulled back forcefully creates a forward movement on the other side of the body. Second, the pullback serves as a counterbalance to the technique being extended, so that if it misses the target, stability can still be maintained.


Reaction force is also applied when striking a target with a technique. When a technique is finely focused, and the body is so firmly connected to the ground that little or no force is accepted back into the body. If a technique quickly recoils after impact, little of the reaction force from the target can transfer back into the striking arm or leg and the duration of the impact is shortened, both of which increase impact force.

Transfer of force

To transfer maximum force to an opponent, you must induce momentum in the opponent in the shortest time possible by applying all your mass into the strike and achieving maximum velocity with the strike. This means a maximum power punch will have the weight of the body behind it and have all the muscles in the body working to accelerate it to maximum velocity, which induces momentum in the opponent in minimum time.
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