Velocity – the distance traveled divided by the time it took. Often referred to as “speed”, although velocity has both a magnitude (speed) and a direction in biomechanics. Velocity = distance/time. Both linear and angular velocity are components of most movement.

Example: The JKD finger jab is one of the fastest (linear velocity/"speed") techniques to execute and therefore was emphasized heavily by Bruce Lee.

Acceleration – the change in velocity over the time of the movement.  Acceleration = change in velocity/change in time.

Example: The JKD one inch punch continues to accelerate into and through (penetration) the target. The subtle wrist “snap” towards the end of the technique can produce increased acceleration by the addition of this body segment to achieve maximum force.

Force – something that causes a change in the state of an object. Force acting on an object can change the state (velocity, acceleration, direction of movement) of the object. Force = mass x acceleration.

Example: The “impact” produced by a punch or kick upon contact with an object is a result of the force generated.

Mass – the amount of matter within an object. Weight and mass are often confused. The weight of an object is a measure of the gravitational pull (force) on the object (mass).

Example: Larger objects may produce more force. A kick may produce more force due to the increased mass of the leg compared with the punch.

Inertia – the natural tendency of a “body” to stay at rest. Inertia can also be described as the resistance to change.

Example: The JKD Ready Stance was optimized to initiate movement from a stable but mobile static position.

Impulse - a force exerted over the contact time. The contact time of the impact affects the amount of force. Impulse = force/time. Impulse can also be defined as the change in momentum.

Example: In Lee’s writings on JKD techniques he discussed the importance of a technique going in toward the target fast but returning even faster. The net effect is to produce a shorter contact time coupled with a large force and does more “damage” to the opponent.

Momentum – the product of an object’s mass and velocity. Momentum = mass x velocity. Conservation of momentum is an important principle that governs impact situations (referred to as “collisions” in biomechanics and physics).

Example: The “impact” produced by a punch or kick upon contact with an object is a result of the force generated. The final impact force is momentum of the strike combined with the momentum of the target/opponent (a “collision” similar to 2 automobiles crashing into each other).