KIN 321: Biomechanics of Human Movement

Comprehensive vocabulary flashcards covering linear kinetics, linear kinematics, work, power, energy, torques, center of gravity, and stability from KIN321 Biomechanics of Human Movement at Oregon State University.

Force

A push or a pull that comes in pairs (equal and opposite reactions) and accelerates or deforms objects; measured in pounds ($\text{lb}$) or newtons ($\text{N}$), where $1.0\,\text{N} = (1.0\,\text{kg})(1.0\,\text{m/s}^2)$.

Internal Forces

Forces that act within the object or system whose motion is being investigated; they occur in action-reaction pairs and cannot produce a change in the motion of the system's center of mass.

External Forces

Forces that act on an object as a result of its interaction with the environment surrounding it, categorized into noncontact forces (such as gravity) and contact forces.

Normal Contact Force

The component of a contact force that acts perpendicular to the two surfaces in contact.

Friction

A contact force with a line of action parallel to the two surfaces in contact that opposes motion or sliding between those surfaces.

Static Friction

Friction force that develops between two contacting surfaces that are not moving relative to each other.

Limiting Friction

The maximum magnitude of static friction developed just before two contacting surfaces begin to slide relative to each other.

Dynamic Friction

Friction force that resists sliding when two contacting surfaces are moving relative to each other; also referred to as sliding or kinetic friction.

Coefficient of Friction

A dimensionless ratio ($\mu_s$ or $\mu_d$) of the limiting friction force or dynamic friction force to the normal contact force, determined by the roughness and hardness of the contacting materials.

Resultant Force

The single net force resulting from the vector addition of all external forces acting upon an object.

Colinear Forces

Forces that share the same line of action and can be combined using algebraic addition according to their directional signs.

Concurrent Forces

Forces that do not act along the exact same line of action but whose lines of action intersect through the same common point.

Static Equilibrium

The mechanical state where all external forces and torques acting on an object sum to zero ($\Sigma F = 0$ and $\Sigma T = 0$), meaning the object is at rest or moving at a constant velocity with zero acceleration.

Free Body Diagram (FBD)

A simplified diagram representing an isolated object or system along with vector arrows illustrating all external forces acting upon it.

Linear Motion

Translation that occurs when all points on an object move the same distance, in the same direction, at the same time.

Rectilinear Motion

A subcategory of linear translation where an object maintains its orientation during movement, so all points move the same distance in straight lines.

Curvilinear Motion

A subcategory of linear translation where an object maintains its orientation during movement, so all points move the same distance along curved paths rather than straight lines.

Angular Motion

Rotary motion where all points on a body or object move along circular paths around a common fixed line or axis of rotation.

General Motion

A combination of linear translation and angular rotation simultaneously, representing the vast majority of human athletic movements.

Distance Traveled

A scalar measure of the actual length of the path followed by an object during its motion from start to finish.

Displacement

A vector quantity defining the straight-line distance and direction from an object's initial position to its final position ($\Delta x = x_f - x_i$ or $\Delta y = y_f - y_i$).

Speed

The scalar rate of motion, calculated as the distance traveled divided by the elapsed time ($s = \frac{\ell}{\Delta t}$); measured in $\text{m/s}$.

Velocity

The vector rate of motion in a specific direction, calculated as displacement divided by the elapsed time ($v = \frac{d}{\Delta t}$); measured in $\text{m/s}$.

Acceleration

The rate of change of velocity over time ($\bar{a} = \frac{\Delta v}{\Delta t} = \frac{v_f - v_i}{\Delta t}$); a vector quantity measured in $\text{m/s}^2$.

Uniform Acceleration

Motion in which the net external force acting on an object is constant and unchanging, resulting in a constant rate of acceleration.

Projectile Motion

The motion of an object projected into the air or dropped, where the only external forces acting on it are gravity ($g = -9.81\,\text{m/s}^2$) and air resistance.

Linear Momentum

The product of an object's mass and its linear velocity ($L = mv$); a vector quantity that is conserved when net external force is zero.

Conservation of Momentum Principle

The principle stating that the total momentum of a system of objects remains constant if the net external force acting on the system is zero ($L_i = L_f$).

Elastic Collision

An impact between objects in which combined momentum is conserved and the objects rebound away from each other without permanent deformation.

Inelastic Collision

A collision where momentum is conserved, but the colliding objects stick together after impact and move together at a shared final velocity ($m_1u_1 + m_2u_2 = (m_1 + m_2)v$).

Coefficient of Restitution

A dimensionless index ($0 \le e \le 1$) of elasticity representing the ratio of separation velocity to approach velocity between colliding objects, or $\sqrt{\frac{\text{bounce height}}{\text{drop height}}}$.

Newton's First Law of Motion

The Law of Inertia, stating that an object at rest stays at rest and an object in motion continues moving at a constant velocity in a straight line unless acted upon by a net external force.

Newton's Second Law of Motion

The Law of Acceleration, stating that a net external force causes an acceleration directly proportional to the force, in the direction of the force, and inversely proportional to the object's mass ($\Sigma F = ma$).

Impulse

The product of the average net force acting on an object and the time interval over which that force acts ($\Sigma \bar{F}\Delta t$).

Impulse-Momentum Relationship

The mechanical principle derived from Newton's Second Law stating that impulse equals the resulting change in momentum: $\Sigma \bar{F}\Delta t = m(v_f - v_i)$.

Newton's Third Law of Motion

The Law of Action-Reaction, stating that if one object exerts a force on another object, the second object exerts an equal magnitude force in the opposite direction on the first object.

Newton's Law of Universal Gravitation

The law stating that every particle attracts every other particle with a gravitational force directly proportional to the product of their masses and inversely proportional to the square of the distance between them: $F = G\frac{m_1 m_2}{r^2}$.

Mechanical Work

The product of force and the displacement of the object along the line of action of that force ($U = \bar{F}(d)$), measured in joules ($\text{J}$), newton-meters ($\text{Nm}$), or foot-pounds ($\text{ft}\cdot\text{lb}$).

Positive Work

Mechanical work performed when the force acting on an object acts in the same direction as the displacement of the object.

Negative Work

Mechanical work performed when the force acting on an object acts in the direction opposite to the displacement of the object.

Kinetic Energy

The mechanical energy an object possesses due to its motion, defined as $\text{KE} = \frac{1}{2}mv^2$.

Gravitational Potential Energy

Energy an object possesses by virtue of its position relative to the earth's surface, defined as $\text{PE} = Wh = mgh$.

Strain Energy

Potential energy stored by an object as a result of its deformation, defined by $\text{SE} = \frac{1}{2}k(\Delta x)^2$.

Work-Energy Principle

The principle stating that the net work done by all external forces acting on an object is equal to the change in mechanical energy of the object ($U = \Delta E$).

Power

The rate of doing work over time, calculated as work divided by time or force multiplied by velocity ($P = \frac{U}{\Delta t} = \bar{F}v$); measured in watts ($\text{W}$) or joules per second ($\text{J/s}$).

Center of Gravity (COG)

The unique point in a body or system around which its mass or weight is evenly distributed in all directions and through which the force of gravity acts.

Stability

The capacity of an object to return to equilibrium or its original position after being displaced, influenced by COG height, base of support area, and object weight.

Torque

The turning or rotational effect produced by an eccentric force acting at a distance from an axis of rotation ($T = F \times r$); measured in $\text{Nm}$.

Centric Force

An external force whose line of action passes directly through the center of gravity of an object, causing pure linear translation without rotation.

Eccentric Force

An external force whose line of action does not pass through the center of gravity of an object, producing both translation and rotation (torque).

Force Couple

A pair of noncolinear, equal-magnitude forces directed in opposite directions that produce pure rotation without linear translation.

Moment Arm

The perpendicular distance ($r$) measured between the line of action of a force and the axis of rotation.