Applied Biomechanics: Interaction of Forces and the System

Vocabulary flashcards covering biomechanical forces, Newton's laws, field and contact forces, friction, stress, strain, elasticity, viscoelasticity, fluid dynamics, and application concepts from Chapter 4 of Applied Biomechanics.

Force

Something with the capability to cause a change in motion of a system, characterized as a push or a pull.

Direction

The specific way in which a force is applied.

Orientation

The alignment or inclination of a force vector in relation to cardinal directions.

Point of Application

The specific location at which a system receives an applied force.

Magnitude

The amount or size of an applied force.

Line of Action

An imaginary line extending infinitely along a force vector through its tip and tail.

Law of Inertia (Newton's First Law)

The law stating that every body perseveres in its state of rest, or of uniform motion in a right line, unless it is compelled to change that state by forces impressed on it.

Law of Acceleration (Newton's Second Law)

The law stating that the alteration of motion is directly proportional to the motive force impressed and is made in the direction of the right line in which that force is impressed.

Law of Action–Reaction (Newton's Third Law)

The law stating that to every action there is always opposed an equal reaction, or the mutual actions of two bodies upon each other are always equal and directed to contrary parts.

Law of Universal Gravitation

The law stating that every body in the universe attracts every other body with a force directed along the line of centers that is directly proportional to the product of their masses and inversely proportional to the square of the separation between them.

Field

An invisible region of influence created throughout space by any object possessing mass, allowing it to act on another object without physical contact.

Strong Nuclear Force

A non-contact force occurring between subatomic particles that prevents the nucleus of an atom from exploding due to the repulsive electric force produced by its protons.

Electromagnetic Force

A non-contact field force existing between electric charges.

Weak Nuclear Force

A field force produced by certain radioactive decay processes that plays an important role in the nuclear reactions by which the sun produces energy.

Gravitational Force

A non-contact field force existing between bodies of mass.

Contact Forces

Forces that result directly from physical contact between two bodies.

External Forces

Extrinsic forces that interact with a system from the outside movement environment; these are the only forces capable of changing the motion of the system.

Internal Forces

Intrinsic forces acting within a defined system; these are the only forces capable of changing the shape of the system.

Action Force

The initially applied force in an interaction between two objects.

Reaction Force

The simultaneous equal counterforce acting in the direction opposite to the action force.

Weight

A vector measure of the gravitational force pulling upon an object's mass, concentrated at its center of mass.

Friction

A force vector parallel to contacting surfaces and opposite to potential sliding direction that resists the sliding of two objects in contact.

Normal Force

The force pressing two surfaces together that acts downward on one surface and upward on another.

Ground Reaction Force

The oppositely directed normal force exerted by the ground against a system pressing down on it.

Static Friction

Friction existing when two contacting surfaces possess potential for movement but are not currently sliding relative to each other, defined by $f_s \le \mu_s F_n$.

Kinetic Friction

Dynamic friction existing when two contacting surfaces are actively sliding relative to each other, defined by $f_k \le \mu_k F_n$.

Rolling Friction

Friction existing whenever one surface is rolling over another without sliding, featuring a coefficient lower than both static and kinetic friction.

Pressure

The magnitude of applied force acting over a given area, calculated as $P = \frac{F}{A}$.

Stress ($s$)

The external force acting to deform a material divided by its cross-sectional area, calculated as $s = \frac{F}{A}$.

Strain ($\varepsilon$)

The resulting magnitude of deformation from applied stress, expressed as the percentage change in length: $\varepsilon = \frac{\Delta l}{l_i} = \frac{l_{final} - l_{initial}}{l_i}$.

Poisson's Ratio ($\nu$)

The ratio of transverse or lateral strain ($\varepsilon_t$) to axial or longitudinal strain ($\varepsilon_a$), calculated as $\nu = \frac{\varepsilon_t}{\varepsilon_a}$, ranging from $0.00$ to $0.50$.

Elastic Modulus ($E$)

The constant relationship of stress to strain for a given material and type of deformation, calculated as $E = \frac{s}{\varepsilon}$.

Tension Stress

Stress occurring when two colinear or parallel forces are applied in opposing directions away from each other, pulling the system apart.

Compression Stress

Stress resulting from two forces applied in opposing directions toward each other.

Shear Stress

Stress caused by two parallel forces that tend to simultaneously displace one part of a system in a direction opposite another part.

Bending

Deformation occurring when off-axis forces create tension stress on one side of a system and compression stress on the opposite side.

Torsion

Deformation caused by forces applied in a manner that rotates part of a system around its longitudinal axis relative to another part.

Young's Modulus ($Y$)

The elastic modulus calculated specifically under conditions of tension stress, expressed as $Y = \frac{F/A}{\Delta l/l_i}$.

Elastic Region

The linear portion of a stress/strain curve where material fully returns to its original shape upon removal of tensile stress.

Yield Point

The threshold point on a stress/strain curve where additional applied stress results in permanent deformation.

Plastic Region

The non-linear portion of a stress/strain curve past the yield point where permanent material deformation persists after stress removal.

Coefficient of Restitution ($e$)

A parameter ranging from $0.00$ (perfectly inelastic) to $1.00$ (perfectly elastic) indicating an object's ability to reform after deformation, calculated as $e = \sqrt{\frac{h_{rebound}}{h_{drop}}}$.

Viscoelastic

A material whose deformation characteristics depend on both the loading rate and the duration of time subjected to a constant load.

Creep

The property of experiencing increasing strain or continued deformation under a constant level of stress.

Stress Relaxation

The eventual decrease in internal stress occurring over time in a viscoelastic material as fluid is no longer exuded.

Pascal's Law

The principle stating that pressure applied to a fluid is transmitted undiminished to every point within the fluid and to the container walls.

Archimedes' Principle

The principle stating that a body submerged in fluid is buoyed up by a force equal in magnitude to the weight of the fluid displaced.

Buoyant Force

The vertical, upward-directed force exerted on an object submerged in a fluid.

Drag Force

The parallel component of dynamic fluid force acting on a system moving through a fluid.

Lift Force

The perpendicular component of dynamic fluid force acting on a system moving through a fluid.

Motive Force

A propulsive force that leads to a change in motion, such as increased velocity or a change in direction.

Resistive Force

A force that prevents changes in motion caused by other external forces or decreases the velocity of a moving system.

Centripetal Force

Any force causing a system to exhibit circular motion.

Centrifugal Force

A fictitious force representing the linear inertia of a system in the absence of sufficient centripetal force to constrain it to a circular path.