kin 346 Linear Kinetics

Effects of Force applied over a period of time

If we want to move, and continually move, force needs to be applied over time This means we need to apply Newton's second Law (?) Important relationship of impulse-momentum What is the definition of momentum? (mass x velocity)*****

Dynamic CoF

In gait, walking or running, CoF is calculated as u= 𝐹𝑦/𝐹𝑧 Fz = vertical force, normal force (vertical GRF) Fy = shear force component Fy is used instead of Fx due it the magnitude of the force...what is Fy?

Rotational Friction

resistance to rotational or twisting movements Examples? How do we increase our rotational CoF? (change the shoes, surface ) Do we try to decrease or limit rotational CoF?

Elastic Force

"When force is applied to a material, the material undergoes a change in its length" written as: 𝐹 = 𝑘∆𝑠 k = constant of proportionality ∆𝑠 = the change in length of the material k refers to the stiffness, or the ability of a material to be compressed or stretched

Impulse

If we think back, momentum is calculated by the mass of an object times (*) its velocity p = mv Due to Newton's second Law we begin with the equation: F = m * a Since we know a = ∆𝑣/∆𝑡, we can rewrite our equation as 𝐹 = 𝑚 ∗∆𝑣/∆𝑡 Further we can rewrite as 𝐹 = ∆(𝑚 ∗𝑣/∆𝑡) With all of that said...the left side of our final equation = impulse (J) J = the measure that is required to change the momentum of an object Units of newton-seconds (N *s) Most important aspect of impulse is the amount of time the force is applied to an object's momentum Ex: seatbelts gives a little bit the force goes to the head

Law of motion

all laws are dervived from sir isaac newton's principle mathematica in 16 1: law of inertia 2: law of acceleration 3: law of action reaction

Law of Inertia part 3

an object at rest (0 velocity) would remain at rest object moving at a constant velocity would continue to do so in a straight line th

Force

it facilitate movement interaction of two objects and produces as a change in the state of an object a vector quantity magnitude = how much force is being applied direction how the force will influence the object newtons (N) point of application line of application

Types of Forces

non contact contact

muscle force continued

sometimes it is assumed muscle force only acts on a single point -That's not accurate though Muscles rarely insert at a single point Force generate by a muscle is represented by a resultant vector, but there are components to it Muscle force can be resolved into Fy and Fx Fy = force generating rotation Fx = force acting towards the joint center The amount of force generated by muscle is difficult to accurately measure Done by either mathematical models or through the use of force transducers

PoA

specifc point where the force is beng applied to an object often determines whether it will result in angular or linear motion, or both PoA of muscular force is at the INSERTION Point in gait poa non muscular force

LoA

straight line of infinite length in the direction horizontal and vertical components

Law III Law of Action -Reaction

to every action there is always an opposed equal reaction forces are never in isolation, always acts in pair***** Sum of F AonB = Sum of F BonA ** jumper lands exerts force on the earth, earth exerts a force back in movement force is generated on an implement o r the ground and the reaction force generally produces the movement

Friction

-Force that acts parallel to the interaction of two surfaces that are in contact during motion/impending motion of one surface as it moves over the other To move the block we need enough force to overcome the weight of the block AND the friction caused by the table on the block This is Frictional/Friction Force (Ff) Ff is due to molecular bonding of the surface

Drag

Component of fluid resistance that always acts to oppose movement Which law of motion? (law of action reaction) *Drag and air resistance are synonymous Depending on an object's mass/size and velocity it can create 2 different types of flows around itself Laminar - uniform flow around the object not creating any turbulence Separated Flow - higher velocities cause the flow of fluid around it to separate causing turbulence

Friction continued

Force of friction is proportional to the normal force between the surfaces 𝐹𝑓=𝜇𝑁 𝜇: coefficient of friction* N: normal force, force perpendicular to the surface (weight of the block) CoF calculated by 𝜇 = 𝐹𝑓/𝑁 CoF is dimensionless*** Dependent on what types of surfaces are interacting

Contact Forces

Force resulting from an interaction between two objects Most important in human movement ground reaction force: GRF joint reaction force: JRF friction fluid resistance inertial force muscle force elastic force

Representation of Forces Acting on a System

Forces acting on a system are shown in free body diagrams This allows to see a simpler version of the system and the forces acting it

Muscle Force

Muscles can only produce a pulling or tensile force Means muscles can produce a unidirectional force...can only pull 1 direction Movements are accomplished by opposing pairs of muscles( biceps brachii and triceps brachii) How does that work with joint types like the hip or shoulder? In analyses all muscle groups acting across a joint need to be accounted for as it is the net force across the joint

Joint Reaction Force

Net force acting across a joint*** Multiple components - compression force - shear force - together equal the JRF *JRF are NOT bone on bone force/contact**** B-o-B force is due to MUSCLE CONTRACTIONS

Inertial Force

One segment exerts force on another segment and causes the other segment to move Not caused by muscle contraction*** Generally happens in a proximal-to-distal manner During swing phase of running the foot begins in a plantarflexed position As we get to terminal swing the foot is now dorsiflexed Muscle activity is almost non-existent during swing at the ankle Other examples? Arms when we walk

Lift Force Component

Perpendicular to drag during movement within fluid Most important in swimming, curveballs, and airplanes*** Produced by any break in the flow of an object Creates pressure differences which cause either a rise or a fall** Why do flaps on airplane wings move up on landing? (create more drag to slow down, going down in vertical position) Creates pressure difference so there is more airflow going over the wing than under How do curveballs curve? Magnus effect

CoF

The greater the CoF the greater the interaction = more friction Prior to a block on a table moving, we must first push with enough force to break the molecular bonds other wise the block is static FsMAX </- usN Once the block is moving, its motion is easier to maintain - we don't have to overcome the initial molecular bonding Fk= 𝜇𝑘N Sliding or rolling friction, types of Kinetic Friction*** 𝜇𝑘 < 𝜇𝑠 𝜇𝑘 is dependent on the relative speed of the object

Static Analysis

Used when a system is at rest (not moving) or is at a constant velocity (not accelerating) ∑F in both x and y components = 0 Net effect of force has to equal 0, otherwise the system's inertia will be overcome We typically see much different occurrences of forces acting on a system Forces act as counter balances to each other A second occurrence that determines when a system is static is when forces are not concurrent This means that forces to not act at the same point Since we're static, there is no motion

Dynamic Analysis

When we see forces that cause accelerations, dynamic analyses need to be used Dynamic analysis is much more complicated than static In DA accelerations and inertial properties of segments need to be accounted for Typically analyses are performed by a method known as inverse dynamics This method begins analysis at the most distal segment (foot) then proceeds proximally Issues with using ID is about the assumptions made fixed mass; constant moment of inertia (we'll get to that later)

Law of Acceleration

change of motion is proportional to the force impressed and is made in the direction of the straight line in which that force is impressed more force = more acceleration summationF =ma summation F = m (delta v)/ delta t) when multiplying mass and velocity, were actually calculating an objects momentum momentum represented by p units of m/s^2*** p=

Fluid resistance density and viscosity

density - units of mass per unit of volume greater density = greater resistance for movement viscosity - resistance of flow water is more viscous than air; air is easier to move thru than water As on object moves through fluid, the object disturbs the fluid - the amount/ magnitude of the disturbance is based on density & viscosity The greater the disturbance, the greater the amount of energy transferred to the object/individual EX (swimming lanes)

Law of inertia continued

directly related to an object's mass greater mass = the greater its inertia -> greater difficulty in moving the object for an object to move, the inertia of the object has to be overcome Ex: 300N-> 500N ( no movement ) 600N -> 500N (yes movement )

law of Inertia

everybody continues in its state of rest or in uniform motion in a straight line unless it is compelled to change that state by forces impressed on it inertia of an object is used to describe its resistance of motion (standing there inertia is body weight )***

GRF

everything we do creates GRF GRF have 3 components x, y, z x = mediolateral; see it as Fx y =anterior/posterior; Fy z = vertical; Fz only examine z why? Sum of the effects of all masses of segments times (*)the acceleration due to gravity higher Newton value is vertical**