KIN 3309 1nd Edition Lecture 17 Outline of Last Lecture I Newton s Laws of Motion II Momentum Weight COM III Contact Forces IV Outline V Free Body Diagrams VI Effects of a Force at an Instant in Time VII Friction VIII Impulse Force Applied over a Period of Time IX Work Force Applied over a Distance X Power XI Energy XII Work Energy Theorem XIII Conservation of Energy XIV Energy Conversion during Gait XV Quiz Outline of Current Lecture I Contact Forces II Free Body Diagrams These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute III Effects of a Force at an Instant in Time IV Impulse Force Applied over a Period of Time V Outline VI Work Force Applied over a Distance VII Power VIII Energy IX Strain Elastic Energy X Work Energy Theorem XI Conservation of Energy XII Energy Conversion during Gait XIII Vectors and Scalars XIV Position Displacement Distance XV Velocity and Acceleration XVI Types of Angles XVII Angular and Linear Position XVIII Angular and Linear Velocity XIX Tangential Acceleration XX Centripetal Radial Acceleration XXI Quiz Current Lecture I Contact Forces a Ground Reaction Force GRF i Force exerted by the ground on a body in contact with it b Joint Reaction Force i Force experienced at a joint c Inertial Force i Force opposite in direction to an accelerating force acting on a body d Muscle Force i Force when muscle fibers generate tension e Elastic Force i The tendency of solid materials to return to their original shape after being deformed f Friction Force i The force acting parallel to the interface of two contacting surfaces during motion or impending motion II Free Body Diagrams a Called a force diagram i To analyze the forces linear and moments angular acting on a body ii Displays magnitude and direction of forces III Effects of a Force at an Instant in Time a IV Impulse Force Applied over a Period of Time a An object with momentum can be stopped if a force is applied against it for a given amount of time b Force applied over a period of time impulse change in momentum c Impulse is a vector quantity the direction is the same as the direction of the force d Symbolized as I e Units kgm s or Ns V Outline a Work b Power c Energy d Work Energy Theorem e Conservation of Energy VI Work Force Applied over a Distance a Work force x distance b The scientific definition of work is using force to move an object a distance when both the force and the motion of the object are in the same direction c W f x cos theta x s i F applied force ii S displacement iii Theta angle between the force vector and line of displacement recall cos 0 1 d Units of work are Joules 1 J 1 Nm e Work is a scalar quantity f Work is not a function of time VII VIII Power a b c d e Power is the rate at which work is done Power work time Power force x velocity Units of power are Watts W J s Power is a scalar quantity Energy a Energy is the capacity of a physical system to perform work b Kinetic Energy KE results from motion c Potential Energy PR results from position in gravitational field d Units of energy are Joules 1 J 1 Nm e Energy is a scalar quantity IX Strain Elastic Energy a Strain elastic energy SE is the capacity to do work due to deformation of a body X Work Energy Theorem a The work done by a resultant force is equal to the change in energy that it produces b This is assuming that no work is done to deform the system i e no strain energy is stored c This also assumes that no work is done to increase rotational kinetic energy XI Conservation of Energy a The total energy of a closed system is constant since energy does not enter or leave a closed system b This only occurs in human movement when the object is a projectile and we neglect air resistance i TE PE KE c Note that gravity does not change the total energy of the system XII Energy Conversion during Gait a Walking and running are characterized by energy conversions i Walking 1 Kinetic potential energy conversion ii Running 1 Kinetic and potential energy are converted to elastic and viceversa XIII Vectors and Scalars a Scalars i Can be described by magnitude ii E g mass distance speed volume b Vectors i Have both magnitude and direction ii E g velocity force acceleration iii Vectors are represented by arrows XIV Position Displacement Distance a Position WHERE i Location in space relative to some reference point ii Linear and angular position s theta b Displacement Distance HOW FAR i Displacement 1 Final change in position 2 Vector quantity ii Distance 1 Sum of all changes in position 2 Scalar quantity XV Velocity and Acceleration a Velocity HOW FAST i Vector quantity b Acceleration HOW QUICKLY IS VELOCITY CHANGING i Vector quantity ii Insight into forces torques XVI Types of Angles a Absolute Angles i An absolute angle is measured from an external frame of reference b Relative Angles i A relative angle is the angle formed between two limb segments c Joint Angles i Anatomical position is zero degrees d Hip Angle i Positive angle for flexion ii Negative angle for extension e Knee Angle i Generally always positive ii Negative angle indicates hyperextension f Ankle Angle i The 90 degree is added to make the normal ankle angle when standing equal to 0 degrees ii Positive angle for dorsiflexion iii Negative angle for plantar flexion XVII Angular and Linear Position a The linear displacement can be determined when the length of the segment radius and the angular displacement are known i r distance from axis of rotation radius ii theta angular displacement here in degrees XVIII Angular and Linear Velocity a The linear velocity can be determined when the length of the segment radius and the angular velocity are known i r distance from axis of rotation radius ii omega angular velocity here in degrees XIX Tangential Acceleration a Similarly tangential acceleration i r distance from axis of rotation radius ii alpha angular acceleration here in degrees XX Centripetal radial Acceleration a Even at constant angular velocities the direction of the linear velocity changes when going around a curve b Since velocity is changing there must be acceleration c This acceleration is towards the center of rotation i Called centripetal acceleration XXI Quiz a If the static coefficient of friction of a basketball shoe on a particular playing surface is 0 58 and the normal force is 911N what horizontal force is necessary to cause the shoe to slide i Force greater than the