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Forces

Forces. Force: force is an action on an object that causes or tends to cause: 1. A change in motion (a, a ) 2. A change in shape (deformation). tension. compression. Forces. Force : force is an action on an object that causes or tends to cause: 1. A change in motion (a, a )

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Forces

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  1. Forces Force: force is an action on an object that causes or tends to cause: 1. A change in motion (a, a) 2. A change in shape (deformation) tension compression

  2. Forces Force: force is an action on an object that causes or tends to cause: 1. A change in motion (a, a) 2. A change in shape (deformation) Forces are vectors! (Magnitude and Direction) origin

  3. FORCE A pull or push or twist: Indirect (gravity, electromagnetic) Direct (impact, muscle tension) F = m•a UNITS: Englishmetric slug•ft/sec2 kg•m/sec2 OR pounds (lb) Newtons (N) oz dynes

  4. FORCE - Gravity Examples of forces Gravity (non-contact) The force created by a mass as an attraction to another mass at a constant acceleration ag = 9.81 m/sec2 or 32 ft/sec2 Fla Fw Fua

  5. FORCE - Inertia Examples of forces Inertial forces - resistance to changes in movement created by the mass and its distribution. AMF

  6. FORCE Examples of forces Muscle Tension (contact force) – Pulling force produced by muscle contraction. Creates rotation (or torque) and affects the stability of joints & bones. Fm Fs

  7. FORCE Examples of forces: Impact – The force registered when an object strikes or is struck by another object or surface. (Force produced by a collision.) Hitting a tennis ball catching a softball Feet striking the ground during running 3.5 x BW

  8. IMPACT FORCE Ndamakong Suh Suh - acceleration, impact force http://www.youtube.com/watch?v=b4iq081DdZU

  9. SPRING FORCE • Examples of forces: Force and energy which is stored when a material is deformed and then released when it returns to shape (recoil). Fs = -kDx where: k is spring constant (a stiffness - resistance) Dx is the change in shape or deformation • Examples of spring tension: • The bounce or recoil of a tennis ball, football, golf ball, etc. when struck • The spring effect in the tissues of the arch of the foot [the calcaneonavicular (spring ligament) and the plantar fascia] • Spring force or elastic force!

  10. SPRING FORCE • “Natural Springs”(can store up to 50%of force, energy needed during running) • (1) arch of the foot: the calcaneonavicular ligament and the plantar fascia • (2) Achilles’ tendon • (3) skeletal muscle (gastrocnemius, soleus, quadriceps) or Achilles’ tendon Arch of foot & plantar fascia Achilles’ tendon and calf muscles

  11. SPRING FORCE • “Natural Springs”(can store up to 50%of force, energy needed during running) • (1) arch of the foot: the calcaneonavicular ligament and the plantar fascia • (2) Achilles’ tendon • (3) skeletal muscle (gastrocnemius, soleus, quadriceps) Calcaneonavicular ligament Plantar fascia, Achilles’ tendon

  12. SPRING FORCE on • Spinal tuning: descending command to activate quads and gastroc + soleus. just before contact of the foot made with the ground. Stiffens (increased k) the muscle spring to store more elastic force and energy small clearance Off (t-1) On

  13. SPRING FORCE – Elastic collision • Spring force or elastic force • Spring oscillation frequency • f = 1/(2p) • √(k/m) • Where: f = frequency • k = spring constant • m= unit mass • To store more force and energy ---> larger bounce • 1) more elastic collision (ability to return completely to original shape quickly • Elasticity: the ability of a material to recoil or “bounce back” once deformed (bent, stretched, compressed • rubber bands, steel, and bones are all elastic!

  14. SPRING FORCE • Spring force http://www.youtube.com/watch?v=2Y57pw_iWlk • Spring oscillation frequency • f = 1/(2p) • √(k/m) • Where: f = frequency • k = spring constant • m= unit mass • To store more force and energy ---> larger bounce • 2) matching the oscillation freq. of the colliding objects • Stiff object with stiff object (golf ball with titanium driver) • More pliable object with more pliable object (leather basketball with wooden floor) • Increases coefficient of elasticity (r) • Driver Golf Ball Collision

  15. SPRING FORCE • Spring oscillation frequency • f = 1/(2p) • √(k/m) • Where: f = frequency • k = spring constant • m= unit mass • 2) matching the oscillation freq. of the colliding objects • Stiff object with stiff object (golf ball with titanium driver) • More pliable object with more pliable object (leather basketball with wooden floor)

  16. Forces Affecting Movement - Fluid Drag (air, water, etc.) • FD = 1/2 CD * AP * r * v2 where: • CD = coefficient of drag - roughness or surface • AP = area exposed normal (perpendicular) to CD & AP = aerodynamics - r(the Greek letter rho) = fluid density • v2 = the square of the velocity of the object relative to the fluid • Fluid drag can slow down an object (as in projectile flight) or can propel an object (like a sailboat) FD

  17. Forces Affecting Movement - Fluid Drag (air, water, etc.) • FD = 1/2 CD * AP * r * v2 where: Fluid drag can slow down an object (as in projectile flight) or can propel an object (like a sailboat or pull-through stroke in swimming) Slow down Fd Propel Fwind vb

  18. Forces Affecting Movement - Fluid Drag (air, water, etc.) • FD = 1/2 CD * AP * r * v2 where: CD = coefficient of drag AP = area exposed High CD = parachute aerodynamics Low CD = lycra bodysuits, Cycling helmet

  19. FD = 1/2 CD * AP * r * v2 Cycling helmet Aggie Wind tunnel A2 testing - Lance

  20. Forces Affecting Movement - Fluid Drag (air, water, etc.) • FD = 1/2 CD * AP * r * v2 where: CD = coefficient of drag AP = area exposed aerodynamics Tour de France cyclists hyper flex hips and keep back parallel to ground and reduce AP Aggie Freight Turbulence slows down object under fluid drag

  21. Forces Affecting Movement - Fluid Drag • FD = 1/2 CD * AP * r * v2 where: • (the Greek letter rho) = fluid density r Struck baseball or golf ball will travel farther at altitude (Coors Field or the International PGA Tournament at Colorado Springs)

  22. Forces Affecting Movement - Fluid Drag (air, water, etc.) • FD = 1/2 CD * AP * r * v2 where: • (the Greek letter rho) = fluid density • Baseball or softball travels farther for a given initial velocity on a hot, humid day than a cool, dry one! • (1) warm air --> lower pressure • (2) water vapor is a gas with (H2O) low molecular weight vs. dry air AND warm air can hold far more water vapor! • H2O = 18 g/mole*; O2 = 32 g/mole; N2 = 28 g/mole • *note: 1 mole of gas occupies 22 .4 liters at STPD

  23. Fluid Drag (air, water, etc.) • FD = 1/2 CD * AP * r * v2 • Lance Armstrong rides at 20 m/sec with a 5 m/sec tail wind on a street in Paris during a time trial. Later in the race he rides at 18 m/sec into a 5 m/sec headwind. What is the % increase in wind-drag with the head wind? • Given: vLA1 = 20 m/sec vLA2 = 18 m/sec vw1 = 5 m/sec vw2 = -5 m/sec Find: % in FD

  24. Fluid Drag (air, water, etc.) • Formula: FD = 1/2 CD * AP * r * v2 • Assume: no ∆ in CD, AP, and r • Then: FDa v2 v1 = 20 - 5 = 15 m/sec v2 = 18 - (-5) = 23 m/sec Solution: use ratios: FD2 / FD1 = ( v2)2 / ( v1)2 FD2 / FD1 = ( 23)2 / ( 15)2 FD2 / FD1 = 529 / 225 FD2 / FD1 = 2.35 % = (2.35 - 1) • 100 = 135% or (529-225)/225 • 100 = 135%

  25. Forces Affecting Movement - Fluid Drag • FD = 1/2 CD * AP * r * v2 where: Cat. 1 - 80 mph Cat. 5 - 160 mph

  26. Forces Affecting Movement - Fluid Drag CS - 50 mph? 75+ mph? • FD = 1/2 CD * AP * r * v2 where:

  27. Forces Affecting Movement • Friction - A resisting force that is normal or perpendicular to the reaction force of an object. weight applied force friction reaction force

  28. Forces Affecting Movement – Friction • Static friction – The friction present when a force attempts to slide an object on a surface but is not strong enough to actually cause movement • Dynamic friction – The friction present once an object starts to move or slide along a surface. • Also called kinetic friction (symbol: Fk) • Rolling friction – The friction that resists the movement of an object rolling on a given surface.

  29. Fa = Applied force parallel to surface (normal force) Fm = Maximum static friction R = Reaction force wt = Object weight

  30. Factors Governing the Magnitude of Friction • The coefficient of friction (m) • General: F = mR • R = wt • cos q where q = deviation from normal force (if weight). R drawn normal or perpendicular to surface • µ = Unitless- indicating the relative ease of an object moves across a surface • µ - Roughness, adherence properties of BOTH surfaces • When walking, riding uphill or downhill • q , cos q decreases, R decreases, friction

  31. The coefficient of friction (m)Incline, decline • F = mR • R = wt • cos qwhere q = deviation from normal force (if weight). R drawn normal or perpendicular to surface • When walking, riding uphill or downhill • q , cos q decreases, R decreases, friction wt q R

  32. Friction Calculations • General: F = mR • Maximum Static Friction: Fm = msR where ms is the coefficient of static friction • Dynamic (Kinetic) Friction: Fk = mkR where mk is the coefficient of dynamic or kinetic friction.

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