Section 4: Kinetics Forces (Linear Velocities, Accelerations) 4-1 - PowerPoint PPT Presentation

Section 4: Kinetics – Forces (Linear Velocities, Accelerations) 4-1

Four Properties of Force • Magnitude – How much force • Direction – The way the force is applied along action line – Gravity – Muscle • Point of Application P i t f A li ti – Where force is applied • Line of Application Li f A li ti – Straight line extending through the point of application 4-2 From: Sterner

Four Properties of Force Four Properties of Force 4-3 From: Sterner

Mechanical Loads on the Body Mechanical Loads on the Body • Compression Compression • Tension • Shear Sh • Stress – Torsion – Bending – Combined Loads 4-4 From: Sterner

How is force calculated? How is force calculated? • Force = mass * acceleration Force = mass acceleration – Mass is expressed in kilograms 2 2 – Acceleration = v f -v i = m/s Acceleration = v v t f - t i – Velocity = displacement = position2 - position 1 Velocity di l t iti 2 iti 1 change of time time 2 - time 1 • Units are in Newtons • Units are in Newtons – 1N = 1m/s 2 in a body of 1 kg mass = kg * m/s 2 4-5 From: Sterner

Force vs Pressure Force vs Pressure • Pressure = F/area Pressure = F/area • Force acting over a given area • Units = lbs/in 2 or N/m 2 2 2 U it lb /i N/ • Ex. – Helmets – Joints 4-6 From: Sterner

Kinetics Kinetics • Joint reaction force – 3 times in single leg Joint reaction force 3 times in single leg stance • 5 times in walking • 5 times in walking • Twice during SLR • Up to 10 times while running 4-7 From: Bajekal

Center of Gravity Center of Gravity • Balance Point Balance Point • Conditions Needed – Linear forces Li f – Rotary Forces • Location • Location – Men = 56% of height – Women = 55% of height Women 55% of height – Height above crotch ~ 6 in. • Stationary or Mobile? Stat o a y o ob e 4-8 From: Sterner

Friction Friction • Force that acts at the interface of surfaces in contact • Occurs in the direction O i th di ti opposite the impending motion • Static Friction • Maximum Static Friction • Kinetic (Sliding) Friction 4-9 From: Sterner

Examples of Altering Normal Reaction Force 4-10 From: Sterner

Sample Problem 13 5 Problem 13.5 The dumbwait er D and it s load have a combined weight of 600 lb while t he weight of 600 lb, while t he count erweight C weighs 800 lb. Det ermine t he power delivered by t he elect ric delivered by t he elect ric mot or M when t he dumbwait er (a) is moving up at a const ant speed of 8 f t / s, (b) has an inst ant aneous velocit y of 8 inst ant aneous velocit y of 8 f t / s and an accelerat ion of 2.5 f t / s 2 , bot h direct ed upward upward. 4-11 From: Jung

− = 2 T 800 0 + − = F T 600 0 = = Fv D 200 lb * 8 ft / s 1600 ft . lb / s = Power 2 . 91 hp p 1 = = 1 . 25 if a a C D 2 2 ( ) − = = 800 2 T ma 800 / 32 . 2 * 1 . 25 C ( ( ) ) + + − = = F F T T 600 600 600 600 / / 32 32 . 2 2 * 2 2 . 5 5 = = Fv 262 . 1 lb * 8 ft / s 2097 ft . lb / s D = P Power 3 3 . 81 81 h hp 4-12 From: Jung