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

P i t f A li ti

• Point of Application

– Where force is applied

Li f A li ti

• Line of Application

– 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

Sh

• Shear
• 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 Acceleration = v v

2

– Acceleration = vf -vi = m/s

2

tf - ti Velocity

di l t iti 2 iti 1

– Velocity = displacement = position2 - position 1

change of time time 2 - time 1

• Units are in Newtons
• Units are in Newtons

– 1N = 1m/s2 in a body of 1 kg mass = kg * m/s2

4-5 From: Sterner

Force vs Pressure Force vs Pressure

• Pressure = F/area

Pressure = F/area

• Force acting over a given area

U it lb /i

2

N/

2

• Units = lbs/in2 or N/m2
• 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

Li f – Linear forces – Rotary Forces

• Location
• Location

– Men = 56% of height – Women = 55% of height Women 55% of height – Height above crotch ~ 6 in.

• Stationary or Mobile?

4-8 From: Sterner

Stat o a y o

• b e

Friction Friction

• Force that acts at the

interface of surfaces in contact O i th di ti

• Occurs in the direction
• pposite 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 / s2, bot h direct ed upward

4-11 From: Jung

upward.

T F T 600 800 2 = − + = − hp Power s lb ft s ft lb FvD 91 . 2 / . 1600 / 8 * 200 = = = a a if p

D C

25 . 1 2 1 = =

( ) ( )

T F ma T

C

5 2 * 2 32 / 600 600 25 . 1 * 2 . 32 / 800 2 800 2 = + = = −

( )

h P s lb ft s ft lb Fv T F

D

81 3 / . 2097 / 8 * 1 . 262 5 . 2 2 . 32 / 600 600 = = = − +

4-12 From: Jung

hp Power 81 . 3 =