Lecture 10 KHS 301 Linear Kinetics: Applications of Newton’s Laws

Law of Acceleration

External forces cause an object to start, stop, speed up, slow down, or change direction

- F = ma

- W = mg

Linear Impulse

Force x Time” in N-sec

- Force is not constant, so impulse = average force x duration

- Vector quantity

- Importance: time of force application for generating momentum

Impulse Importance

time of force application for generating momentum

Forces that occur over very short time intervals = ?

= impulsive forces

Linear Momentum

Mass x Δ velocity (kg m/s)

- an increase in mass or velocity increases momentum

- vector quantity

- Δ momentum is proportional to F in the direction of motion & decreases when F is in opposite direction

Collisions

example of change of momentum

- Δ momentum is proportional to F in the direction of motion & decreases when F is in opposite direction

An increase in impulse =

An increase in momentum =

Why decrease momentum?

- Catch a ball (decrease momentum of ball)

- Landing from jump

- Deceleration from sprint & COD

- Blocking, tackling

- Exercise/RT: ↑ time under tension ---endurance, hypertrophy, ROM specificity

Using impulse distribution to reduce impact

Protective equipment: headgear, pads, landing pits/surfaces, mats, mouthguards, boxing gloves

Landing: position of COG, sequential contact

Alter Force/time distribution

Expand time with less impact force

- increases interval over which momentum is 0

An increase in Direction of Impact force

= decreases impact force

Stiff landing =

increases impact force vs soft landing

Injury factor women vs men in stiffness

Women are more stiff than men

- ECC strength is important

Ukemi

"art of falling"

- forward break fall

- backward break fall

- side breakfall

- handstand break fall

relaxed, normal breathing

Arm Slap

- ↑ contact area with ground & dissipates energy

- force applied to arm decreases loading on other body parts and alters F/T distribution

Application of Judo/ JJ Breakfall

Dissipate impact force, decrease head acceleration, and momentum during fall

- hand contact is associated with head deceleration (timing key!)

Reducing Impact via impulse

- increase impact time to reduce average force

- impulse and momentum are identical except F/T curve varies due to landing surface

Cushioned Materials change F/T curves and decrease impact, so ...

decrease in impulsive force

- Rugby pads decrease peak impact force by 35-70% during tackling

Reducing Impact: Headgear

• Protection (greater density of foam) decreases impact to head, head acc., & injury

• increases energy attenuation

Study: Cricket Leg Guards

- increase time of runs by 0.5 sec

- Widest pads had greatest negative effects by increasing hip ABD and decrease hip EXT

- Stride Length decreased, stride width increased

- Pads increased impact force and braking pulse during running

- Found pad design + weight was cause

- pads made them slower

Study: Ice Hockey Gear

- simulated ice hockey game

- body temperature, lactate, sweat loss, HR, and power decrements in pads compared to non-pads

Application: Hockey Goalie Gear

Studied butterfly save technique of goalies

• GRF = 1.45 x BW

• Hips extended by 20 degrees & adducted 10 degrees

• Hip internal rotation angles approached max values of 37 degrees ± 11 degrees

• increased pad width by 1 inch did not affect motion

Conservation of Momentum

A collision does not create or destroy momentum, but the sum of the momentum between colliding objects remains constant

Momentum before collision =

momentum after collision

An increase in momentum of one object is proportional to...

a decrease in momentum of another object

Transfer of Momentum

Momentum from 1 body is transferred to another

Ways in which momentum can be transferred

– 1 part of body to another(kinetic chain)

– Different bodies

– Body-to-object

– Object-to-object

concussions

Complex patho-physiological process affecting the brain, induced by traumatic biomechanical forces

- Headache, disoriented, emotional changes, amnesia, loss of consciousness, irritability, sensitive to light, cognitive impairment, sleep disturbance, drowsiness

- 1.6 - 3.8 million per year

Concussion Biomechanics

Linear & angular acceleration of head based on Newtonian concepts of force, torque, &conservation of momentum during collisions

Impact Frequency: High school vs College Football

College Football is more frequently: 322 to 1,354 impacts per season

- High school being around 652 per season

- about 22 per game and about 15.9 per practice

Concussion Threshold

75 to 90 g, 5580 rad/s^2?

- Linear Acc. = 74 to 168 g

- Angular Acc. = 5583 to 9516 rad/s^2

High School Concussion Rate

3.6 - 5.6%

College Concussion Rate

4.8 - 6.3%

What percent of impacts are less than 75g

98%

Is the human body a rigid object?

No, it is a series of linked rigid objects

Effective Mass

the portion of a body's mass that is involved with a collision

- depends on force during impact, time of force application, & speed of collision

How Does Effective Mass Increase or Decrease?

Depends on how tight the rigid objects are during impact

An increase in neck strength and tension can decrease concussion risk by how much?

35%

A greater coupling of head and torso causes---

an increase in effective mass

Concussions make up what percentage of soccer injuries?

22%

• 25-30% of concussions come from heading ball

• Max ball velocity > 28-30 m/s

• Soccer players avg 6-12 headings per game

• Estimated that a pro soccer player may play~300 games with >2000 headings in career

Long term Brain Damage from Soccer Heading

- Inconclusive,

- Some show:

-- Cognitive Issues

-- Cerebral atrophy

-- Increase Blood damage markers

- however nothing long-term

Females Soccer Players have how much percent less neck strength than males?

50%

Olympic Boxers - a hook punch ----

increases hand velocity of 11.0 ± 3.4 m/s & punch force of 4405 ± 2318 N

- Avg neck load = 855 ± 537 N

- Linear head acc = 71.2 ± 32.2 g

- Rotational acc = 9306 ± 4485 r/s2

Do Boxers punch with more rotational or linear acceleration?

Rotational Acceleration

Contact Adaptation: Boxing, Football, and Contact does what?

Increase Bone Mineral Density

Helmets increase mass and foam thickness =

decrease head acceleration and increase impact distribution