Linear Kinetics of Human Movement

What are Newton's Three Laws of Motion?

1: an object at rest/motion will remain at rest/motion unless acted upon by an unbalanced force 2: force x mass = acceleration 3: for every action there is an equal and opposite reaction

What is friction?

A force acting over the area of contact between two surfaces (ALWAYS! cannot be one) (ex: hoof and ground) • Direction is opposite of motion or motion tendency ==> always opposite of impending motion Ex: Moving a fridge on vinyl v. carpet ==> vinyl = easier b/c there's less friction

Discuss the Born to Run book that was talked about in class.

Based off the Tarahumara tribe in NW Mexico Main mode of transportation = run/jog/quick walk Runs for survival, communication, sport, etc. Runs barefoot or deer skin/tire sandals Drinks corn beer = increases hydration and glycogen (carb)

What are ways an athlete aims to increase or decrease the coefficient of friction with their attire?

Basketball shows wiping them = decrease ms Shaving body = decreases ms

If horse force = carriage force, how can the cart move?

Cart muse overcome the friction forces in order to move Friction force exert the same force as cart (just in opposite direction)

Change 2 surfaces =

Change coefficient Ms or Mk

Barefoot or forefront runners = _____________ impact peak

DECREASES

Decrease GRF = ____________ risk of injury

DECREASES

Move easier = ____________ Ms

Decrease

Player A is 92 kg and collides into player B. Player A's acceleration is 4.9 m/s2. How much force does player B exert back on to player A at the time of the collision?

F = 450.8 kgm/s^2 Not considering direction Applies to Newton's 3rd Law

Player A is 92 kg and collides into player B. Player A's acceleration is 4.9 m/s2. Figure out how much force player A exerts on to player B.

F = m x a F = 92kg x 4.4 m/s^2 F = 450.8 kgm/s^2 Applies to Newton's 2nd Law

How do we calculate friction?

F = mR m = coefficient of friction R = normal reaction force

(T/F) In accordance with Newton's second law of motion, ground reaction forces are sustained with every footfall during running.

FALSE; In accordance with Newton's THIRD law of motion, ground reaction forces are sustained with every footfall during running.

(T/F) Kinetic friction is always MORE than static friction.

FALSE; Kinetic friction is always LESS than static friction

Describe Fa and Fm in this scenario: Larger applied force Maximum static friction force No motion but motion impending

Fa = Fm Overcome maximum friction force = increase anymore and motion will occur

Describe Fa and Fs in this scenario: Small applied force Small friction force No motion

Fa = Fs Fs = small static friction

Describe Fa and Fk in this scenario: Larger applied force Motion occurring

Fa > Fk

How do you calculate the magnitude of kinetic friction force?

Fk = mkR

How do you calculate maximum static force?

Fm = msR

The μs between a basketball shoe and a court is 0.56, and the normal reaction force acting on the shoe is 350 N, how much horizontal force is required to cause the shoe to slide?

Fm= μs*R F = 0.56 x 350N = >196 N must be more than the max static force (196)

What are ways an athlete aims to increase or decrease the coefficient of friction with their equipment?

Gymnast chalk = increase ms (stick more)

Increase impact peak = ______________ likelihood of injury

INCREASES

What is the principle of conservation of momentum?

In the absence of external forces, the total momentum of a given system remains constant. M1 = M2 (mv)1 = (mv)2

Increase m = ______________ friction force

Increase

Move harder = ________ Ms

Increase

(T/F) In accordance with the law of reaction, the weight of a box sitting on a table generates a reaction force by the table that is equal in magnitude and opposite in direction to the weight.

TRUE

(T/F) Most impacts fall in between perfectly elastic and perfectly plastic.

TRUE

What is momentum?

Quantity of motion possessed by a body • Measured as the product of a body's mass and its velocity;

Is friction larger in a static or dynamic system?

Static

If a person had barely any squat/push off phase and landed with their leg straight, how would this impact: a) Impulse b) Time in air c) GRF d) Impact peak

a) Lowers impulse b) Less time in air c) Lowers GRF d) Increases impact peak ***look at PP diagram

Change weight =

change momentum

Momentum =

mass x velocity

What kinds of impacts are there?

• Perfectly elastic impact - in which the velocity of the system is conserved; (e = 1)(superball bounce is close...) • Perfectly plastic impact - in which there is a total loss of system velocity; (e = 0)(spaghetti hits a wall)

Describe the difference between kinetic and static friction.

Kinetic friction = horizontal force + motion Static friction = horizontal force, NO motion

How does a counter-movement vertical jump affects COM and GRF?

Lowers COM = Decreases GRF = applied forced to ground and pushes off

Are rearfoot or midfoot runners more optimal during running? Why?

Midfoot runners because they have a less GRF