Chapter 3 Chapter 3

Fundamental Concepts

andPrinciples of Mechanics

MechanicsMechanics

• Science concerned with the effects of forces acting on objects (body) • body: focus of the analysis

• human body• individual body segment• specific tissue / anatomical site• balls, pucks• implement: bat, stick, club

MechanicsMechanics

• Science concerned with the effects of forces acting on objects (body)• Rigid-body mechanics• Deformable body mechanics• Fluid mechanics• Relativistic mechanics• Quantum mechanics

Rigid Body MechanicsRigid Body Mechanics

• Acceptable for analyzing gross movements

• Assumptions• body does not deform by bending,

stretching or compressing• segments are rigid links joined by

frictionless hinges at joints

S ta tic s-a t res t

-con s tan t ve loc ity

K in em atics-d esc rip tion o f m otion

K in e tic s-s tu d y o f th e fo rces th a t cau se

or ten d to cau se th ech an g es in m otion

D yn am ics-ch an g in g m otion (acce le ra tion )

R ig id B od yM ech an ics

Basic dimensions in Basic dimensions in mechanicsmechanics

• Describe someone out for a run

Basic dimensions in Basic dimensions in mechanicsmechanics

• Describe someone out for a run• Kinematics

• How far did she run?• How long to run that far?• How fast was she? • How big is she?

• Kinetics• What friction under her feet?• What forces on her joints?• What tension in her muscles?

LengthLength

• Measure to describe• location at a particular point in her run• how far she ran

• Feet, inches, miles• Systeme Internationale d’Unites (SI)

• meter• 1 m = 3.28 feet = 39 inches

TimeTime

• Measure to describe how long it takes her

• seconds, minutes, hours, days, months, years

• Systeme Internationale d’Unites (SI)• second (s)

Length & time = motionLength & time = motion

• Space to move in and time during which to move

• speed & velocity==> length per unit of time • miles per hour• m / s

• acceleration• m/s/s

InertiaInertia

• Inertia• resistance to a change in state of

motion

• Who is harder to start or stop moving• Olympic weight lifter• Olympic gymnast

InertiaInertia

• Inertia• resistance to a change in state of

motion

• Who is harder to start or stop moving• Olympic weight lifter: has more inertia• Olympic gymnast

Inertia and MassInertia and Mass

• Inertia• resistance to a change in state of motion

• Mass• the quantity of matter a body possesses• quantifies inertia (the measure of

inertia)• Greater mass, greater inertia

resistance to change state of motion

• units are kilogram (kg) or slug (English)• Not the same as weight

Importance of Inertia & Importance of Inertia & MassMass

• Provide another example of changing motion in • sport• exercise• workplace

Importance of Inertia & Importance of Inertia & MassMass

• Provide another example of changing motion in • Sport: size expectations of different

positions (ie interior defensive linemen)• Exercise: alter mass to be moved to

increase load on NMS system (ie push-up)• Workplace: alter mass of components to

reduce load (ie cement bags, engine blocks)

3 basic 3 basic dimensionsdimensions• Length

• Time• Mass

• Force: defined from the above• a push or pull acting on a body

All that is needed to describeand explain the motion of objects

ForceForce• Force: a push or pull acting on a body that

causes or tends to cause a change in the linear motion of the body

• Characteristics of a force• magnitude• direction• point of application.• line of action

• Net Force: resultant force (overall effect of multiple forces acting on a body)

• Example: push from side and front = at angle

Sir Isaac Newton1642-1727

Proposed fundamental laws that are the basis of modern mechanics

3 laws of motionlaw of gravitation

Click here to see moreon Newton

Newton's First Law of Newton's First Law of MotionMotionNewton's First Law of Newton's First Law of MotionMotion

• Law of inertia - a body in motion will stay in motion and a body at rest will stay at rest unless acted upon by a net external force.

See video next slideNewton I & whiplash injuryHomework: read & summarize

Newton I in action (with nasty consequences)

HANS deviceHANS device

The HANS® (Head And Neck Support) Device was originally designed to limit the extreme front to back andside-to-side movement of the head and neck during a violentcrash. Using a collar and yoke system made of carbon fiberand Kevlar, the device is connected to the helmet with aseries of quick connect tethers. The HANS® Device is wornaround the neck and down the front of the shoulders,underneath the safety belts of the shoulder harness. Thisallows for normal movement of the head and helmet, but limits the extreme movement of the head and neck that is so common in the rapid deceleration during a crash

Click image for more info

Newton's 3rd Law of Newton's 3rd Law of MotionMotionNewton's 3rd Law of Newton's 3rd Law of MotionMotion

• Law of action-reaction - for every action, there is an equal and opposite reaction

• There are two bodies involved when force is exerted

• The force on each body is the same size, but in opposite directions

• true even when the bodies are of significantly different mass

• Sumo with my boys Gymnast on bar Skipvideo

Newton III in actionNewton III in action

Newton's 2nd Law of Newton's 2nd Law of MotionMotionNewton's 2nd Law of Newton's 2nd Law of MotionMotion

• Law of acceleration - an unbalanced force applied to a body causes an acceleration of that body with a magnitude proportional to the force, in the direction of the force, and inversely proportional to the body's mass.

Newton & Swimming

F = ma• Unbalanced force causes a

change in motion (an acceleration)• CAUSE-EFFECT relationship

During running, if a runner swings his arms across his body, there is a compensatory increase in pelvic rotation.

It is more efficient and better for the pelvis and pelvic musculature if the runner moves his arms parallel to the motion in which he is running.

Click hereto go toa sitewith

practice problemson forces, and

a review of Newton’s 3 Laws

Units of ForceUnits of Force• Metric system (systeme

internationale)• Newton (N)

• the amount of force necessary to accelerate a mass of 1 kg at 1 m/s2

• English system• pound (lb)

• the amount of force necessary to accelerate a mass of 1 slug at 1 ft/s2

• equal to 4.45 N

It is important to know It is important to know how how Newton’s 3 lawsNewton’s 3 laws apply.apply.

Click here to go to a site on Newton’s Laws suggested by Mike Dawson (Spring, 2000). See if you

can pick out the errors in the presentation.

Of Newton...Of Newton...

Alexander Pope wrote:

Nature and Nature's Laws lay hid in night:

God said, "Let Newton be!" and all was light

Quiz on Newton’s Laws

Newton's Law of Newton's Law of Universal GravitationUniversal GravitationNewton's Law of Newton's Law of Universal GravitationUniversal Gravitation

• Law of gravitation - all bodies are attracted to one another with a force proportional to the product of their masses and inversely proportional to the square of the distance between them.

Click here forsome problems

to solve

g = Gravitational g = Gravitational accelerationacceleration

• The acceleration of gravity• - 9.81 m/s2 (- 10 m/s/s)

• Negative sign??? • indicates that the acceleration caused

by gravity is directed downward or toward the center of the earth

• The earth sucks

Newton and the Apple: click here to see more

Amusement Park Physics

Term

inal

Velo

city

WeightWeight• Weight

• the amount of gravitational force exerted on a body

• F = m g• W = m g

• W: weight (wt) (units: N or lbs)• m: mass of the body (units: kg)• g: gravitational acceleration (units: m/s/s)

• As the mass of a body increases, its’ weight increases proportionally

Weight is a forceWeight is a force

• As a force, weight characterized by • magnitude

• proportional to mass

• direction• ALWAYS downward

• point of application• at center of gravity of the body

• Units: Newtons or pounds (mass???)

A problem often A problem often encountered in KNR encountered in KNR 282.282.

Click here for another great site on Newton’s Laws

We all get heavier as we get older because there's a lotmore information in our heads."

- Vlade Divac