Download Chapter 3

Chapter 3
Fundamental
Concepts
and
Principles of
Mechanics
Mechanics
• 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
Mechanics
• 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 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
Rigid Body
Mechanics
Statics
-at rest
-constant velocity
Dynamics
-changing motion (acceleration)
Kinematics
-description of motion
Kinetics
-study of the forces that cause
or tend to cause the
changes in motion
Basic dimensions in
mechanics
• Describe someone out for a run
Basic dimensions in
mechanics
• 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?
Length
• 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
Time
• Measure to describe how long it takes
her
• seconds, minutes, hours, days, months,
years
• Systeme Internationale d’Unites (SI)
• second (s)
Length & 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
Inertia
• Inertia
• resistance to a change in state of motion
• Who is harder to start or stop moving
• Olympic weight lifter
• Olympic gymnast
Inertia
• 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 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 & Mass
• Provide another example of changing
motion in
• sport
• exercise
• workplace
Importance of Inertia & Mass
• 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 dimensions
• Length
• Time
• Mass
All that is needed to describe
and explain the motion of objects
• Force: defined from the above
• a push or pull acting on a body
Force
• 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 Newton
1642-1727
Proposed fundamental
laws that are the basis
of modern mechanics
3 laws of motion
law of gravitation
Click here to see more
on Newton
Newton's First Law of Motion
• 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 slide
Newton I & whiplash injury
Homework: read & summarize
Newton I in action (with nasty consequences)
HANS device
Click image for more info
The HANS® (Head And Neck
Support) Device was originally
designed to limit the extreme
front to back and
side-to-side movement of the
head and neck during a violent
crash. Using a collar and yoke
system made of carbon fiber
and Kevlar, the device is
connected to the helmet with a
series of quick connect tethers.
The HANS® Device is worn
around the neck and down the
front of the shoulders,
underneath the safety belts of the
shoulder harness. This
allows 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
Newton's 3rd Law of Motion
• 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
Skip
video
Newton III in action
Newton's 2nd Law of Motion
• 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 here
to go to
a site
with
practice problems
on forces, and
a review of
Newton’s 3 Laws
Units 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 how
Newton’s 3 laws 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...
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 Universal
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 for
some problems
to solve
g = Gravitational acceleration
•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
Terminal
Velocity
Weight
• 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 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 encountered
in KNR 282.
We all get heavier as we get older because there's a lot
more information in our heads."
- Vlade Divac
Click here for another great site on Newton’s Laws