1. Fundamental Concepts and Principles of Mechanics
Chapter 3
Fundamental Concepts
and
Principles of Mechanics

2. 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

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

4. 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

6. Basic dimensions in mechanics
Describe someone out for a run

7. 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?

8. Length Measure to describe Feet, inches, miles
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

9. Time Measure to describe how long it takes her
seconds, minutes, hours, days, months, years
Systeme Internationale d’Unites (SI)
second (s)

10. 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

11. Inertia Inertia Who is harder to start or stop moving
resistance to a change in state of motion
Who is harder to start or stop moving
Olympic weight lifter
Olympic gymnast

12. Inertia Inertia Who is harder to start or stop moving
resistance to a change in state of motion
Who is harder to start or stop moving
Olympic weight lifter: has more inertia
Olympic gymnast

13. Inertia and Mass Inertia Mass
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

14. Importance of Inertia & Mass
Provide another example of changing motion in
sport
exercise
workplace

15. 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)

16. 3 basic dimensions All that is needed to describe
Length
Time
Mass
Force: defined from the above
a push or pull acting on a body
All that is needed to describe
and explain the motion of objects

17. 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

18. 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

19. 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.
Newton I & whiplash injury
Homework: read & summarize
See video next slide

21. Newton I in action (with nasty consequences)

22. HANS device 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
HANS device
Click image for more info

23. 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

24. Newton III in action

25. 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

26. F = ma  Unbalanced force causes a change in motion (an acceleration)
CAUSE-EFFECT relationship

27. 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.

28. Click here
to go to
a site
with
practice problems
on forces, and
a review of
Newton’s 3 Laws

29. Units of Force Metric system (systeme internationale) English system
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

30. 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.

31. 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

32. 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

33. g = Gravitational acceleration
The acceleration of gravity
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
Amusement Park Physics
Newton and the Apple: click here to see more

35. Terminal
Velocity

36. 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

37. 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???)

38. 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