Linear Kinetics Objectives

1
Linear Kinetics Objectives

• Identify Newtons laws of motion and gravitation
  and describe practical illustrations of the laws
• Explain what factors affect friction and discuss
  the role of friction in daily activities and
  sports
• Define impulse and momentum and explain the
  relationship between them
• Explain what factors govern the outcome of a
  collision between two bodies
• Discuss the interrelationship among mechanical
  work, power, and energy
• Solve quantitative problems related to kinetic
  concepts

2
Linear Kinetics Outline - The Relationship
between force and motion

• Read Chapter 12 in text
• Classification of forces
• Types of forces encountered by humans
• Force and motion relationships three ways to
  look at it
• Instantaneous effect Newtons law of
  acceleration (Fma)
• Force applied through time (Impulse-momentum)(Ft
  mv)
• Conservation of Momentum
• Force applied through distance (work-energy) (Fd
  1/2mv2)
• Conservation of Energy
• Self-study problems
• Sample problems 2 p 392 3 p 396, 4 p 397,
  5 p 402, 6 p 405, 7 p 408
• Introductory problems, p 411 1,3,5,7,8,10
• Homework problems (Due Wednesday, April 13)
• Additional problems, p 412 6,8,9

3
Effect of forces on the system (can be total
human body, or a part of the body)

• Action vs reaction
• Internal vs external
• Motive vs resistive
• Force resolution horizontal and vertical
  components
• Simultaneous application of forces determining
  the net force through vector summation

4
External forces commonly encountered by humans

• Gravitational force (weight mg)
• Ground Reaction Force (GRF)(Figure 12-4, p 386)
• Vertical
• Horizontal (frictional)
• Frictional force (coefficient of friction) (pp
  389-395)
• Elastic force (coefficient of restitution) (pp
  399-402)
• Free body diagram - force graph (p 63)

5
Force Plates Measurement of ground reaction
forces
6
Coefficient of friction, resistance to
sliding Cfr Frf /Nof Sample Prob 2, p 392
7
Coefficient of Restitution (liveliness or
bounciness)
8
Coefficient of restitution (liveliness or
bounciness)
9
Free body diagrams
10
Instantaneous Effect of Force on an Object

• Remember the concept of net force?
• Need to combine, or add forces, to determine net
  force
• Newtons third law of motion (F ma)
• Inverse dynamics estimating net forces from the
  acceleration of an object
• Illustrations from Kreighbaum Figures F.4, F.5,
  and F.6 (pp 283-284)

11
(No Transcript)
12
(No Transcript)
13
(No Transcript)
14
Force Applied Through a Time Impulse-Momentum
Relationship (pp 295-399)

• Force applied through a time
• Impulse - the area under the force-time curve
• Momentum - total amount of movement (mass x
  velocity)
• An impulse applied to an object will cause a
  change in its momentum (Ft mv)
• Conservation of momentum (collisions, or impacts)
• in a closed system, momentum will not change
• what is a closed system?

15
Impulse area under force- time curve Net
impulse (Ft) produces a change in momentum (?mV)
Sample problem 4, p 397
16
Vertical impulse While Running Area
under Force-time curve
17
Anterioposterior (frictional) component of GRF
impulse Is area under Force-time curve Positive
and Negative impulse Are equal if Horizontal
comp Of velocity is constant
18
Conservation of momentum when net impulse is
zero (i.e. the system is closed), momentum does
not change
Sample prob 3, p 396
19
Force Applied Through a Distance Work, Power,
Energy (pp 403-409)

• Work - force X distance (Newton-meters, or
  Joules)
• On a bicycle Work F (2?r X N)
• On a treadmill Work Weightd X per cent grade
• Running up stairs Work Weightd
• Power - work rate, or combination of strength and
  speed (Newton-meters/second, or watts)
• On a treadmill P Weightd X per cent grade/
  time
• On a bicycle P F (2?r X N) / time
• Running up stairs P Weightd /time (See next
  slide)
• Energy - capacity to do work
• kinetic, the energy by virtue of movement (KE
  1/2 mv2 )
• gravitational potential, energy of position (PE
  weight x height)
• elastic potential, or strain, energy of condition
  (PE Fd)

20
Sample prob 6, p 405
Power running up stairs Work rate (weight X
vertical dist) time
21
Work while running on treadmill
From McArdle and Katch. Exercise Physiology
Note that grade tan ? X 100, and tan ? and sin
? are very similar below 20 grade
22
Homework Calculating Power on a Treadmill

• Problem What is workload (power) of a 100 kg
  man running on a treadmill at 10 grade at 4 m/s?
• Solution
• Power force x velocity
• Force is simply body weight, or 100 x 9.8 980 N
• Velocity is vertical velocity, or rate of
  climbing
• Rate of climbing treadmill speed x percent
  grade 4 m/s x .1 .4 m/s
• Workload, workrate, or power 980N X .4 m/s
  392 Watts
• Note 4 m/s 9 mph, or a 6 min, 40 sec mile
• Calculate your workload if you are running on a
  treadmill set at 5 grade and 5 m/s.
• Answer for 200 lb wt (91 kg) is 223 Watts

23
Conservation of Energy

• In some situations, total amount of mechanical
  energy (potential kinetic) does not change
• Stored elastic energy converted to kinetic energy
• diving board
• bow (archery)
• bending of pole in pole vault
• landing on an elastic object (trampoline)
• Gravitational potential energy converted to
  kinetic energy
• Falling objects
• Videodisk on pole vault

24
Energy conservation Case I elastic potential
(strain) and kinetic
Potential energy (FD) Kinetic energy (1/2mv2)
remains constant
25
Energy conservation Case II gravitational
potential and kinetic
Potential energy (Wh) kinetic energy (1/2mv2)
remains constant
26
Conservation of energy gravitational potential
and kinetic
Sample problem 7, p 408
27
Three ways to minimize impact force of 2
colliding objects

• Force-time, or impulse-momentum relationship (Ft
  mv)
• Increase time through which force is applied
• Force-distance, or work-energy relationship (FD
  ½ mv2)
• Increase distance through which force is applied
• Force-area, or pressure concept (P F/a)
• Increase area over which force is applied

28
Linear Kinetics Formulae