Transcript Forces and COM

Linear Kinetics Objectives
• Identify Newton’s 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
Linear Kinetics Outline - The Relationship
between force and motion
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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 – Newton’s law of acceleration (F=ma)
– 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
Effect of forces on the system (can be total
human body, or a part of the body)
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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
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)
Force Plates –
Measurement of ground
reaction forces
Coefficient
of friction,
resistance to
sliding:
Cfr = Frf
/Nof
Sample Prob
# 2, p 392
Coefficient of Restitution (liveliness or bounciness)
Coefficient of restitution (liveliness or bounciness)
Free body diagrams:
Instantaneous Effect of Force on
an Object
• Remember the concept of net force?
• Need to combine, or add forces, to
determine net force
• Newton’s 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)
Force Applied Through a Time: ImpulseMomentum Relationship (pp 295-399)
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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?
Impulse: area
under forcetime curve
Net impulse (Ft)
produces
a change in
momentum
(mV)
Sample problem
#4, p 397
Vertical
impulse
While
Running:
Area under
Force-time
curve
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
Conservation of momentum: when net impulse is zero
(i.e. the system is closed), momentum does not change
Sample prob
#3, p 396
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)
Power running up stairs:
Work rate = (weight X vertical dist) ÷ time
Sample prob
#6, p 405
Work while running on treadmill:
From McArdle and Katch. Exercise Physiology
Note that %grade = tan θ X 100,
and tan θ and sin θ are very
similar below 20% grade
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
Conservation of Energy
• In some situations, total amount of mechanical energy
(potential + kinetic) does not change
– Stored elastic energy converted to kinetic energy
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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
Energy conservation – Case I : elastic potential (strain) and kinetic
Potential energy (FD) +
Kinetic energy (1/2mv2)
remains constant
Energy conservation – Case II : gravitational potential and kinetic
Potential energy
(Wh) + kinetic
energy (1/2mv2)
remains constant
Conservation of energy: gravitational potential and kinetic
Sample problem
#7, p 408
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
Linear Kinetics Formulae