biomechanics 4.ppt

8/9/2019 Biomechanics 4.ppt

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Work, Power, and Energy

Explaining the Causes of Motion

Without Newton


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Objecties

! "efine #echanical work

! "istinguish the differences between

positie and negatie work! "efine energy

! "efine kinetic energy


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Objecties

! "efine graitational potential energy

! "efine strain energy

! Explain the relationship between#echanical work and energy

! "efine power


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$ntroduction

! %he explanations for the causes of #otiondescribed in this chapter do not rely onNewton&s laws of #otion but rather on the

relationship between work, energy, andpower

! 'o#e analyses and explanations are

easier if based on work and energyrelationships rather than Newtonian#echanics


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Work

! Product of force and the a#ount ofdisplace#ent in the direction of that force

! Means by which energy is transferred fro#one object or syste# to another

! ( ) *+d- ( ) work done on an object

- * ) aerage force applied to an object- d ) displace#ent of an object along the line of

action of the force


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Work

! (nits for work are units of force ti#e units

of length +ft.lb or N#

!$nternational units/joule

+0 is the unit of#easure#ent for work

! 10 ) 1N#


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Work

! %o deter#ine the a#ount of work done

on an object we need to know three

things2

13 4erage force exerted on the object

53 "irection of this force

63 "isplace#ent of the object along the line of

action of the force during the ti#e the forceacts on the object


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Work

! "iscus thrower exerts an aerage force of

1777N against the discus while the discus

#oes through a displace#ent of 38# in

the direction of this force

! 9ow #uch work did the discus thrower do

to the discus:


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Work

! 4 weightlifter bench;presses a 1777N barbell/

c# aboe the chest/?owers the

barbell to >c# aboe the chest/?ifts the barbellback to the starting position =>c# aboe the

chest/4erage force while lowering and lifting

1777N upward

! 9ow #uch work did the lifter do on the barbell

fro# the start until the finish of the lift:


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Work

! 9ow #uch work during the lowering:

! 9ow #uch work during lifting:

! Work can be positie or negatie- Positie work is done by a force acting on an

object if the object is displaced in the sa#edirection as the force/Exa#ples:

- Negatie work is done by a force acting on anobject when the object is displaced in thedirection opposite the force acting on it/Exa#ples:


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Work

! 'a#ple Proble# @31 +text p3 17>

! 4 therapist is stretching a patient/

%herapist pushes on the patientAs foot with

an aerage force of 577N/Patient resists

the force and #oes the foot 57c# toward

the therapist

! 9ow #uch work did the therapist do on

the patientAs foot during this stretch:


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Energy

! Capacity to do work

! Many for#s +e3g3 heat, light, sound,

che#ical

! $n sports pri#arily concerned with

#echanical energy

- Binetic/energy due to #otion

- Potential/energy due to position


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Binetic Energy

! Moing object has the capacity to do work

due to its #otion

! Mass and elocity of an object affects

kinetic energy and the capacity to do work

! Binetic energy is proportional to the

suare of the elocity


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Binetic Energy

! BE ) D#

- BE ) kinetic energy

- # ) #ass

- ) elocity

! (nits for kinetic energy are units of #ass ti#es

elocity suared, or kg+#Fs or Gkg+#FsH# or

N# or 0oules

! (nit of #easure#ent for kinetic energy is the

sa#e as the unit of #easure#ent for work


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Binetic Energy

! 9ow #uch kinetic energy does a baseball

thrown at I7#iFhr +6>3I#Fs hae: 4

baseball #ass is 1@>g +31@>kg3

! "eter#ining the kinetic energy of an

object is easier than deter#ining the work

done by a force, because we can #easure

#ass and elocity #ore easily than wecan #easure force


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Potential Energy

! Energy an object has due to position

- Jraitational/Energy due to an objectAs

position relatie to the earth

- 'train/Energy due to the defor#ation of an

object


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Jraitational Potential Energy

! Kelated to the objectAs weight and itseleation or height aboe the ground orso#e reference point

! PE ) Wh or PE ) #gh- PE ) graitational potential energy

- W ) weight

- # ) #ass- g ) acceleration due to graity

- h ) height


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Jraitational Potential Energy

! 9ow #uch graitational potential energy

does a =77N ski ju#per hae at the top of

a L7# ju#p:

!


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'train Energy

! Energy due to the defor#ation of an object

! Kelated to stiffness, #aterial properties,

and its defor#ation

! 'E ) Dkx

- 'E ) strain energy

- k ) stiffness or spring constant of #aterial

-x ) change in length or defor#ation of the

object fro# its undefor#ed position


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'train Energy

! 9ow #uch strain energy is stored in a

tendon that is stretched 377># if the

stiffness of the tendon is 17,777NF#:

! $n hu#an #oe#ent and sports, energy is

possessed by athletes and objects due to

their #otion +kinetic energy, their position

aboe the ground +potential energy, andtheir defor#ation +strain energy


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Work/Energy Kelationship

! Kelationship exists between work and

energy/Work done on an object can

change total #echanical energy

! "iscus exa#ple

- What was the elocity of the discus at the end

of the period of work:


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! Work done ) BE PE 'E

! Work done ) BE 7 7

! Potential energy is ero because thedisplace#ent of the discus was horiontal

! # ) 5kg

! ( ) 8770

! i ) 7#Fs

! f ) :


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! %he work done by the external forces

+other than graity acting on an object

causes a change in energy of the object

! ( ) E

! ( ) BE PE 'E


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"oing Work to $ncrease Energy

! $n sports and hu#an #oe#ent, we are often

concerned with changing the elocity of an

object

! Changing elocity #eans changing kineticenergy

! ?arge change in kinetic energy +and thus a large

change in elocity reuires that a large force be

applied oer a long displace#ent

- 'i#ilar to i#pulseF#o#entu# relationship


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! Kules for shot putting indicate that the put

#ust be #ade fro# a =ft dia#eter circle

! 'ie of the ring thus li#its how #uch work

the athlete can do to the shot by

constraining the distance oer which the

putter can exert a force on the shot

! Early 57thcentury, shot;putters began their

put fro# the rear of the ring


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! %echniue has now eoled with

shoulders turned toward rear of the circle

in the initial stance/4llowed greater

displace#ent of shot before release

! Work done on the shot increased/

Jreater height +potential energy and

elocity +kinetic energy of the shot atrelease/Kesulted in longer put


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! 'a#ple Proble# @35 +text p3 117

! Pitcher exerts an aerage horiontal force of

177N on a 31>kg baseball during deliery of a

pitch/9and and ball #oe through a horiontaldisplace#ent of 13># during the period of force

application/$f the ballAs horiontal elocity was

ero at the start of the deliery phase, how fast

will the ball be going at the end of the delieryphase when the pitcher releases the ball:


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! # ) 31>kg

! * ) 177N

! d ) 13>#! i ) 7

! f ) :

! ( ) E


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"oing Work to "ecrease Energy

! When you catch a ball, its kinetic energy isreduced +or absorbed by the negatie work youdo on it

! our #uscles do negatie work on your li#bsand absorb energy when you land fro# a ju#p orfall

! 4erage force you #ust exert to absorb energy

in catching a ball or landing fro# a ju#p or falldepends on how #uch energy #ust be absorbedand the displace#ent oer which the force isabsorbed


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"oing Work to "ecrease Energy

! 'afety and protectie euip#ent used in #any

sports utilies the workFenergy principle to

reduce potentially da#aging i#pact forces

! Exa#ples of shock absorbing or energyabsorbing #aterials

- ?anding pads +gy#nastics, high ju#ping, and pole

aulting increase displace#ent of the athlete during

the i#pact period- 'and +long ju#per, water +dier, #idsole #aterial in

shoes +runner


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Conseration of Mechanical Energy

! %otal #echanical energy of an object isconstant or consered when no externalforces +other than graity act on the

object +e3g3 projectile #otion! "rop a 1kg ball fro# a height of @3L1#-

Potential energy +PEi of the ball just

before letting go is the sa#e as the kineticenergy +BEf of the ball just before hittingthe ground


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! We can deter#ine how fast the ball was

going just before it hits the ground

! PEi) Bef

! #gh ) D#f

! We could also use the euation fro#

Chapter 5- f ) 5gy +p3 88


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! Pole aulting- %otal #echanical energy at the instant of

takeoff should eual the total #echanical

energy at bar clearance- Qaulters kinetic energy at takeoff is

transfor#ed into strain energy as the polebends, and this strain energy is then

transfor#ed into potential energy- 9eight of a pole ault largely dependent on

running speed


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Power

! Kate of doing work

! $n sports, excelling reuires not just the

ability to do a large a#ount of work, but

also the ability to do that work in a short

a#ount of ti#e

! Power can be thought of as how uickly or

slowly work is done


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Power

! '$ units for power are watts +W

! 1W ) 10Fs

! P ) (Ft- P ) power

- ( ) work done

-t ) ti#e taken to do the work

! P ) *+dFt

! P ) *


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Power

! Power can be defined as aerage forceti#es aerage elocity along the line ofaction of that force

! Co#bination of force and elocitydeter#ines power output/What is thebest tradeoff:

! Cycling/9igher gear +higher pedal forcesand slower pedal rate ersus ?ower gear+lower pedal forces and higher pedal rate


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Power

! Characteristics of #uscles deter#ine the opti#altradeoff between force and elocity

! 4s a #uscleAs elocity of contraction increases,its #axi#u# force of contraction decreases

! $f the #uscleAs elocity of contraction is #ultipliedby its #axi#u# force of contraction for thatelocity, the #uscleAs power output for eachelocity can be deter#ined

! Maxi#u# power occurs at a elocityapproxi#ately one;half the #uscleAs #axi#u#contraction elocity +depending on specific#oe#ent and training status


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Power

! Places a constraint on perfor#ance

! "uration of actiity influences the power outputthat an indiidual can sustain

! Oly#pic weightlifter perfor#ing a clean and jerk+high force and high elocity generates a QEK?4KJE power output, but only for a brief interalof ti#e

! 'printer, #iddle distance runner, #arathonrunner/Power output progressiely decreasesas the length of the actiity increases


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'u##ary

! Work done by a force is the force ti#es thedisplace#ent of the object along the line ofaction of the force acting on it

! Energy is the capacity to do work

! Energy can be diided into potential +positionand kinetic +#otion

! Potential energy can be diided into graitationaland strain

! %he work done by a force +other than graitycauses a change in energy of an object

! Power is defined as the rate of doing work

biomechanics 4.ppt

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