2 energy and its conservation (1)

8/19/2019 2 Energy and Its Conservation (1)

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Energy and its Conservation

Physics

Mrs. Coyle


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Part I

• Mechanical Energy

– Potential

– Kinetic

• Work Energy Theorem


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Energy and Work

• Energy is the ability to do work.

• Work is the energy transferred toor from a system by a force thatacts on it.


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Video ink! Kinetic "c#l$t#re

• htt$!%%www.yo#t#be.com%watch&v'W

c()*+t#,o-feat#re'related

http://www.youtube.com/watch?v=WcR7U2tuNoY&feature=relatedhttp://www.youtube.com/watch?v=WcR7U2tuNoY&feature=relatedhttp://www.youtube.com/watch?v=WcR7U2tuNoY&feature=relatedhttp://www.youtube.com/watch?v=WcR7U2tuNoY&feature=related


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Energy

• "ymbol! E

•

"calar

• *nits!

–/0 /o#le

–cal0 calorie

–kcal0 kilocalorie 1Cal2


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Mechanical Energy

• Potential Energy

• Kinetic Energy


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Kinetic Energy0 KE= 1 mv2

2

Energy of Motion


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Problem 34 bird of mass +kg is traveling at a s$eed of

5m%s. What is its kinetic energy& 14! 65/2

If the s$eed of the bird do#bles whatha$$ens to its kinetic energy&


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7#estion

• Can Kinetic Energy be ,egative&


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Work8Energy Theorem

W'∆KE

W'KEf 8KEi

The net work done on an ob9ect ise:#al to the change in its kineticenergy.


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Problem +If (obin ;ood a$$lies a 3


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Problem 6

What is the

work done bygravity as an> kg ob9ectfalls from

rest at 3


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Work Aone by the Bravitationalorce

"ystem is an ob9ect and the earth.

4s an ob9ect falls!W' mgh

4s an ob9ect is thrown #$!W' 8mgh


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Potential Energy0 PE'mgh

• Energy of $osition

• "tored energy

• "ymbol! * or PE

• *nit! /o#le

• Com$ared to a(eference Point

1base level2


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Problem @ 4 flower $ot of mass


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Part II

• Conservation of Energy


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Conservation of Energy

• Energy can neither be created nordestroyed. It can only change fromform to form.

• Mechanical Energy0 PE and KE

•Conservation of Mechanical Energy

PE1 + KE1 = PE2 + KE2


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What ha$$ens when friction is$resent&

• When friction is $resent0 the workdone by the frictional force W'fd istransferred to heat energy.

• riction is a non8conservativeF force.


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4 orce is ,on8ConservativeFif!

• the work it does on an ob9ect that movesbetween two $oints de$ends on the $athtaken.F

• the work it does on an ob9ect that movesthro#gh a ro#nd tri$ is non8Gero.F

• EHam$le! friction0 tension0 normal force0$ro$#lsion forces.


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4 orce is ConservativeF if!

• the work it does on an ob9ect that movesbetween two $oints de$ends only on the$osition of these two $oints and not on the$ath.F

• the work it does on an ob9ect that movesthro#gh a ro#nd tri$ is Gero.F

• EHam$le! gravity0 force of a s$ring.


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Problem <4 6kg watermelon sits ona table 3.+m above thegro#nd.

a2What is its PE at the to$

com$ared to the gro#nd&

b2 What is its KE at the to$&

c2 With what s$eed will it hitthe gro#nd&

4! 65/0 =/0 @.m%s


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Problem 5 4 $enny is at the to$ of

the Em$ire "tateD#ilding 6>3m abovethe gro#nd. It is thenreleased. With what

s$eed will it hit thegro#nd&

4! >).6 m%s


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Problem )8(oller Coaster

4 roller coaster car started from $oint 4

at a height of 3==m. What is its s$eed

at $oint D&

4! @@.)m%s


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Problem > 8 Pend#l#m

a2 What is the$otential energy ofthe bob at $oint 4com$ared to the

gro#nd&

b2 What is the s$eed

of the bob at thebottom&

4! +.6/0 3.3m%s

A

2 energy and its conservation (1)

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