Force 2.10-2.12 Student Copy

8/17/2019 Force 2.10-2.12 Student Copy

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2.10 – 2.11


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Form 4© COPYRIGHT BY SUPER EDUCATION GROUP (JO

2.10 – 2.11

PREPARED BY JJ R!EON P"#$ 1© COPYRIGHT BY SUPER EDUCATION GROUP (JOHOR JAYA)


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F

% %

F&'

F

'

2.10 – 2.11

2.10,2.11 UNDERSTANDING WORK, ENERGY AND EFFICIENCY

A Wor

1. Work is the product of applied force and !"#$%&'ement in the direction of the applied force.

2. When the work is done ener(y is transferred from one object to another.

3. The work done is equal to the amount of ener(y transferred.

4. The SI unit for work is )o*%e.

The formulae of work

WORK = FORCE X DISPLACEMENT

W = F ! s

No wor "# !one w+en

1- The object is stationary

2) The direction of motion of the object is perpendicular to that of the applied force


W : work in Joule/J

F : force in Newton/N

s : displacement in

Work done

Force and displacement in the samedirection

Force and displacement in different directions

W = F.s W = Work F = Force s = displacement

W = F X sW = F s cos θ W = work F = force s = displacement θ = angle between force

and displacement

F

F Y F Y

F

F&


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D%*"+$m$,- / m

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Diagram (a) Diagram (b)

Diagrams (a) and (b) shows a boy pshing a load and a weightlifter lifting aload of !" kg

a) #alclate the work done

i. by the boy

W = F.s = 20 x 2 = 40 Nm or 40

ii. by the weightlifter

W = F.s = m!h

= "0 x #0 x 2 = #200 Nm or #200

$. %&man is plling a bo' with a force of " at an angle of !"o from the hori&ontal.#alclate the work done to mo*e the bo' to a distance of + m.

Wor$ = %omponent of force x displacement&'n the direction of displacement)

= (0 cos "0o x = 2( x = *(

PREPARED BY JJ R!EON P"#$ /© COPYRIGHT BY SUPER EDUCATION GROUP (JOHOR JAYA)


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2.10 – 2.11

"!ample 1

"!ample 2

#$ %

&$$

s ' ( m

"!ample 3

"!ample 4

) ' &$$ %

S ' $.# m


For+$ F

s


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2.10 – 2.11

PREPARED BY JJ R!EON P"#$ © COPYRIGHT BY SUPER EDUCATION GROUP (JOHOR JAYA)


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%

For+$ F

2.10 – 2.11

Ener(y

1. "ner*+ is the potential to do work.

2. "ner*+ cannot be created nor be destro+ed.

3. "!ist in ,arious forms such as $otent"&% ener(y, "net"' ener(y- electrical ener*+- sound ener*+-

nuclear ener*+- heat and chemical ener*+

"ner*+ is !e"ne! as the capacit+ to do work.

Wor "# !one when ener(y "# 'on/erte! from one form to another.

The unit of work is Nm or o*%e-

A.Wor !one &n! t+e '+&n(e "n "net"' ener(y

1. inetic ener*+ is ener*+ of an object due to its motion.

2. /efer to the fi*ure abo,e-

3. "!ample 0 small car of mass 1$$ k* is mo,in* alon* a flat road. The

resultant force on the car is 2$$ %.

a What is its kinetic ener*+ of the car after mo,in* throu*h 1$ m b What is its ,elocit+ after mo,in* throu*h 1$ m

Solution : Given : m = 100 k ! F = "00 N

a. #inetic ener$! %k = Fs

= "00 & 10= "000 J

'. (elocit$! v ) * mv " = "000

v = +.," m s

-1


Work ' )s

' mas

' m ,2

The formulae of inetic ener*+- "k ' m,2

Through, v 2 = u2 +2as

u = 0

and, as = ½ v 2


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2.10 – 2.11

. Wor !one &n! (r&/"t&t"on&% $otent"&% ener(y

h ' 1.( m

1. 5ra,itational potential ener*+ is ener(y o &n o3)e't !*e to "t# $o#"t"on. $o##e##e! 3y &n o3)e't !*e

to "t# $o#"t"on "n & (r&/"t&t"on&% "e%!-

2. /efer to the fi*ure abo,e

W ' F s ' m( h

where- F 4 m(

So, Gr&/"t&t"on&% ener(y, E$ 4 m(+



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2.10 – 2.11



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2.10 – 2.11

5r"n'"$%e o 'on#er/&t"on o ener(y

the principle of conser,ation of ener*+.

1. "ner*+ cannot be created or destro+ed but can be chan*ed from one form to another form.

2. E6&m$%e 6 a thrown ball upwards will achie,e a ma!imum hei*ht before chan*in* its direction and falls

3. "!ample in calculation 6 0 coconut falls from a tree from a hei*ht of 2$ m. What is the ,elocit+ of

coconut just before hittin* the earthGiven : = "0 m! u = 0 ! = . ms-" ! v =

% p = %k

m = * mv "

m2.32"03 = *mv "

v " = ,"! v = 1. m s-1

5ower

1. 7ower is the rate of doing or!"

rate of energ# transfor$ation"

4erefore! power! 5 =ti$eta!en

or!done

! so! 5 =t

W

Were! 5 : power in watt/W

W : work in 6oule/J

t : time to do work in seconds/s

2. 8nit6 9oule per second 9 s:1 or Watt W

3. 0 wei*htlifter lifts 1#$ k* of wei*hts from the floor to a hei*ht of 2 m abo,e his head in a time of $.# s.

What is the $ower (ener&te! b+ the wei*htlifter durin* this time

* ' ;.# ms:2


7a&imum 5otential ener$

#inetic ener$ decrease potential ener$ decrease

and potential ener$ and kinetic ener$

8ncrease increase

7a&imum

kinetic ener$


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2.10 – 2.11

E"'"en'y

1.


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2.10 – 2.11

,amy releases a $ kg metal ball from a bilding -" m high (ake acceleration



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2.10 – 2.11 de to gra*ity as 1" ms/$)

a) %t the height of -" m0 the metal ball has (gra*itational potential energykinetic energy)

b) 2st before the metal ball hits the grond0 the ma'imm energy that it has is (gra*itationalpotential energykinetic energy).

c) #alclatei) he energy of the metal ball at the height of-" m.

3gra*itational = mgh

= ($) (1") (-")= 4"" 2

ii) the kinetic energy of the metal before it hits the grond.

+$inetic = +!ra,itational= -00

d) What is the principle sed in c ii)5

The principle of conser,ation of ener!y

. % motor lifting a weight 1 kg to a height of -." m in - s. he inpt energy spply to the motor in onesecond is $" 2. #alclate

a) power of the motor

o/er = /or$ donetime ta$en

= m!h t

= ) )&4.0) 4

= #0 /att

b) the efficiency of the motor


6etal ball

-" meter


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2.10 – 2.11

+fficiency = seful ener!y output x #001 +ner!y input

= #0 x #00120

= (0 1

2.10 A55RECIATING T7E IM5ORTANCE OF MA8IMISING T7E EFFICIENCY OF DE9ICES

1. .

The unwanted ener*+ produced in the de,ice *oes to waste

inetic ener*+

"ner*+ loss due to "ner*+ loss "ner*+ loss "ner*+ loss due to friction at

friction in as heat as sound other parts in the

mo,in* parts en*ine


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2.10 – 2.11


Force 2.10-2.12 Student Copy

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