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2.10 UNDERSTANDINGWORK, ENERGY, POWER AND EFFICIENCY

LEARNING OUTCOMES :

A student is able to:

Explain the process of define work (W) as the product of an applied force (F) and

displacement (s) of an object in the direction of the applied force ie W = Fs

State that when work is done energy is transferred from one object to another

Define kinetic energy and state that Ek= mv2 Define gravitational potential energy and state that Ep = mgh

State the principle of conservation of energy

Define power and state that P=W/t

Explain what efficiency of a device is

Solve problems involving work, energy, power and efficiency

SECTION A. Choose the correct word in the bracket.

1. Work is the product of applied force and (distance/displacement) in the directionof the applied force.

2. When the work is done (force/energy) is transferred from one object to another.

3. The work done is equal to the amount of (temperature/energy) transferred.

4. The SI unit for work is (joule/watt).

SECTION B. Fill in the blank with the correct answer.

postion unchanged energy motion

1. Kinetic energy is the energy of an object due to its motion

2. Gravitational potential energy is the energy of an object due to its position in thegravitational field

3. The principle of conservation of energy states that energy can be transferredfrom one form to another but it cannot be created or destroyed.

4. The principle of conservation of energy explains that the total amount of energyalways remains unchanged.

SECTION C. State or false for each of the following statements.

1. Power is the rate of doing work (True/False)

2. The efficiency of a device is the percentage of the energyinput that is transformed into useful energy.

(True/False)

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SECTION D : State the transformation of energy.

Light energy Gravitational potential energy

Sound energy Elastic potential energy

Statement From Transformation of energy

1. Durian falls to the ground Gravitationalpotential energy

Kinetic energy+ sound energy

2. A bulb connected to a dry celllights up

Electrical energy heat energy+ light energy

3. A bell rings when the switch ispressed

Electrical energy Sound energy

4. The motion of an arrow whenan archer releases the string

Elastic potentialenergy

Kinetic energy+ sound energy

SECTION E : Answer all questions.

Work done

Force and displacement in the samedirection

Force and displacement in different directions

W = F.s W = WorkF = Forces = displacement

W = FX. sW = F s cos W = work

F = forces = displacement = angle between force

and displacement

F

s

2

Fx

FY FYF

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Diagram (a) Diagram (b)

1. Diagrams (a) and (b) shows a boy pushing a load and a weightlifter lifting aload of 60 kg

a) Calculate the work done

i. by the boy

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

ii. by the weightlifter

W = F.s= mgh= 60 x 10 x 2= 1200 Nm or 1200 J

2. Azman is pulling a box with a force of 50 N at an angle of 60o from the horizontal.Calculate the work done to move the box to a distance of 3 m.

Work = Component of force x displacement(In the direction ofdisplacement)

= 50 cos 60o x 3= 25 x 3= 75 J

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Displacement = 3 m

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3. Samyreleases a 2 kg metal ball from a building 40 m high (Take accelerationdue to gravity as 10 ms-2)

a) At the height of 40 m, the metal ball has (gravitational potentialenergy/kinetic energy)

b) Just before the metal ball hits the ground, the maximum energy that it hasis (gravitational potential energy/kinetic energy).

c) Calculatei) The energy of the metal ball at the height of40 m.

Egravitational = mgh

= (2) (10) (40)= 800 J

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

Ekinetic = Egravitational= 800 J

d) What is the principle used in c ii)?

The principle of conservation of energy

5. A motor lifting a weight 1 kg to a height of 4.0 m in 4 s. The input energysupply to the motor in one second is 20 J. Calculate

a) power of the motor

Power = work donetime taken

P = mght

= (1) (10)(4.0)4

= 10 watt

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Metal ball

40 meter

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b) the efficiency of the motor

Efficiency = Useful energy output x 100%Energy input

= 10 x 100%20

= 50 %

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2.11 APPRECIATING THE IMPORTANCE OF MAXIMIZING THE EFFICIENCY OFDEVICES

LEARNING OUTCOMES :

A student is able to:

Recognize the importance of maximizing efficiency of devices in conserving

resources

A. Fill in the box with the correct terms.

Kinetic Friction in engine Resistance in electrolyte

Resistance Electrical ChemicalLight

Device Conversion of energy Loss of energy Loss of energy due to

Bulb Electrical Heat

+ Light

Heat Resistance

Petrol engine Chemical Kinetic Heat and sound Friction inengine

Electric fan Electrical Kinetic Heat Resistance incoil + bearing

Battery Chemical Electrical Heat Resistance inelectrolyte

B. Underline the correct statement below.

Statement Answer 1. Most of the energy in mechanical devices loss as heat andsound.

(True/False)

2. A major cause of inefficiency in machines is due to friction. (True/False)

3. The output energy of devices always more then input energy. (True/False)

4. Maximizing the efficiency of devices makes the best use of theinput energy and reduces energy wastage.

(True/False)

5. Maximizing the efficiency of devices helps to conserve energy

resources.

(True/False)

6. When energy transformations take place, not all of energy isused to do useful work.

(True/False)

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LEARNING AREA : FORCE AND MOTION

2.12 UNDERSTANDING ELASTICITY

LEARNING OUTCOMES :

A student is able to:

Define elasticity

Define Hookes Law

Define elastic potential energy and state that Ep = kx2

Determine the factors that affect elasticity

Describe applications of elasticity

Solve problems involving elasticity

SECTION A : Fill in the blacks with the correct word or terms.

extension Force constant work energy transferred

Elastic limit elastic potential energy Car spring support elasticity

A cushion directly proportional Spring balance

1. The property materials that can return to its original shape or sizes when the externalforce no longer acts are known as elasticity.

2. Elastic limit of a spring is the maximum force that can be applied to a spring suchthat the spring will be able to be restored to its original length when the force isremoved.

3. Hookes Law states that the extension of a spring is directly proportional toapplied force provided that the elastic limit is not exceeded.

F = k x

Where F = Force on the springk = Force constant springx = extension of the spring

4. A spring that is stretched or compressed stores elastic potential energy.

5. When a force acts on a spring, work is done. The work done is stored aselastic potential energy.

6. Applications of elasticity

a) A cushion is used to support our body when we sit on it them.

b) Car shock absorbers enable passengers in a car to feel comfortableeven when it travels on a bumpy road.

c) Spring balance is used to measure mass/weight which is stretched when a loadis hung on it.

SECTION B : Factors that affect the elasticity of a spring. Complete the table below.

The larger the smaller

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The shorter elasticity

Factors Relationship to elasticity

1. Length The shorterthe spring, the larger the force constant of thespring.

2. Diameter of spring(coil)

The larger the diameter of a spring (coil), the smallertheforce constant of the spring.

3. Diameter of spring wire The largerthe diameter of spring wire, the larger the force

constant of the spring.4. Type of material The elasticity of a spring depends on the material it ismade of.

SECTION C : Answer all the question.

1. The force-extension graphs for two springs, R and S are shown below. Complete the

table that follows..

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Statement Spring R Spring SThe gradient of the graph 8 N cm- 4 N cm-1

The force constant of spring 8 N cm-1 4 N cm-1

Force needed to extend 1 cm of thespring

8 N. 4 N.

The area under the graph when thespring is extented by 5 cm

1 N m 0.5 N m

E lastic potential energy when the

spring is extented by 5 cm

1 J 0.5 J

From the graph, we may conclude that

Spring R is more stiff compared to spring S.

When each spring is extented by 5 cm, spring R stores more elastic

potential energy than spring S

2. The figures below shows the arrangement of apparatus in an experiment to

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Extension, x (cm)

Force, F (N)

0

20

10

40

SR

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determine the relationship between the extension, e of a spring T with weight, W.The relationship ofe and Wis shown in the graph.

a) Based on the graph,i. Mark with a cross (x) the elastic limit of the spring.

ii. Name the law that is related to the graph before the elastic limit isexceeded.

Hookes Law

iii. State the relationship between W and e before the elastic limit.

Wis directly proportional to e

iv. Determine the value ofe when W= 8 N. Show on the graph,how you determine the value ofe.

3.3 cm

b) The spring stores energy when it is extended.

i) Name the type of energy stores in the spring.

Elastic potential energy.

ii) Calculate the force constant of the spring in SI unit.

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Force constant of the spring = Force .Extension of the spring

= 100.04

= 250 N m-1

iii) Calculate the area under the graph, when the spring isextended from o cm to 4 cm.

Area of the graph = (10)(0.04)= 0.2 J

iv) Calculate the energy stored in the spring when it is extended by 4 cm.

E = Fx= (10)(0.04)= 0.2 J

v) What is the relationship between area under the graph and the energystores in the spring.

Area of the graph is equal to energy of the spring

c) Another spring T is added parallel with the spring T as shown in the figurebelow.

Sketch the graph of W against e for this experiment on the graph.

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Spring T

Spring T