2.10 UNDERSTANDING WORK, 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 = ½ 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 direction of 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 ____________

2. Gravitational potential energy is the energy of an object due to its ____________ in the gravitational field

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

4. The principle of conservation of energy explains that the total amount of energy always remains ___________.

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 energy input that is transformed into useful energy.

(True/False)

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(..…………./ 29 X 100 = ………%)

SECTION D : State the transformation of energy..

Light energy Gravitational potential energySound energy Elastic potential energy

Statement From To1. Durian falls to the ground Kinetic energy

+ sound energy2. A bulb connected to a dry cell lights up

Electrical energy heat energy +

3. A bell rings when the switch is pressed

Electrical energy

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

Kinetic energy + sound energy

SECTION E : Answer all questions.

Work done

Force and displacement in the same direction

Force and displacement in difference direction

W = F.s W = Work F = ___________ s = ___________

W = FX . sW = __________ W = work F = force s = displacement θ = angle between ______ and _____________

F

s s

Fx θ

F

θ

2

F Fx

FY FY F

Diagram (a) Diagram (b)

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

a) Calculate the work done

i. by the boy

ii. by the weightlifter in lifting the load.

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.

3

Displacement = 3 m

3. Samy releasing 2 kg metal ball from a building of 40 m height (Take the acceleration due to gravity as 10 ms-2.

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

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

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

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

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

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

a) power of the motor

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Metal ball

40 meter

b) the efficiency of the motor

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2.11 APPRECIATING THE IMPORTANCE OF MAXIMIZING THE EFFICIENCY OF DEVICES

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 electrolyteResistance Electrical Chemical

Light Heat

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

Bulb Electrical →+________

Heat ____________

Petrol engine Chemical → _________ Heat and sound ___________ ___________

Electric fan ________ → Kinetic _______ Resistance in coil + bearing

Battery ________ → Electrical Heat _____________

B. Underline the correct statement below.

Statement Answer1. Most of the energy in mechanical devices loss as heat and

sound.(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 the input 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 is used to do useful work.

(True/False)

6

(.……./ 17 X 100 = ………%)

LEARNING AREA : FORCE AND MOTION

2.12 UNDERSTANDING ELASTICITY

LEARNING OUTCOMES :

A student is able to: Define elasticity Define Hooke’s Law Define elastic potential energy and state that Ep = ½ kx2 Determine the factors that affect elasticity Describe applications of elasticity Solve problems involving elasticity

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

extension Force constant work energy transferredElastic limit elastic potential energy Car spring support elasticityA cushion directly proportional Spring balance

1. The property of materials that can return to its original shape or size when the external force no longer acts on it is known as ______________.

2. ___________ of a spring is the maximum force that can be applied to a spring such that the spring will be able to be restored to its original length when the force is removed.

3. Hooke’s Law states that the extension of a spring is ________________to applied force provided that the elastic limit is not exceeded.

F = k xWhere F = Force on the spring

k = ________________spring x = ______________of the spring

4. A spring that is stretched or compressed stores __________________ _____________.

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

6. Applications of elasticity

a) ________________is used to support our body when we sit on it them.

b) ________________ enable passengers in a car to feel comfortable even when it travels on a bumpy road.

c) _______________is used to measure mass/weight which is stretched when a load is hung on it.

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

7

(.……./ 38 X 100 = ………%)

The larger the smallerThe shorter elasticity

Factor Relationship to elasticity1. Length the spring, the larger the force constant

of the spring. 2. Diameter of spring

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

3. Diameter of spring wire the diameter of spring wire, the larger the force constant of the spring.

4. Type of material The of a spring depends on the material it is made 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 ____N cm-1 ____ N cm-1

The force constant of spring ____N cm-1 ____N cm-1

Force needed to extend 1 cm of the spring

______N. _______N.

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

100_N cm____N m

50 _N cm____N m

E lastic potential energy when the spring is extented by 5 cm

___J ____J

From the graph, we may conclude that

Spring ___ is more stiff compared to spring____.

When each spring is extented by 5 cm, spring _____stores more elastic

potential energy than spring ____

2. The figures below shows the arrangement of apparatus in an experiment to 8

Extension, x (cm)

Force, F (N)

0

20

10

40

SR

5

determine the relation ship between the extension, e of a spring T with weight, W. The relationship of e and W is 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 is exceeded.

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

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

b) The spring stores energy when it is extended.

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

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

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

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

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

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

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

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Spring T

Spring T