Energy , Work & Power

Energy, Work and Power

T H E M E T W O : N E W T O N I A N M E C H A N I C S

C h a p t e r 7

Energy,Work & Power



C h a p t e r 7

2. Where do cars get energy?

From Food.

1. How does our body get energy?

From Petrol

YUMMY!!!YUMMY!!!



C h a p t e r 7

Definition:Definition:Energy is the capacity to do work.

Without Energy = no lightno electricityno water from tapno bus, no train!!!

Without Energy = CLASS WILL BE VERY QUIET!!!!!! can’t talk, can’t walk, can’t breath!!!

Without energy, no living, non-living things would work.

SO!!!!!!!!!!! Without energy: Everything dies no life.

What is Energy???What is Energy???

Txbk pg 5



C h a p t e r 7

Energy

Defined as: capacity to do WORK F x D

D = distance moved in the same direction as force applied

1 Joule = 1 Newton meter1 J = 1 Nm

Any body or system that can do work possess Energy.



C h a p t e r 7Quiz Which of the following are forms of

energy?

a. Soundb. Nuclearc. Elastic Potentiald. Chemical Potentiale. Joules

Is Energy MATTER?

NO! Energy does not occupy space and has no mass.



C h a p t e r 7Forms of EnergyForms of Energy

Energies in Action

thermalthermal

lightlight

soundsound

kinetickinetic

nuclearnuclear

ENERGYENERGY

Potential (stored)

Potential (stored)

chemicalchemical

electricalelectrical

GravitationalGravitational

ElasticElastic



C h a p t e r 7Kinetic Energy

Energy a body possess due to its motion:

Ek = KE = mv2

Ek = KE (J)m = mass (kg)v = speed of the body (ms-1)

1

2



C h a p t e r 7

Usain Bolt holds the Olympic record of 9.69s for his 100-m race. Assuming his mass is 70kg. What is the kinetic energy KE that he possess? Velocity=

69.9

100

t

D= 10.32 m/s

KE = ½ mv2

= ½ (70) (10.32)2

= 3727 J = 3730 J (3 s.f.)

30-second Quiz 1



C h a p t e r 730-second Quiz 2

Velocity= 5 km/h = = 1.389 m/s

KE = ½ mv2

= ½ (2000) (1.389)2

= 1929 J = 1930 J (3 s.f.)

A car with mass of 2000 kg is travelling with a speed of 5 km/h on PIE in a jam. What is its kinetic energy KE?

s

m

6060

10005



C h a p t e r 7Gravitational Potential

EnergyEnergy a body has due to its position.

Ep = PE = mgh

Ep = GPE (J)m = mass (kg)g = gravitational field strength (N/m)h = height (m)



C h a p t e r 730- second Quiz 3A box of mass 20 kg is being pushed up a

slope of 15m long with constant speed of 30 m/s as shown in Figure. a)What is the gain in gravitational potential energy?

20kg5 m

15 m

PE = mgh

= 20(10)(5)

= 1000J

m = 20g = 10 N/kgh = 5m



C h a p t e r 7

• Substances that can be burnt contain chemical potential energy.

• Thermal energy of an object = total kinetic energy of the atoms or molecules in the object.

*** Heat is the transfer of thermal energy from one body to another.

• Molecular kinetic energy is known as internal energy.

Other Types of Energy



C h a p t e r 7

COE



C h a p t e r 77.2 Conservation of Energy

Energy cannot be created or destroyed in any process, but only changes from one form to another or transferred from one body to another

Total amount of energy CONSTANT



C h a p t e r 7

BEFORE AFTER

Gravitational energy

Kinetic energy

Thermal energy and Sound

Amount of energy

no change

Conservation of Energy

Pile-driver for constructing buildings



C h a p t e r 7Conservation of Energy

Conservation_of_Energy_demolition_ball.wmv

• Conversion between PE and KEFor example, in the roller coaster,

As carriages move downwards, – PE KE.As carriages move upwards, – KE PE



C h a p t e r 7Conservation of Energy

KE = 0, PE = max

Assume negligible air resistance

KE = 0, PE = max

KE = max, PE = 0

Both KE + PE

Both KE + PE



C h a p t e r 7

Conversion of energy

Which one of the following correctly describes the energy conversion that occurs after a bungee jumper jumps from the bridge to the instant when the chord is extended to the maximum?

A. EPE KE GPEB. GPE KE EPEC. GPE EPE KED. KE GPE EPE

15 –second Quiz 4

EPE = elastic PE

KE = kinetic energy

GPE = gravitational PE

ans B



C h a p t e r 7Electricity for Singapore

high pressure steam

steam

air for combustion

Oil or gas

turbine

exhaust gases

condenser

water

4) Generator produces electricity

water for cooling

1) Fuel (oil or natural gas) is burnt

1) Fuel (oil or natural gas) is burnt

Water runs through pipes to boilerWater runs through pipes to boiler

2) water turned into steam under intense pressure.

2) water turned into steam under intense pressure.

3) Turbine turned by steam

3) Turbine turned by steam



C h a p t e r 7

• Worksheet 7 A Q3

• Text book Pg 127

• Try 7B Q 3 (3mins)

• Try 7B Q 4 (3mins)

Conservation of Energy



C h a p t e r 7

3 m

1 –min Quiz 5i

Loss of GPE = mgh

= 70 x 10 x 3

= 2100 J

An acrobat of mass 70 kg jumps down on to the seesaw and lift his partner upward. (Assume negligible air resistance and frictions Take g=10 N kg-1)(i) Calculate the loss of gravitational potential energy when the acrobat touches the see saw.



C h a p t e r 7

3 m

1 –min Quiz 5ii

Gain in KE = loss in PE

½ mv2 = 2100

½ x 70 x v2 = 2100

v = 7.75 m/s

An acrobat of mass 70 kg jumps down on to the seesaw and lift his partner upward. (Assume negligible air resistance and frictions Take g=10 N kg-1)(ii) What is the speed of the acrobat just before touching the see saw? loss in PE =

2100



C h a p t e r 7

3 m

1 –min Quiz 5iii An acrobat of mass 70 kg jumps down on to the seesaw and lift his partner upward. (Assume negligible air resistance and frictions. Take g=10 N kg-1)(iii) Given that his partner has a mass of 60 kg, how high would he reach? By Conservation of Energy,

Amount of energy transferred to partner = 2100 JGain in PEpartner = Loss in KE

mgh = 210060 x 10 x h= 2100

h = 3.5 m



C h a p t e r 7

Green arrow acceleration g (no change)

Blue arrow Velocity (highest at bottom, lowest at top)

Brown arrow Spring’s force when stretched (greatest at bottom)



C h a p t e r 7GPE

GPE + KE

Elastic PE (max stretch)

GPE + KE + Elastic PE

KE + Elastic PE (just before max stretch)

Green arrow acceleration g Blue arrow Velocity Brown arrow Tension



C h a p t e r 7What have we covered so far??Give examples of the various form of energy

state the principle of the conservation of energy

Give the formula for calculating KE and GPE?

apply the relationships for KE and GPE to new situations or to solve related problems

Next:

Apply relationship

Work Done = force x distance moved

in direction of force

to new situations or to solve related problems



C h a p t e r 7

• Work is done when a force produces motion.

• Work = force × distance moved in the direction of the force

• SI unit: joule (J).

F d

Initial position

Final position

7.3 Work

W = F × d

d must be in the direction that force F is applied



C h a p t e r 710-second Quiz 6In which of the following cases is work

done?

A. A person pushing a wooden box forward.

B. A person pushing the wall of a building.

C. A farmer carrying a bag of rice.D. Two opposing teams of people pulling a tug-of-war rope which is stationary.

F x DWORK =ans A



C h a p t e r 7

• No work is done unless a force causes an object to move in direction of applied force.

wall

No work is done!!

Wall did not move even though force is applied

No work is done!!

7.3 Work



C h a p t e r 7 7.3 Work




Fd = 1.5 mInitial

positionFinal position

Fr = 6N

On horizontal plane, Force required = Force toto move the block overcome friction

= 6 N

Hence, Work Done = F x D (D in direction of F)

= 6 x 1.5 = 9.0 J






C h a p t e r 7 2 min- Quiz 7An object of mass 20 kg is pulled up a slope of 15m long with a constant speed. The height of the slope is 5m. The frictional force between the object and the slope is 30 N. a)What is the work done to overcome friction?b)What is the total work done in pulling the object up the slope?

Fr = 30N

20kg5 m

15 m

Rope



C h a p t e r 7 2 min- Quiz 7a)What is the work done to overcome friction?b)What is the total work done in pulling the object up the slope?

a)WD to overcome friction

= 30 x 15 = 450 J

Total WD

= WD to overcome friction + WD to lift 20 kg up 5m

= 450 + 1000 = 1450 J

b) WD to lift 20 kg up 5 m = mgh = 20 x 10 x 5 = 1000J

20kg 5 m

15 m

Fr = 30N

Rope



C h a p t e r 7

Try 7B Q 7 ans : a) 2400 J,

b) 2400 J (explain)

c) KE = 1200 J

Those finished, do

Q 9: ans a) 0N, b) 100N, c) 150J, e) 50W

Q11: ans a) 2 ms-2 b) 60J

Work




W = F × d

WD against another force

WD to change object speed

• Work is done when a force produces motion.

• SI unit: joule (J).

eg, 1. Against gravity pull

2. Against elastic forces

3. Against friction etc



C h a p t e r 7 7.3 WorkW = F × d

WD against another force1. against gravitational force on object

W = mg

h

Initial position

Final position

Force to overcome gravity pull, F = W = mg

WD against gravity pull = F x d= W x h= mgh

F




WD against another force2. against elastic forces

WD to stretch spring= Fe x extension d

Fe




WD against another force3. against friction

F

d

Initial position

Final position

Fr

WD to overcome friction = Fr x d




WD to change speed of object

Work done on object to change its speed.

F

du m/s v m/s

Frictionless

KEi = ½ mv2

KEf = ½ mv2




WD against another force

WD to change object speed

eg, 1. Against gravity pull

2. Against elastic forces

3. Against friction etc



C h a p t e r 71- min Quiz 8A bullet of mass 50g was travelling at a speed of 200ms-1 before striking a sandbag. It travelled through 20cm of the sandbag before stopping.

What was the total resistive force produced by the sandbag?

Conservation Of Energy, Loss in KE = WD by bullet to move

through 20 cm of sandbag½ mv2 = F x d

½ (0.05) (200)2 = F x (0.20)F = 5000 N

Sandbag



C h a p t e r 7

Go through Wksht 7B

Q11: ans a) 2 ms-2 b) 60J

Work



C h a p t e r 7 7.4 Power

Walking leisurely Chased by dogTook 1 secTook 30 secs

Height risen 10

m

Feel more tired running upstairs compared to walking upstairs.

70kg70kg



C h a p t e r 7

Defined as:Rate of work done or Rate of energy conversion

Refers to how fast work is done

or how fast energy is converted

7.4 Power




Walking leisurely Chased by dogTook 2 secsTook 30 secs

Height risen 10

m

WD = F x d= mg x h= 7000 J

70kg70kg

WD = F x d= mg x h= 7000 J

SAME WD But running

took less time then walking!

Work is done fasterWork is done slower




Walking leisurely Took 30 secs

Height risen 10

m

WD = 7000 J

70kg70kg

Work is done fasterWork is done slower

SAME WD But running

took less time then walking!

Chased by dogTook 2 secs

WD = 7000 J

Running guy has more power!



C h a p t e r 7

Defined as:Rate of work done or Rate of energy conversion

SI unit : watt (W)

• Other units: Joule per second (J s−1)

taken timedonework

taken timechangeenergy

Power = =

t

d x F

t

Energyin==

7.4 Power



C h a p t e r 7

A man of mass 60 kg takes 1 min to run up a flight of stairs from X to Y as shown. What is his working power?

1- min Quiz 9

A.60 J

B.80 J

C.60 W

D.80 W

ANS: C



C h a p t e r 7

Height h = 6 m, m= 60 kg, t = 1 min = 60sGain in GPE = mgh

= 60 x 10 x 6 = 3600 J

t = 60 s

Power = =

A man of mass 60 kg takes 1 min to run up a flight of stairs from X to Y as shown. What is his working power?

1- min Quiz 9

Δ in Energy Time

360060

= 60 W



C h a p t e r 7

A girl weighing 400N runs up a flight of stairs of vertical height 5 m in 4 s. What is her gain in potential energy and the power developed?

1- min Quiz 10

Gain in GPE Power developed

A. 1600 J 400 W

B. 2000 J 500 W

C.16 000 J 4000 W

D.20 000 J 5000 W

ANS: B



C h a p t e r 7

Height h = 5 m, W= 400 N, t = 4sGain in GPE = mgh

= 400 x 5 = 2 000J

t = 4 s

Power = =

A girl weighing 400N runs up a flight of stairs of vertical height 5 m in 4 s. What is her gain in potential energy and the power developed?

1- min Quiz 10

Δ in Energy Time

20004

= 500W



C h a p t e r 7

Height h = 5m, mass = 200 kgWD in raising the water = mgh

= 200 x 10 x 5 = 1 x 104J

Everyday t = 24 x 60 x60 = 8.64 x 104 s

Power = =

A windmill is used to raise water from a well. The depth of the well is 5 m. The windmill raises 200 kg of water every day. What is the useful power extracted from the wind?

1- min Quiz 11

Work Done Time

1 x 104 8.64 x 104 = 0.116 W

0.116 W



C h a p t e r 7Efficiency

Useful Energy Output Energy

Input

= x 100%

Efficiency



C h a p t e r 7

• Try Worksheet 7C • Q3 ans: a) 200J, b) 20 W• Q6 ans: 7.2 x 105 J, b) 3.6 x 104 W

Those finished can try • Q8• Q4 ans: a) 32J, b) 4 m/s, c) 1600 N

7.3 Work and Power



C h a p t e r 7Challenge

Where did all these energy come from?



C h a p t e r 7

is defined as the Capacity to do:

Conservation of Energy F x D

is related to is related to

is defined as

Work Done

Time

Change in Energy

Time

OR

is defined as

Others: Thermal Magnetic Nuclear

etc

is in the forms of

defined as

Energy body has due to its

motion

Energy body has due to its state, shape or position

KE = ½mv2

GPE = mgh

KEPE

ENERGY (J) WORK (J)

POWER (W)

SUMMARY



C h a p t e r 7

Green arrow acceleration g (no change)

Blue arrow Velocity (highest at bottom, lowest at top)

Brown arrow Spring’s force when stretched (greatest at bottom)



C h a p t e r 7GPE

GPE + KE

Elastic PE (max stretch)

GPE + KE + Elastic PE

KE + Elastic PE (just before max stretch)

Green arrow acceleration g Blue arrow Velocity Brown arrow Tension

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