Work and Energy 4450

8/2/2019 Work and Energy 4450

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Work, Energy, Powerand Efficiency

MISS AIDA


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Work

What does WORK mean to you?

Are you doing WORK when

Lifting weights? Walking with a big bag of grocery in your

hand?

Completing your homework assignment?

Writing an essay?


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Physics concept of WORK

WORK is done only when a constantforce applied on an object, causes theobject to move in the same direction as

the force applied.


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What IS considered as work done inPhysics:

You push a heavy shopping trolley for 10 m

You lift your school bags upwards by 1 m


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What is NOT considered as work done:

You push against a wall

Jumping continuously on the same spot

Holding a chair and walking around theclassroom


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WORK can be calculated by:

Work done = Constant x Distance movedforce (N) in the direction

of force (m)

W = F x s

Units: [J] [N] [m]

SI Unit for Work is JOULE (J)


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More Examples of WORK

You are helping to push your mothers heavy

shopping cart with a force of 50 N for 200 m.What is amount of work done?

Work done, W = F x s= 50 x 200

= 10,000 Jor

10 kJ (kilo-Joules)


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Energy Quick Re-cap

Energy is the capacity to do work- work isdone , energy is transferred from one objectto another.

SI Unit: Joule (J)

Common ones:

Kinetic

Potential Electric

Chemical

Solar

Nuclear


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Kinetic Energy (KE)

A form of energy that a body in motionpossess.

A body a rest, will it possess any KE? Examples:

Bullet shot out from pistol

Helicopter flying at 120km/h


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Kinetic Energy (KE)

The amount of KE of a moving bodydepends on:

Mass of body (kg)

Velocity (ms-1)

When either mass or velocity of movingbody is increased, KE will also increase.


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Kinetic Energy (KE)

Formula:

SI Unit: Joule [ J ] same as Work Done

Kinetic Energy = x Mass x (Velocity)2

KE = x m x v2

Units: [ J ] [kg] [ms-1

]2

2

1

2

1


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Kinetic Energy (KE)

KE = m v2

Mass = m kg

Velocity, V


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Examples of KE

Find the KE of an empty van of mass 1000kg moving at 2m/s.

Find the KE of van when it is loaded with goods to give a totalmass of 2000kg, and moving at 2m/s.

Find KE of unloaded van when it speeds up to 4m/s.

KE of van at 2m/s = x 1000 x (2)2

= 2000 J = 2 kJ

KE of van at 2m/s = x 2000 x (2)2

= 4000 J = 4 kJ

KE of van at 2m/s = x 1000 x (4)2

= 8000 J = 8 kJ


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Kinetic Energy (KE)

Formula: KE = mv2

From the formula, what can you infer

about the change in KE when

Mass doubles

Velocity doubles

KE doubles

KE increases by FOUR times


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Examples of KE

A motorcycle accelerates at 2m/s2 from rest for5s. Find the KE of motorcycle after 5s. Mass ofmotorcycle is 200 kg.

Velocity of motorcycle after 5s, a = (v-u)t

v = 2(5) + 0 = 10m/s

KE of motorcycle at 10m/s = x 200 x (10)2

= 10,000 J = 10 kJ


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Potential Energy

Potential energy is the energy possessedby an object as a result of its POSITION orCONDITION.

Common kinds:

Gravitational PE


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Gravitational PE

Energy that can be possessed by an objectdue to its POSITION.

In Physics, ground level is normally assumed to be at ZEROGPE.

Any object that is at ground level has ZERO GPE.

If object is lifted a certain height above ground, its GPE hasincreased.


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Gravitational PE

Examples:

When a chair lifted from ground a distance of1m

You sitting on the 3rd storey of this building


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Gravitational PE

Can be calculated with:

GPE = mass gravitational height above

acceleration ground level

= m g h

Units:

[J] [kg] [m/s2] [m]

SI Units of GPE : Joule [J]

Ground,0 GPE

Distance fromground, h

Object on top ofbuilding, of mass, mg

earth


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Examples of GPE

You lifted a set of books of mass 3kg, for 2m. Whatis the GPE gained by the books? Take g=10m/s2.

Find the work done by you to lift the books.

GPE = mgh

= 3 10 2= 60 J

Work done, W = F

d (F = weight of books)= (m g) d= 3 x 10 x 2= 60 J (Note: same as GPE)


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Conservation & Conversionof Energy


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Energy of an object can be thought of asthe sands in an hourglass!

Energy always remain same or fixed inquantity!

But this sand can change position, from the top to bottom and

bottom to top! Likewise energy can change in

form eg. From KE PE

Conservation of Energy


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Conservation of Energy Note that energy CANNOTbe created

nor destroyed!

So what does this mean when viewed incontext of the Earth?


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Conservation of Energy

Conversion of energyis the term usedto denote change in energy from oneform to another.

Eg. Burning candle: Chemical Heat, Light

Fuel: Chemical Heat KE Electricity

Nuclear explosion: Nuclear

Heat, light Spring: Elastic PE KE


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Conversion of Energy For O-Levels, we are only concerned with:

KEGPE And such situations are only found

when a moving object is at the same

time undergoing changes in height


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Conversion of Energy Eg. of KE PE

Roller-coaster

Falling object


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Free Falling objectmodel An object in free fall means the object is

falling freely, under the influence of gravity

When the object is at the highest position,

the GPE is at maximum and KE is zero.

When the object is falling, the GPE decreases

as it loses height, and the KE increasesAt the lowest position, the KE is at maximum

and GPE is zero.
http://../PED582/Stella%20Samples/ConsEner-1.stm


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A fresh durian of mass 5 kg is found growing at the end of atree branch 20 m above ground. When ripe, the durian will byitself drops to the ground below. Let gravity = 10m/s2.

Find the energy of the fresh durian? What form is it?

GPE. GPE = mgh = 5 x 10 x 20 = 1000J

Find the GPE and KE of the durian when it is 5m above ground.Sum up both the GPE and KE and compare the value withabove. What can you infer from the results?

GPE = 5 x 10 x 5 = 250J. s = vt, v = gt

s = gt2, t = sqrt 3

KE = mv2 = (5)(10sqrt3)2 = 750J v = 10(sqrt 3)

Sum of energies = 250 + 750 = 1000J Same as above => energy is conserved.

Eg. of Conservation of Energy


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Eg. of Conversion of Energy

A car of 800 kg is moving at an average speed of 5 m/s.The traffic light changed to red and so the driver steppedon the brakes to bring the car to a quick, sudden andscreeching halt.

Find energy of moving car and what form of energy isthis?

KE. KE = mv2 = x 800 x 52 = 10,000 J.

What energy does the car possesses when it stops? None.

What happened to the original energy of the moving car?

KE has changed to Sound and Heat Energy.


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Power (P)

Power is defined as the rate at which work is done,or the amount of work done per second.


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POWER can be calculated by:

Power = Work Done

Time Taken

P = W

t

Units: [W] = [J][s]

SI Unit for Power is watt (W)

Power (P)


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Power (P)

Example:

A 40kg boy climbs a vertical height of 3.5 m in 4 s

a) Calculate the gain in gravitational potential energy of the boy.b) Determine the power generated by the boy [ take g= 9.8m/s2

a) Gravitational potential energy gain,Ep = mgh

= 40 x 9.8 x 3.5= 1372 J

b) Power Generated, P = Ep / t=1372 / 4

=343 W


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Efficiency

The efficiency of a device is defined as thepercentage of the energy inputthat is transformed into useful energy

Efficiency = useful energy output x 100%Energy input


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Work and Energy 4450

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