Transcript
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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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Physics concept of WORK
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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Physics concept of WORK
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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Physics concept of WORK
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.
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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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