Download Work

Work, Energy & Power
1
Specific Instructional
Objectives
At the end of the lesson, students should be able to:
– Show understanding of the Physics concept of Work
– Correctly identify Work from given situations
– Recall and show understanding of the formula to
calculate work done
– Solve related problems involving work
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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 constant
force applied on an object, causes the
object to move in the same direction as
the force applied.
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Physics concept of WORK
• What IS considered as work done in
Physics:
– 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 the
classroom
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Physics concept of WORK
WORK can be calculated by:
Work done = Constant x
force (N)
Distance moved
in the direction
of force (m)
W = F x ∆d
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 mother’s heavy
shopping cart with a force of 50 N for 200 m.
What is amount of work done?
Work done,
W = F x ∆d
= 50 x 200
= 10,000 J
or
10 kJ (kilo-Joules)
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More Examples of WORK:
• Jack put on his bag-pack of weight 120 N. He
then starts running on level ground for 100 m
before he started to climb up a ladder up a
height of 10 m. How much work was done?
From Physics point of view, no work is done on pack at
level ground. Reason: Lift is perpendicular to movement.
Work is done on pack only when Jack climbs up the ladder.
Work done, W = F x ∆d
= 120 x 10
= 1200 J or 1.2 kJ
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Specific Instructional
Objectives
At the end of the lesson, students should be able to:
– Show understanding of the Physics concept of Kinetic
Energy (KE)
– Recall and show understanding of the formula
– Distinguish situations involving KE
– Solve related problems involving KE
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Energy – Quick Re-cap
• Energy is the capacity to do work
• SI Unit: Joule (J)
• Many forms
• Common ones:
–
–
–
–
–
–
Kinetic
Potential
Electric
Chemical
Solar
Nuclear
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Kinetic Energy (KE)
• A form of energy that a body in motion
possess.
• 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 body
depends on:
– Mass of body (kg)
– Velocity (ms-1)
• When either mass or velocity of moving
body is increased, KE will also increase.
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Kinetic Energy (KE)
• Formula:
Kinetic Energy =
KE =
Units:
[J]
1
2
1
2
x Mass x (Velocity)2
x m x v2
[kg] [ms-1]2
• SI Unit: Joule [ J ] … same as Work Done
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Examples of KE
• Find the KE of an empty van of mass 1000kg moving at 2m/s.
KE of van at 2m/s = ½ x 1000 x (2)2
= 2000 J = 2 kJ
• Find the KE of van when it is loaded with goods to give a total
mass of 2000kg, and moving at 2m/s.
KE of van at 2m/s = ½ x 2000 x (2)2
= 4000 J = 4 kJ
• Find KE of unloaded van when it speeds up to 4m/s.
KE of van at 2m/s = ½ x 1000 x (4)2
= 8000 J = 8 kJ
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Kinetic Energy (KE)
• Formula: KE = ½ mv2
• From the formula, what can you infer
about the change in KE when…
– Mass doubles KE doubles
– Velocity doubles KE increases by FOUR times
– Mass triples KE triples
– Velocity triples KE increases by NINE times
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Examples of KE
• A motorcycle accelerates at 2m/s2 from rest for
5s. Find the KE of motorcycle after 5s. Mass of
motorcycle is 200 kg.
Velocity of motorcycle after 5s,
a = (∆v)
t
v = 2(5) + 0 = 10m/s
KE of motorcycle at 10m/s = ½ x 200 x (10)2
= 10,000 J = 10 kJ
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Work Done By Gravity
• When you pick up a 5kg object a distance
of 2m, how much work do you do?
• How much work does gravity do?
• If you drop it, how much work will gravity
do?
• What will be its kinetic energy as it hits the
ground?
• How much work will the object do on the
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ground?
Potential Energy
• Potential energy is the energy possessed
by an object as a result of its POSITION or
CONDITION.
• Two common kinds:
– Gravitational PE
-- Elastic PE
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Elastic PE
• Energy that can be possessed by an object
due to its CONDITION. Examples:
• “Slinky” … when stretched or compressed
• Spring … when stretched or compressed
• Rubber band … when stretched
• Balloon with air … when compressed
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Gravitational PE
• Energy that can be possessed by an object
due to its POSITION.
• In Physics, the reference level is defined to be at ZERO GPE.
• Any object that is at reference level has ZERO GPE.
• If object is lifted a certain height above the reference level, its
GPE has increased.
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Gravitational PE
• Examples:
– When a chair lifted from ground a distance of
1m
– 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
=
m  g  h
g
earth
Units:
[J]
[kg]
[m/s2]
[m]
SI Units of GPE : Joule [J]
reference level
Object on top of
building, of mass, m
Distance from
Reference level, h
Ground,
0 GPE
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Reference Level
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Example of GPE
• You lifted your bookbag to the top of your desk.
What can you say about the GPE of your bag?
– Zero, increase, decrease
• Lift the same bag on the Moon. What happens to
GPE?
– Zero, increase, decrease
• Will the GPE be the same on Earth and Moon?
– Same, less on Moon, more on Moon?
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Examples of GPE
• You lifted a set of books of mass 3kg, for 2m. What
is the GPE gained by the books? Take g=10m/s2.
GPE = mgh
= 3  10  2
= 60 J
• Find the work done by you to lift the books.
Work done, W = F  d
= (m  g)  d
= 3 x 10 x 2
= 60 J
(F = weight of books)
(Note: same as GPE)
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Conservation & Conversion
of Energy
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Conservation of Energy
• Energy of an object can be thought of as
the sands in an hourglass!
• Energy always remain same or fixed in
quantity!
• 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
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PE
KE
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Conservation of Energy
• Conversion of energy is the term used
to denote change in energy from one
form to another.
• Eg.
– Burning candle: Chemical  Heat, Light
– Fuel: Chemical  Heat  KE  Electricity
– Nuclear explosion: Nuclear  Heat, light
– Spring: Elastic PE  KE
– Dropping an Object Gravitational PE → KE
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Conversion of Energy
• For O-Levels, we are only concerned with:
• KE  GPE
• 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 object
model
• 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 increases
At the lowest position, the KE is at maximum
and GPE is zero.
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Throwing an Object in The
Air
• KE
• GPE
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Eg. of Conservation of Energy
• A fresh coconut of mass 5 kg is found growing at the end of a
tree branch 20 m above ground. When ripe, the coconut will by
itself drops to the ground below. Let gravity = 10m/s2.
• Find the energy of the coconut? What form is it?
– GPE. GPE = mgh = 5 x 10 x 20 = 1000J
• Find the GPE and KE of the coconut when it is 5m above
ground. Sum up both the GPE and KE and compare the value
with above. What can you infer from the results?
–
–
–
–
GPE = 5 x 10 x 5 = 250J. V = sqrt(2g∆d) = 17.3m/s
KE = ½ mv2 = ½ (5kg)(17.3m/s)2 = 750J
Sum of energies = 250 + 750 = 1000J
Same as above => energy is conserved.
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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 stepped
on the brakes to bring the car to a quick, sudden and
screeching halt.
• Find energy of moving car and what form of energy is
this?
– KE. KE = ½ mv2 = ½ 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?
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– KE has changed to Sound and Heat Energy.