Transcript 7.1 Work and Energy

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7.1 Work and Energy
7.1 Work and energy
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Work
a
Meaning of work
When a force moves something, work is
done.
In everyday language,
work may mean anything
that people do.
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a
meaning of work
What is work then?
In physics,
work is done whenever
A transfers energy
to B by forcing B
to have a displacement.
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a
meaning of work
Work is calculated by:
force in the
Work = direction of  displacement
displacement
W = F//s
F
s
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a
meaning of work
3 things happen when work is done:
1 Application of force
2 Displacement of object
3 Energy transfer
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a
meaning of work
Work
 Scalar (has magnitude only, no direction)
 Unit: N m or J
1 J of work is done when a force of 1 N
moves a distance of 1 m in the direction of
the force.
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a
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meaning of work
If there is no motion along the force direction,
no work is done on the object even though a
force is exerted on it.
F
E.g.: Carrying a load is tiring,
but no work is done on it.
Why?
There is no displacement
(s = 0) along the
direction of force.
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standing
still
E
b
work (non-parallel cases)
When force makes  with displacement,
 only component of the force in the
direction of displacement counts

F
s
W = Fs cos 
F sin 

F
F cos 
this component
// forward motion
W = F//s = (F cos )s = F (s cos ) = Fs//
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W = Fs cos 
When  = 0o, W = Fs
When  = 90o, W = 0
When  = 180o, W = -Fs
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3 Energy transferred and
work
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Sign of work:
the sign of work does not indicate direction.
+ve work:  in energy of the object.
ve work:  in energy of the object.
Work done by resistive force e.g. friction
Energy is dissipated in the form of heat and sound.
7.1 Work and energy
3 Energy transferred and
work
Sign of work:
e.g. when braking a car, the braking force
does ve work on the car.
F
F
 KE of car 
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Mechanical energy
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Energy exists in many different forms.
But we shall only study the different forms of
mechanical energy.
kinetic
energy
Gravitational
potential energy
7.1 Work and energy
Elastic potential
energy
a
Kinetic energy (KE)
 A moving object has
kinetic energy.
 The faster it moves, the
more KE it possesses.
kinetic energy
chemical energy
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b
Gravitational potential
energy (PE)
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When you lift a box
up, it gains
gravitational potential
energy (重力勢能).
put the box higher
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box possesses
more PE
c
Elastic potential energy
(彈力勢能)
Another form of
potential energy
When an elastic
object is stretched,
compressed or bent
object gains elastic potential energy
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3
Energy transferred and 16
work
Work is done whenever energy is
transferred from one form to another.
energy in
one form
work
done
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energy in
another form
3 Energy transferred and
work
energy in
one form
work done
energy in
another form
potential
energy
10 J
work done 10 J
kinetic
energy
10 J
Work done = energy transferred
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Example 1
Work done in lifting a suitcase
A man lifts a 10-kg suitcase on his shoulder
through a vertical distance of 1.5 m.
(a) How much work does he do?
What is the energy transfer
in this process?
(b)
Does he do work when
he transports the
suitcase by carrying it
on his shoulder?
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1.5 m
10 kg
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Example 1
Work done in lifting a suitcase
(a) Force = weight of suitcase
= 10  10 = 100 N
100 N
Work = 100 × 1.5 = 150 J
Chemical energy of the
man is converted to the
potential energy of the
suitcase.
7.1 Work and energy
100 N
1.5 m
10 kg
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Example 1
Work done in lifting a suitcase
(b) Force on suitcase is at right
angle to the motion.
Displacement in the
direction of force is zero.
 work done is zero.
5m
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Example 2
Work done in pulling a suitcase
How much work does he do? What is the
energy transfer in this process?
20 N
30
10 kg
Man's chemical
energy 
suitcase's KE
5m
20 N
30
10 kg
Component of force in the direction of
motion = (F cos ) s = 20 cos 30
Work = 20 cos 30  5 = 86.6 J
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Work done by tension of the
string = 0
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Q1 Which of the following...
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Which of the following statements about
work is INCORRECT?
A
Work = applied force in the direction of
displacement  displacement
B
Since displacement and force are vectors,
work done is also a vector.
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Work is done when a force moves
something.
D
Our muscles do work when we walk.
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Q2 A block of weight W...
A block of weight W slides down an inclined
plane over a distance d.
Find work done by gravity on the block.
A
Work = W  d  sinθ
B
Work = W  d  cosθ
C
Work = W  d
d
θ
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Q3 The following screw...
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The following screw jack is used to raise heavy
objects by turning the handle.
30 cm
load
50 kg
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handle
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Q3 The following screw...
What is the work done in
one complete revolution if
the turning force is 20 N?
The work done of the
turning force
= force × displacement
= _______
20  2    ________
0.3
37.7
= ________________
J
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30 cm
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The End
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Return
7.1 Work and energy