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4.1 Work and energy
Forms of energy
Forms of energy
Mechanical energy
Check-point 1
Work and energy transferred
Check-point 2
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Book 2 Section 4.1 Work and energy
Forms of energy
When people play on a
trampoline, several forms
of energy are involved.
What are they?
• Elastic potential energy
• Kinetic energy
• Gravitational potential energy
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Book 2 Section 4.1 Work and energy
1 Forms of energy
Different forms of energy:
light
sound
electrical energy
chemical energy
nuclear energy, etc.
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Book 2 Section 4.1 Work and energy
1 Forms of energy
E.g. Forms of energy involved in trampoline:
elastic potential energy
kinetic energy
mechanical
energy
gravitational potential energy
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Book 2 Section 4.1 Work and energy
2 Mechanical energy
a Kinetic energy (KE)
A moving object has
kinetic energy.
faster  more KE
When running:
kinetic energy
chemical energy
Book 2 Section 4.1 Work and energy
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2 Mechanical energy
b Potential energy (PE)
i Gravitational potential energy
When you lift a box
up, it gains
gravitational potential
energy.
put the box higher
box possesses
more PE
Book 2 Section 4.1 Work and energy
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2 Mechanical energy
b Potential energy (PE)
ii Elastic potential energy
Another form of PE.
When an elastic
object is stretched,
compressed or bent…
gains elastic potential energy
Total mechanical energy = KE + PE
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Book 2 Section 4.1 Work and energy
Check-point 1 – Q1
When a cannonball leaves the cannon, its
kinetic
________energy
is increasing while its
potential energy is decreasing.
__________
These two forms of energy are collectively
mechanical energy.
called ___________
Book 2 Section 4.1 Work and energy
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Check-point 1 – Q2
When a falling ball hits the ground and
changes its shape before it bounces
elastic potential energy
upwards, its ________________
increases.
If the temperature of the ball increases, its
internal energy also increases.
________
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Book 2 Section 4.1 Work and energy
Check-point 1 – Q3
Describe the energy changes that take place
when Simon sets off from the mark and
starts running.
Some chemical energy stored in his
body changes into kinetic energy,
potential energy and internal energy.
Book 2 Section 4.1 Work and energy
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3 Work and energy transferred
Process of energy transfer:
Two bodies of different
temperatures in contact:
hot
cold
Energy transferred  Heat
Force exerted on a object:
F
Energy transferred  Work
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Book 2 Section 4.1 Work and energy
3 Work and energy transferred
a Definition of work
What is work then?
Work is done when a force
exerts an object over a
displacement parallel to
the force.
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Book 2 Section 4.1 Work and energy
a Definition of work
Work is calculated by:
Work =
force in the
direction of  displacement
displacement
W = Fs
F
s
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Book 2 Section 4.1 Work and energy
a Definition of work
Note the 3 things happen when work is
done:
1 Application of force
2 Displacement of object
3 Energy transfer
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Book 2 Section 4.1 Work and energy
a Definition of work
Work:
Scalar (has magnitude only, no direction)
Unit: N m or Joule (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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Book 2 Section 4.1 Work and energy
a Definition of work
Sign of work:
+ve work: Force acts in the same direction
as the displacement.
 object gains energy
ve work: Force acts in the opposite
direction as the displacement.
 object loses energy
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Book 2 Section 4.1 Work and energy
a Definition of work
Example 1
Work done on a football
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Book 2 Section 4.1 Work and energy
3 Work and energy transferred
b Work (non-parallel cases)
When force makes  with displacement,
 only component of the force in the
direction of displacement is counted

F sin 
F
F cos 
s
parallel to
forward motion
W = F// s = (F cos )s = Fs cos 
Book 2 Section 4.1 Work and energy
P.21
b Work (non-parallel cases)
When force F is at right angles to
displacement s (F ⊥s):
 cos 90 = 0
 Fs cos  = 0
 No work is done on the load
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Book 2 Section 4.1 Work and energy
b Work (non-parallel cases)
Example 2
Work done on a dog moving
in different ways
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Book 2 Section 4.1 Work and energy
3 Work and energy transferred
c Work done against a force
F and s are in opposite direction
 Work done is -ve
E.g.: Work done against friction is –ve.
The table loses energy from the work
done against friction.
Book 2 Section 4.1 Work and energy
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c Work done against a force
Example 3
Work done by a football
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Book 2 Section 4.1 Work and energy
Check-point 2 – Q1
A car engine delivers a driving force of
10 000 N. How much work does the car
engine do in travelling a distance of 100 m?
Work = Fs
= 10 000  100
= 1000 kJ
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Book 2 Section 4.1 Work and energy
Check-point 2 – Q2
A block of mass M slides down an inclined
plane over a distance d.
d

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Book 2 Section 4.1 Work and energy
Check-point 2 – Q2
(a) Calculate the work done by the gravity
on the block.
Component of the block’s weight
down the plane
= Mg sin 
By W = Fs,
work done by the gravity on the block
= Mg sin   d
= Mgd sin 
Book 2 Section 4.1 Work and energy
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Check-point 2 – Q2
(b) Describe the energy change of the block.
gravitational potential energy of
The ___________________
kinetic
the block is changed to its _________
energy.
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Book 2 Section 4.1 Work and energy
The End
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Book 2 Section 4.1 Work and energy