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# 5. Work, energy and power

```5. Work, Energy and Power
5. Work, Energy and Power
5. Work, energy and power–The
objectives
1. Define
Work, energy, Potential energy
Power, Efficiency
2. Differentiate between Gravitational potential
energy, Elastic PE and Electric PE.
3. Be competent in using following equations
W=F.x
πππ
π
W= P.οV
π¬=
πππ
π
π(πΏππ −πΏππ )
π
π¬ = πππ
πΎ
π
π· = = π­. π½
4. Be able to analyze Force-distance graph
π¬=
πΎ=π¬=
A warm-up Question for you
Which block
will
move
higher?
Explain your
The video
1.Work (W)
Work = Force x Distance travelled
in the direction of force
Units: J (kgm2s-2)
Scalar
1. 2019 May P11
Q17
2. 10M.1.HL.TZ1.5 Which
of the following is
a correct definition of work?
A. Product of force and distance
B. Product of force and distance
moved in the direction of the force.
C. Product of power and time
D. Product of force and displacement
3. Find the work done when
the plate rotates twice
4. What is the work done
by the centripetal force in
one circle?
The mass at the end of a
pendulum is made to
move in a horizontal circle
of radius r at constant
speed. The magnitude of
the net force on the mass
is F.
What is the direction
of F and the work done
by F during half a
revolution?
1.Work done on a gas
• Work done = Force x
Distance
to
force
direction.
• In gases, more useful
concepts are pressure
and volume. Not the
force and distance.
• Hence,
we
change
W=F.x into a different
form
W= F. οx
= (F/A). (A. οx)
Hence, W= P.οV
2018 Nov P12 Q16
Force- Distance graph
βπ
βπ‘
π=
βπ  = π. βπ‘
Area under graph= The work done = The energy used.
15M.1.HL.TZ2.4
A girl is standing on a
moving skateboard.
She
pushes
backwards on the
ground at intervals as
shown on the graph.
How much kinetic energy is gained by the girl
during the period represented on the graph?
Frictional forces are negligible.
A. 200 J
B. 400 J C. 600 J D. 1200 J
2 .Energy (E)
The ability of doing
work is called
Energy.
• When you do work, you
lose energy.
• When work is done on
you, you gain energy
The ball drop activity.
Part 1: teacher demo
Explain why the smaller ball ends up
with higher energy?.
Extension 1: Two balls are identical
except that one ball has hard outer
crust. When the two balls are thrown
at a window at same speed, the
hardball may break the glass while
the softball does not. Explain this
phenomena using newtons laws.
Students: Get the collision moment.
Use app jianying for video analysis
Extension 2: Explain the
concept of egg drop in
terms of Newtons laws
Kinetic Energy
• Energy due to the
motion of an object is
called Kinetic energy.
1. 2020 May 13 Q18
2. 2021 May 13 Q18
Kinetic Energy- Deriving the equation
• Kinetic
energy=The
work done on the object
• W= F.x
• But, F=ma
• So, W= K.E.=mas
• Using linear motion
equation of π£ 2 = π’2 + 2ππ
16N.1.HL.TZ0.7
An object of mass 2kg is thrown
vertically downwards with an initial
kinetic energy of 100J. What is the
distance fallen by the object at the
instant when its kinetic energy has
doubled?
A. 2.5m
B. 5.0m C. 10m
D. 14m
3. 2019 March P11 Q10
4. 2020 May 13 Q16
An astronaut is moving at a
constant velocity in the absence of a
gravitational field when he throws a tool away
from him. What is the effect of throwing the tool
on the total kinetic energy of the astronaut and
the tool and the total momentum of the
astronaut and the tool?
19M.1.SL.TZ2.7
Potential energy
The energy due to
position or the
compressed/
stretched state of
an object is called
potential energy.
Gravitational Potential Energy: Deriving the equation
• The energy a mass
processes due to the
position
in
a
gravitational field is
called the GPE.
• But, F=ma
• So, W= mg.h
Q1. An object has a weight of
2
6.10 &times; 10 N. What is the
change
in
gravitational
potential energy of the object
when it moves through 8.0 m
vertically?
A. 5 kJ
B. 4.9 kJ
C. 4.88 kJ
D. 4.880 kJ
Elastic Potential Energy
ππ2
π¬=
2
Work done from Extension X1 to
Extension X2
π(πΏ22 − πΏ12 )
πΎ=π¬=
2
A compressed spring
is used to launch an object along
a horizontal frictionless surface.
When the spring is compressed
through a distance x and
released, the object leaves the
spring at speed v. What is the
distance through which the
spring must be compressed for
the object to leave the spring
at v/2?
18N.1.SL.TZ0.8
A.
C.
π
π
π
π
B.
π
π
D. π π
An increasing force acts on a
metal wire and the wire extends
from an initial length l0 to a new
length l. The graph shows the
variation of force with length for
the wire. The energy required to
extend the wire from l0 to l is E.
The wire then contracts to half its
original extension. What is the
work done by the wire as it
contracts?
A.
B.
C.
D.
0.25E
0.50E
0.75E
E
Electric potential energy
Electric potential Difference
The energy conversion
happens for 1C charge is
called
the
potential
difference between two
points.
π = βπ¬ = βπ½. πΈ
An
electron
is
accelerated through a potential
difference of 2.5 MV. What is
the change in kinetic energy of
−ππ
the electron? (π = π. ππππ πͺ )
17M.1.SL.TZ1.19
A. 0.4μJ
B. 0.4 nJ
C. 0.4 pJ D. 0.4 fJ
Power (P)
• The rate of doing
work
is
called
power.
• Power=
Work/
Time (P=W/t)
• Scalar quantity. Unit
Power Equations.
Efficiency
P=W/t
P=Fx/t
P= FV
Efficiency= Useful work
Total work
Efficiency= Useful Power
For electric
Total Power
circuits:
P=VI=I2R=V2/R
5. Work, energy and power–The
objectives
1. Define
Work, energy, Potential energy
Power, Efficiency
2. Differentiate between Gravitational potential
energy, Elastic PE and Electric PE.
3. Be competent in using following equations
W=F.x
πππ
π
W= P.οV
π¬=
πππ
π
π(πΏππ −πΏππ )
π
π¬ = πππ
πΎ
π
π· = = π­. π½
4. Be able to analyze Force-distance graph
π¬=
πΎ=π¬=
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