Transcript 7.4 Power - OoCities

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7.4 Power
7.4 Power
Warm-up
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We often see 'horsepower' on cars,
vacuum cleaners and other machines or
appliances. Why do we use these terms?

We compare the work done by
machines with that by horses.

It would be easier to compare the
performance of different engines.
It is no use but just gimmick.
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Warm-up
2 Johnny tests cars A, B and C of the same
mass by driving them over distance d on a
slope. Which engine is the most powerful?
 A: takes 3 minutes to finish d
B: takes 5 minutes to finish d
C: can hardly go up the slope
Why?
When A, B & C go up a slope,
work done against gravity: same;
their time taken to go up: different
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0 Introduction
The 2 cars are similar but one is much
more powerful than the other.
What does this mean?
How is power measured?
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Definition of power
The rate at which energy is transferred or
work is done.
energy transferred
Power =
time taken
work done
= time taken
W
(P =
)
t
Unit: W (1 W = 1 J s1)
E.g. an engine with 1000 W output means:
do 1000 J of work in 1 s
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Definition of power
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Typical power outputs:
Athlete: 400 W
Small car
engine:
25 kW
Washing machine: 250 W
7.4 Power
Lamma Power Station:
3305 MW
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Power and velocity
We can express power in another form:
Fs
s
W
P=
=
= F
t
t
t
s
v=
t
(for an object moving
at constant velocity)

 P = Fv
Power = force  velocity
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Example 10
Measuring human power
An athlete of mass 60 kg runs up a flight of
steps in 60 s. If the vertical height of the
top of the steps above ground is 50 m, find
the average power of the athlete.
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Example 10
Measuring human power
W
P=
t
Data:
mass = 60 kg
mgh
=
t
time= 60 s
height = 50 m
60  10  50
=
60
= 500 W
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Example 11
Power of a car
A car travels at a constant v (30 m s1).
Find the power output of the engine if
the total drag force on the car is 800 N.
30 m s1
800 N
(air resistance +
frictional force)
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Example 11
Power of a car
At constant speed, net force on car = 0
30 m s1
800 N
800 N
(air resistance +
frictional force)
forward force
due to engine
Power = force  velocity
= 800  30
= 24 000 W (or 24 kW)
7.4 Power
Q1 A crane raises a 10-kg…
A crane raises a 10-kg mass to a height of
10 m at a constant speed of 0.1 m s–1.
Calculate the output power of the crane.
A
1000 W
B
100 W
C
10 W
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Q2 In a marathon, the winner...
In a marathon, the winner and the first
runner-up have the same mass. Compared
with the first runner-up, the winner has
more
A
energy.
B
power.
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Q3 If a lift transports 10 people... 14
If a lift transports 10 people from the ground
to the 20th floor in 20 s, what is the useful
power of the lift?
Given: Each storey is 3 m high
Mass of each person = 65 kg
Mass of lift = 200 kg
7.4 Power
Q3 If a lift transports 10 people... 15
Distance travelled by the lift
3
60
= 20 × _______
= ________
m
PE gained by people (PEpeople)
= mpeoplegh
65 ) × 10 × _______
60
= (10 × _____
390 000 J
= ___________
( 390 000 )
Useful power =
(
)
20
19 500 W
= _________
7.4 Power
Data:
G/F to 20/F
(each storey
3 m)
time= 20 s
10 persons
(each 65 kg)
mass of lift
= 200 kg
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The End
7.4 Power
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7.4 Power