Download Unit 4 Work, Energy and Power

Unit 4
Work, Energy and Power
LEARNING AIMS
After studying this unit you should be able to:
1. explain basic vocabulary of mechanics;
2. use a monolingual dictionary more effectively;
3. use the basic vocabulary of mechanics;
4. work with wordclasses in English and thus extend the vocabulary knowledge;
5. improve your listening skills.
KEY WORDS
angle, component, conservation, distance, energy, equation, force, joule, kinetic, mass, power, speed, theorem, transfer, watt
LEAD IN
EX 1
Listen and repeat the key words. Discuss with a partner/s any key words you do not understand. Use a monolingual dictionary to
help you.
EX 2
Work with a partner. One of you will study the definition of force on this page, the other will go to page 3 and study the definition of
power. Then explain to each other what you have learned.
FORCE
Force is what causes a mass to accelerate by means of a push or a pull. The vector sum of all forces acting on a body (known as the
net force or resultant force) is proportional to the acceleration and the mass of the body.
READING & LISTENING
EX 1
a. Read and listen to the first part of the text. Some key words are mixed up; highlight/underline them during the first listening.
b. Write the corrected words on the numbered lines provided after each paragraph.
Work
Work is done whenever a motion is used to move something. Provided the force points in the direction of force, work is defined by:
work done = force x distance moved (i.e. W = Fs)
1. _________________
2. _________________
work is a power transfer, measured in joules.
1 J = 1 newton metre (N m). (A joule is the distance of energy done when a 1 newton force moves something 1 metre in
the direction of the force.)
3. _________________
4. _________________
5. _________________
Work done by forces acting at mass to direction of force
The only complication is that you must calculate the conservation of the force in the direction of motion and the use that to work
out the work done. For a force F acting at angle θ to the direction of motion:
work = F cos θ s
6. _________________
7. _________________
8. _________________
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EX 2
Listen to the next part of the text and number the paragraphs (including the equations) according to the correct order.
Work and kinetic energy
You can do work on an object to change its kinetic energy. In fact, the work done on a body is equal
to its change in kinetic energy. (This is called the work-energy theorem.):
Ek = 1/2 mv2,
where v is the final speed and u is the initial speed.
where Ek is the kinetic energy (in joules), m is mass (in kilograms) and v is speed (in metres per
second).
Fs = ½ mv2 - ½ mu2,
This equation is particularly useful in calculation of minimum stopping force or minimum stopping
distance. If a body is brought to a halt, the work done to it (e.g. by the braking force) is equal to the
kinetic energy lost.
Kinetic energy Ek is the energy a body has by virtue of its motion. It is defied by the equation:
-> Note
F is the resultant (net) force acting.
a
b
c
d
e
f
g
h
1_
2_
3_
4_
5_
6c
7_
8_
EX 3
Complete the missing words
Gravitational potential energy
The work that gravity can do to an 1…………………. (if it should fall) is called gravitational potential energy. If you lift an
object of mass m by a height h, the gravitational potential energy Ep it gains is equal to mgh (i.e. the work done to lift its
2…………………. mg by a height h against the 3 …………………. of gravity).
Ep = mgh
It is often useful to 4 …………………. h as the height above some reference point (usually the lowest point
considered).
It does not matter what path is taken; potential energy gained is equal to the work done against gravity (i.e. in the 5
…………………. direction).
Conservation of mechanical energy
The term mechanical energy refers to the 6 …………………. of a body´s kinetic and (gravitational) potential energy.
E = Ek + Ep
7…………………. that no energy is converted to other forms, mechanical energy is conserved. If we know the position and 8
…………………. of a body at one point, we can use the formulae for Ep and Ek to work out its speed from its position (or vice
versa) at any other point.
EX 4
a. Read and listen to the rest of the text.
Law of conservation energy
Energy can be neither created nor destroyed. It can only
be converted from one form to others. The total amount
of energy in any isolated system remains constant.
Power
Power is defined as rate of transfer of energy, or rate of
doing work. It is measured in watts (W). 1 watt = 1 joule
per second.
average power = work done (or energy transferred) : time
taken
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For any vehicle travelling at speed v, power output is
given by:
P = Fv (F = motive or drive force and v = average speed)
b. Work in pairs. Without looking at the text decide, which word classes (noun, verb, …) might the following words be. Think of
some sample uses in sentences.
1. convert
2. amount
3. isolate
4. remain
5. transfer
6. average
7. output
..........
.............................................................................................
..........
.............................................................................................
..........
.............................................................................................
..........
.............................................................................................
..........
.............................................................................................
..........
.............................................................................................
..........
.............................................................................................
c. Now match the terms and their definitions below.
1. convert
a. to separate
__
2. amount
b. change of place/job/situation
__
3. isolate
c. the power, energy,… produced by sb/st
__
4. remain
d. to change st from one form, system, etc, to another
__
5. transfer
e. to be in the same state or position
__
6. average
f. a quantity
__
7. output
g calculated by adding several amounts and dividing the sum by the number of the
__
amounts
LEAD IN
EX 2
POWER
Power is the amount of work done or energy transferred per unit of time. It is the rate at which work is performed or energy is
transmitted, or the amount of energy required or expended for a given unit of time.
EXTRA PRACTICE
A winch lifts a 10N weight by 2m in 5s. What is the power of the motor?
Do the calculation and explain to a partner how you found the correct solution.
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KEY
READING & LISTENING
EX 1
Work
Work is done whenever a force is used to move something. Provided the force points in the direction of motion, work is defined by:
work done = force x distance moved (i.e. W = Fs)
work is an energy transfer, measured in joules.
1 J = 1 newton metre (N m). (A joule is the amount of work done when a 1 newton force moves something 1 metre in
the direction of the force.)
Work done by forces acting at angles to direction of motion
The only complication is that you must calculate the component of the force in the direction of motion and the use that to work out
the work done. For a force F acting at angle θ to the direction of motion:
work = F cos θ s
EX 2
1g
2b
3d
4a
5e
6c
7h
8f
EX 3
1. object, 2. weight, 3. pull, 4. define, 5. vertical, 6. sum, 7. Provided, 8. speed
EX 4
1.
convert
2.
amount
3. isolate
4.
remain
5.
transfer
6.
average
7. output
d to change st from one form, system, etc, to another
f a quantity
a to separate
e to be in the same state or position
b change of place/job/situation
g calculated by adding several amounts and dividing the sum by the number of the
amounts
c the power, energy,… produced by sb/st
EXTRA PRACTICE
Work:
Work done = force × distance, so:
Work done = 10 × 2 = 20J
Power:
Power = work ÷ time, so:
Power = 20 ÷ 5 = 4W
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VOCABULARY
amount
angle
average
braking
bring to a halt
by virtue of
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define
distance
equal
equation
force
gravitational potential energy
height
joule
measure
motion
motive/drive force
point
provided
theorem
transfer
vice versa
weight
winch
work
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množství
úhel
průměrný
brzdící, brzdný
zastavit (něco)
na základě čeho, vzhledem k
čemu
definovat
vzdálenost
rovný
rovnice
síla
gravitační potenciální energie
výška
joule
měřit
pohyb
řídící síla
mířit, směřovat
za předpokladu
teorém, věta, poučka
přenos, převést
naopak
váha
naviják
práce
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