Energy, Work, & Power
SP3. Students will evaluate the forms and transformations of
energy.
a. Analyze, evaluate, and apply the principle of conservation
of energy and measure the components of work-energy
theorem by
• describing total energy in a closed system.
• identifying different types of potential energy (PE).
• calculating kinetic energy (KE) given mass and velocity.
• relating transformations between PE and KE.
f. Analyze the relationship between temperature, internal
energy, and work done in a physical system.
g. Analyze and measure power.
What is energy & why do we need it?
• Energy – the ability to do work
• Work – moving an object by exerting a
force
• Force – a push or a pull
• Whenever force is used to move an
object, energy is required
– EX. When you walk, your muscles pull on
your bones in order to move your body.
This requires your muscles to use energy.
What is energy & why do we need it?
• Without energy, there could be no
motion
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–
–
–
–
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Atoms and molecules could not move
Stars could not shine
Planets could not orbit around stars
Animals could not walk, run, swim, or fly
The wind could not blow
Messages could not be sent from your
brain to your body
Is work being done when you hold a book
over your head?
• No
• But it did take work to get
it there
• Whenever work is being
done, energy is being used
• The energy was
transferred from your
body to the book
How much work is being done?
• We can measure the amount of work being
done to move an object
• We need to know 2 things in order to measure
work:
1. The amount of force being used
2. The distance of the movement
• We can calculate work using the following
formula:
Work = Force x Distance (W = F d)
Calculating Work – Which Units?
• When calculating work, use the following
units:
– Force is measured in newtons (N)
– Distance is measure in meters (m)
– Work is measured in joules (J)
• 1 joule equals 1 newton multiplied by 1 meter
• You do about 1 joule of work when you lift an
apple above your head
Calculating with Work –Problem #1
• If you lift a 3 N book 2 meters off the floor,
how much work did you do?
• W=Fxd
• W = 3N x 2m
• W=6J
Calculating with Work –Problem #2
• A man pushes a couch a distance of 0.75 m. If
113 J of work is done, what is the magnitude
of the force applied??
• F=W÷d
• F = 113 J / 0.75 m
• F = 151 N
Calculating with Work –Problem #3
• It requires 3480 J to move a 675 N object how
far?
• d=W÷F
• d = 3480 J / 675 N
• d = 5.16 m
Calculating with Work –Problem #4
• The 3rd floor of a house is 8 m above street
level. How much work is needed to move a
150 kg refrigerator to the third floor?
• W = Fd
• F = mg (150 kg x 9.8 m/s2)=1470 N
• W = 1470 N x 8 m
• W = 11760 J
Calculating with Work –Problem #5
• A toy truck is pushed across a table 0.80 m
north and pulled back 0.80 m south. If a
constant horizontal force of 15 N was applied
in both directions, what is the net work?
• Displacement = 0 (same distance N and S)
• No motion = no work done
Question
• When you carry a heavy bag of groceries from
your car to your kitchen, what does work, your
arms or your legs? Explain why.
• Your legs, because they move you and the
groceries from the car to the kitchen. Your
arms only lift and hold the groceries.
Energy Is Also Measured In Joules
• Since energy is required to do work, it is
measured using the same unit (joules)
• The amount of energy required to do work is
ALWAYS greater than or equal to the amount
of work being done
• EX. If you do 6J worth of work to lift a book,
you need at least 6J of energy to do it
Doing Work Gives Energy to Objects
• When work is done on an object:
– Energy is transferred from the object doing the
work to the object having work done on it
– The object doing the work loses energy
– The object having work done on it gains energy
• EX. A student pushing a desk across the floor
is doing work on the desk – energy is
transferred from the student to the desk and
the student loses energy
Why do we get tired after doing work?
• We give up our energy to all the objects we touch
and move around
• In any energy transfer in our body, some energy is
changed to heat and transferred to our
environment – the energy is LOST from our body
• Staying alive requires a lot of energy
– Heart beating, brain sending messages, cells moving
substances in and out, muscle contractions, etc.
• As our body’s energy gets low, we get tired
What is power?
• Power – the rate at which work is done
• More power means…
– More work is done in the same amount of time
– The same amount of work is done in less time
• EX. A person that is a more powerful runner is
faster and can run farther in the same amount of
time as a less powerful runner (more work in
equal time)
• EX. A car with a more powerful engine can
accelerate to 60mph faster than a car with a less
powerful engine (equal work in less time)
Calculating Power
• Power is measure in watts (W)
• We need to know 2 things in order to measure
power:
1. The amount of work being done
2. The amount of time it takes to do the work
• We can calculate power using the following
formula:
Power = Work / Time (P = W / t)
• 1 watt equals 1 joule divided by 1 second
Calculating Power – Sample Problem 1
• If an engine does 100,000 joules of work in 10
seconds, how much power did it use?
• P=W/t
• P = 100,000J / 10s
• P = 10,000W
• The engine used 10,000 watts of power
Calculating Power – Sample Problem 2
• Because work equals force multiplied by distance,
another way to write the power formula is:
Power = (Force x Distance) / Time
P = (F x d) / t
• P = W / t is the same as P = (F x d) / t
Calculating Power – Sample Problem 2
• If an engine exerts 3500 newtons of force to
move a car 50 meters in 10 seconds, how
much power did it use?
• P = (F x d) / t
• P = (3500N x 50m) / 10s
• P = 175,000J / 10s
• P = 17,500W
• The engine used 17,500 watts of power
A Watt Measures Work Done and
Energy Used In an Amount of Time
• A Watt equals 1 Joule per second
• The more watts, the more work is done each
second
• Joules also measure energy, so a watt also
measures energy use per second
• EX. A 100W light bulb uses 100J of energy
each second that it is on
More about Energy
Different Forms of Energy
1. Electrical
2. Chemical
3. Radiant
4. Thermal
5. Mechanical
KINETIC ENERGY (KE)
-The energy of MOTION
-All moving objects have Kinetic Energy!
-Depends on: MASS and SPEED of the object
-Equation:
Kinetic Energy (KE) = ½ x mass x velocity squared
KE = 1/2mv2
-Joule (J) = SI unit for Energy
Sample Problem
-A jogger whose mass is 60kg is moving at a speed of 3 m/s. What is the
jogger’s Kinetic Energy?
Mass = 60kg
Velocity = 3 m/s
KE = ??J
KE = 1/2mv2
KE = 1/2(60kg)(3m/s)2
KE = 270 J
POTENTIAL ENERGY (PE)
-The energy of REST
-Objects at REST have POTENTIAL ENERGY
-Potential Energy is CHANGED into KINETIC ENERGY when MOTION
occurs
TYPES OF POTENTIAL ENERGY
1. Elastic Potential Energy = Energy stored by something that can be
STRETCHED or COMPRESSED
Ex: Rubber band
2. Chemical Potential Energy = Energy stored in CHEMICAL BONDS
Ex: Food, Natural gas
3. Electrical Potential Energy = Energy stored due to ELECTRICAL
CHARGES
4. Nuclear Potential Energy = Energy stored in the NUCLEI OF ATOMS
TYPES OF POTENTIAL ENERGY
5. Gravitational Potential Energy: Energy stored by objects due to their
POSTION ABOVE EARTH
-Ex: Anything with the potential to FALL
-Depends on: MASS and HEIGHT above ground
-Equation:
GPE (J) = Mass (kg) X gravity (m/s2) X height (m)
GPE = mgh
GPE Example Problem
-What is the GPE of a ceiling fan that has a mass of 7kg and is 4m above
the ground?
GPE = mgh
Gravity = 9.8 m/s2
Mass = 7 kg
Height = 4 m
GPE = (7kg)(9.8m/s2)(4m)
= 274 kgm2/s2 = 274 J
To INCREASE GPE
1: INCREASE object’s HEIGHT
2. INCREASE object’s MASS
The Change of GPE to KE
-As objects fall, GPE is changed into KE
-KE is LARGEST right before the object
hits the ground, thus GPE is the SMALLEST right
before hitting the ground
-Objects with MORE GPE move FASTER because
they have more KE
Converting between KE and PE
-MECHANICAL ENERGY = Total amount of POTENTIAL and KINETIC
energy in a system
Mechanical energy = PE + KE
What happens to the mechanical energy as PE and KE are converted into
each other?
ME stays the same!! As PE and KE are
converted, the FORM of energy changes, but the
TOTAL AMOUNT STAYS THE SAME
The Law of Conservation of Energy
-States that: ENERGY CAN’T BE CREATED OR DESTROYED!!
-So does this mean the total amount of energy in the Universe is the
same at all times???
Friction and Air Resistance
These forces can cause some mechanical energy to change into
THERMAL ENERGY!!
TEMPERATURE
-TEMPERATURE = Measure of the AVERAGE KINETIC
ENERGY of the particles in an object
-As temp. INCREASES = Average SPEED of particles
INCREASE = KE INCREASES
-SI UNIT: Kelvin (K) or Celsius (0C)
THERMAL ENERGY
Sum of the KINETIC and POTENTIAL energy of all the
particles in an object
THERMAL ENERGY & TEMPERATURE
-As KE INCREASES =
TEMPERATURE INCREASES
THUS
-As TEMP INCREASES =
THERMAL ENERGY INCREASES
THERMAL ENERGY & MASS
-As MASS INCREASES = total KE INCREASES
THUS
-AS MASS INCREASES = THERMAL ENERGY
INCREASES
HEAT
-Thermal energy that FLOWS from something at a HIGHER
TEMP to something at a LOWER TEMP
-Form of ENERGY
-Measured in JOULES (J)
***Always flows from warmer to cooler***
SPECIFIC HEAT
-Amount of HEAT needed to RAISE THE TEMP of 1kg of
a material by 10C
-Measured in: J/(kg 0C)
-As a substance absorbs HEAT, its TEMP change depends
on the composition of the substance. This is called the
Specific Heat of the substance!
Ocean water
v/s
Beach Sand
(see next
slide)
SPECIFIC HEAT EXAMPLE PROBLEM
1 kg of sand takes 6x less heat to raise it 10C than 1 kg of
water
Which has the higher specific heat??
WATER
Which can absorb more heat with changing its
temp?
WATER
TRANSFERRING THERMAL ENERGY
-3 Ways to transfer thermal energy from place to place
1. CONDUCTION
2. CONVECTION
3. RADIATION
CONDUCTION
-The transfer of energy by COLLISIONS or TOUCH
-This happens because particles are in CONSTANT
MOTION
-Thermal Energy is transferred by collisions between
molecules with more KE to molecules with less KE
**HEAT is transferred by COLLISIONS, not by movement
of matter**
HEAT CONDUCTORS
1. Heat moves faster by conduction
in SOLIDS and LIQUIDS than in
gases
WHY?? Gases are more
spread
out/collisions are
less frequent
2. The BEST conductors = METALS
WHY?? All the e – aren’t
bonded to atoms, so
they move more
freely
CONVECTION
-The transfer of thermal energy in a FLUID (gas or liquid) by
movement of WARMER and COOLER fluid from place to
place
-Uses CONVECTION CURRENTS to transfer the
energy from WARM TO COOL
-Still uses collisions to transfer as well
-As a fluid increases its
temp = it EXPANDS and
its DENSITY
DECREASES
WHY??
-Particles have more KE = move faster = spread out
RADIATION
-The transfer of energy by ELECTROMAGNETIC WAVES
-Waves can travel through space even though there is
no matter THUS Radiation is the only form of energy
transfer in space
-Energy = RADIENT ENERGY
RADIATION
-Radiation can be ABSORBED, REFLECTED, or
TRANSMITTED thru an object
-This depends on: MATERIAL OF OBJECT
-LIGH-COLORED = Reflect more
-DARK-COLORED = Absorb more
-Radiation can be
TRANSMITTED thru:
1. SOLIDS
2. LIQUIDS
3. GASES
INSULATORS
-A material in which HEAT FLOWS SLOWLY
-Good Conductors = BAD INSULATORS
-Good Insulators = Wood, plastic, fiberglass, air
-Gases are usually the BEST INSULATORS
CONTROLLING HEAT FLOW
-You can use various materials to control heat flow
Ex: Jacket/sweater
-Living organisms have special features to help control the
flow of heat
Ex: Fur, Blubber, Scales (reflect), Color
Transferring Energy
Review: 1st: Discussed transferring energy as
HEAT
-Heat flows from an object that is warmer
(more ke) to an object that is cooler (less ke)
New Stuff: 2nd way to transfer energy
-Work = transfer of energy that occurs
when a force makes an object move