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Title: Energy

  • Chapter 8 Physics Matters
  • (By James Trefil and Robert Hazen)

Some initial questions
What does it mean to do work?
Can you do a work out without ever actually doing
any work?
Why is it dangerous to speed/tailgate when
driving a car?
Other questions we will eventually be able to
Why does the roller coaster work without a motor
on the car?
How come a ball never bounces as high as it was
Work is done whenever a force is exerted on an
object over a distance
- It is the transfer of energy that results in
moving an object over a distance.
Work Force x Distance
Newton-meters are known as Joules (J)
How much work do you to to lift a 5 kg book from
the floor onto a table if the table is 3m high?
How far will you lift a 20N bag if you do 100 J
of work on it?
Work and Energy
  • When you do work you transfer energy or use
  • Energy is a systems ability to do work
  • Since work is measured in Joules and work is a
    transfer of energy, energy must also be measured
    in Joules.

There are many different forms of energy, what
are they?
Is this Work?
Power is the rate at which energy is used, or the
rate at which work is done.
- The faster energy is used or transferred, or
the faster work is done on an object, the more
power that is expended by the machine doing the
For what everyday things do we tend to pay
attention to power?
One major type of thing are the bulbs that light
our rooms. How do we differentiate between
different types of bulbs?
Watts! Wattage is a unit of measurement that
tells us how much energy the bulb will use in a
certain amount of time.
Power is measured in Watts (joules per
second). - So, a more powerful bulb will use
stored energy faster then a weaker bulb with
the same amount of stored energy.
The English system form of power was based on how
much a single horse can pull.
This was known as horsepower!
1 horsepower ? 746 Watts
A space shuttle has over 37 million horsepower!
How powerful is an elevator that can lift a mass
of 400kg up a building of height 100 m in 20
Lindsey wants to hike up this 400 m mountain and
she has a mass of 48 kg. If she takes the path
that you see and hikes a distance of 620 m, how
much work does she do against gravity to get up
the mountain?
Does Andi, who has the same mass as Lindsey, do
more or less work if instead of hiking, she rock
climbs up a sheer cliff strait up to the
Question Continued
Who is more powerful if Lindsey takes 3 hours to
hike the path, but Andi, being a professional
rock climber (maybe I should have said Maggie),
takes only an hour to scale the cliff?
When both Lindsey and Andi each reach the top of
the mountain, do they have any energy left?
Yes! But what type of energy to do they have?
Potential Energy
  • Basically it is energy stored in a system
  • Both girls have what is called Gravitational
    Potential Energy.
  • Since gravity could exert a force on either of
    them to make them fall to the ground from the top
    of the mountain, there is potential for work to
    be done on them, and hence, potential energy.

Gravitational Potential Energy
Which is mass x acceleration due to gravity x
Potential Energy
How much potential energy does each girl have?
Gravitational Potential Energy
  • Independent of Path to get there

Kinetic Energy
If Philip is waiting at the top of the mountain
to show Lindsey the awesome new set of wings he
built, and he has a mass of 55 kg, how much
potential energy does he have?
Can he convert his potential energy into anything?
Yes! If he jumps off the cliff, gravity will do
work to pull him toward the ground, converting
his potential energy into kinetic energy.
Kinetic Energy
Kinetic Energy is the energy an object has
because it is moving.
It is similar to momentum as they both result
from the movement of masses, but they differ in
the fact that KE is a scalar quantity. Also,
as velocity increase, momentum also increases
linearly, but KE increases as a squared function
(if v goes up by 2, KE goes up by 4)
Kinetic Energy
How much Kinetic Energy will Philip have as he
hits the ground, if he jumps of the mountain at
the sheer cliff face, and his new wings fail
Notice any Coincidences?
Conservation of Energy
Just like with momentum, the total energy of a
system is never lost It is conserved!
This means, that in the case of Philip jumping
off a mountain, he converted his Potential Energy
at the top of the mountain completely into
Kinetic Energy by the time he reached the ground.

If energy is always conserved, why wont our
experiments always work?
Conservation of Energy
If this cliff is 150 m high, and the person in
the picture has a mass of 65 kg, how much PE does
he have at the top of the cliff? How much KE
does he have at the top of the cliff?
How much KE does he have as he hits the water?
How much PE does he have?
How much PE and KE does he have half way down the
Conservation of Energy
What is his velocity as he hits the water?
Conservation Equation
The total energy in a system is equal to the sum
of the potential energy and the kinetic energy at
any given time.
Conservation of Energy
This means that when something falls from a
height, its initial potential energy is equal to
its final kinetic energy.
  • As an object falls, it is speeding up but losing
    height. Therefore, it is gaining kinetic energy
    at the same rate that it is losing potential
  • In fact, gravity is doing work on the object to
    convert its PE into KE!

Conservation of Energy
There are two more important things to
realize. For a given system
When PE is at its maximum, KE is at its minimum,
and when KE is at its maximum, PE is at its
The Work-Energy Theorem
The work done on an object is equal to the
combination of the changes in both kinetic and
potential energy.
Spring Potential
Just like how increasing the height of an object
will store potential energy in the object based
on how much gravity might be able to move it,
elastic things can also store potential energy.
This is known as Elastic Potential Energy.