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Work & Power Notes
Ch 8
Work
• Work is done when a force is applied
over a distance.
• W = F•d
in N (Newtons)
in m (meters)
Work is measured in Joules (J)
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1 J = 1N•1m
Discuss with your table:
• Is holding a heavy book considered work?
• Not in physics! But why?
Lifting a book is work:
• Work is done on the book when:
1. A force is applied
2. The book moves due to the force
• If you just hold the book, you are NOT
doing work on it, even though you may
become tired.
• The work that is done is on your muscles
as they flex or stretch.
Think about it!
Work is done by lifting a book…
Q: How much more work is done when lifting
two books the same distance?
A: Twice as much: 2F•d
Q: How much more work is done when lifting
two books twice as far?
A: 4 times as much as the original book: 2F•2d
Discuss with your table group…
• When carrying a box up stairs, you do the
same work (W = F d ) whether you run or
walk.
– Force (weight of the box) is the same.
– Distance up the stairs is the same.
• Why are you more tired after running up
the stairs than when you walk?
• Hint: What’s different between the two
situations?
Power!
• Power is the rate at which work is
done.
• P=W
t
• Power is measured in Watts (W)
• 1 watt = 1 joule
1 second
Types of Energy Notes
Chapter 8
When work is done
to lift a hammer,
the hammer
acquires the ability
to do work on the
object it hits when
it falls.
When an archer pulls back
on a bowstring, the bent
bow acquires the ability to
do work on the arrow.
• In both cases, “something” has been acquired
that enables the object to do work.
– Compression of atoms
– Physical separation of attracting bodies
– Rearrangement of electrical charges
Mechanical Energy (ME)
• Energy that enables an object to do work
• ME is most often
– Potential Energy (PE) -- The energy due to an
object’s position
– Kinetic Energy (KE) -- The energy due to an
object’s motion
• Energy is measured in Joules (J)
Potential Energy (PE)
• Gravitational Potential Energy (GPE)
• Is the energy of position above the earth’s
surface
• GPE = mg • h
• where: m = mass, g = acceleration due to
gravity, and h = height above the Earth’s
surface
Potential Energy
• Potential energy is energy that has the
potential to do work.
• The energy is considered in a “stored” state.
• Chemical energy in fuels, electric batteries,
and the food we eat is potential energy due to
the positions of the electrical charges at a
submicroscopic level!
Potential Energy (PE)
• PE is stored energy
• To elevate an object, work must be done to lift
the object against the pull of gravity
• How much work must be done to elevate the
object is also the amount of PE it has!
• Remember that weight = mass•gravity, or
• w = mg … and this is the force (F) of an
object due to its mass!
Gravitational Potential Energy (GPE)
• Is the energy of position above the earth’s
surface
• GPE = Work
• GPE = F • d
• GPE = mg • h
• where: m = mass, g = acceleration due to
gravity, and h = height above the Earth’s
surface
Kinetic Energy (KE)
• If an object is moving, it can do work!
• Kinetic energy is the energy of motion, and
depends on the mass of an object and its speed.
• KE = ½ mv2
• KE = Work required to start or stop an object
from moving
• KE = W
• ½ mv2 = F•d
Conservation of Energy
• Whenever work is done, energy changes.
• Law of Conservation of Energy: Energy is not
created or destroyed; energy is only
transformed.
• ME = KE + PE
• The total ME is conserved, even if PE and KE
change.
ME is conserved
• Consider a cat on a ledge.
• The cat has PE due to its position
• If the cat jumps off the PE changes and is
transferred to KE as the cat gains speed.
• Just before the cat hits the ground its energy
is all in the form of KE