Transcript Work and Energy

Chapter 5: Work and Energy
Today’s Objectives
What do you think?
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
List five examples of things you have done
in the last year that you would consider
work.
Based on these examples, how do you
define work?
Work
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In physics, work is the magnitude of
the force (F) times the magnitude of
the displacement (d) in the same
direction as the force.
W = F•d
Work Done by a Constant Force
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What are the SI units for work?
• Force units (N)  displacement units (m)
• N•m are also called joules (J).
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How much work is 1 joule?
• Lifting an apple weighing about 1 N from
the floor to the desk, a distance of about
1 m, is equivalent to 1 J of work.
Question?
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How much work is done in pushing
this Barack Obama Ice Statue with
20 N of Force a distance of 10
meters?
Answer: Work = Fd
(20 N) x (10 m) =
200 N m
x
Question 2
How much work is done
carrying a 10 kg bag a
distance of 10 meters?
No work on it. Why?!
Work only depends on Force
that is in the same direction
as the movement!!!
Work
How would you calculate the work in
this case?
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What is the component of F
in the direction of d?
• F cos 
If the angle is 90°, what is the
component of F in the direction
of d?
• F cos 90° = 0
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If the angle is 0°, what is the
component of F in the direction
of d?
• F cos 0° = F
More Accurate
Equation
W = F d cos()
•
•
Work Done by a Constant Force
Work done by forces that oppose the
direction of motion, such as friction, will
be negative.
Centripetal forces do no work,
because they are always
perpendicular to the direction
of motion.
How
mumuch work is done by friction if the vacuum is 25 kg and
How
the kinetic coefficient of friction is .4 when the vacuum is dragged
across a rug 4 m?
Work is a Scalar
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Work can be
positive or
negative but
does not have
a direction.
What is the
angle between
F and d in each
case?
Now what do you think?

Based on the physics definition, list
five examples of things you have
done in the last year that you would
consider work.
(Show your List to Mr. P. before you leave.)
• Energy is traditionally defined as the
ability to do work.
• Energy appears in many forms. Light,
Electricity, Nuclear Energy, Heat, and Mechanical
Energy are all examples.
• Mechanical Energy is energy due to
position or movement.
2 Types of Mechanical Energy
Kinetic Energy – Energy due to Motion
Any moving object has the ability to do work
on another object. Therefore every moving
object has Kinetic Energy.
M = mass
2
V = velocity squared
2 Types of Mechanical Energy
Potential Energy - energy due to position.
Any object can have potential energy due to its
position or because of its surroundings.
Familiar examples of potential energy:
• A wound-up spring
• A stretched elastic band
• An object raised to some height over the ground
Water Mill
Gravitational Potential Energy
• Work is done in order to lift an
object up off the ground:
• W = Fd = (mg)h
• By doing work on the object,
we have given it energy.
• We therefore define the
gravitational potential energy:
P.E. = mgh
m: mass
g: 9.8 m/s
2
h: height
Potential Energy
This potential energy can become kinetic energy
if the object is dropped.
Potential energy is a property of a system as a
whole, not just of the object (because it depends
on external forces).
We usually measure h from the ground. So at
the ground, h = 0, which means the P. E. is zero.
Potential Energy
Potential energy can also be stored in a spring
when it is compressed; the figure below shows
potential energy changing into kinetic energy.
Weekly Homework
Due on Tuesday
Pg. 184 #’s: 1, 7, 8, 9, 12,
19, 21, 23, 33
Recap – so far…
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Work = Fd(cos )
Energy is the ability to do work.
Kinetic Energy – Energy due to
motion – K.E. = 1/2mv2
Potential Energy – Energy due to
position – (gravitational P.E. = mgh)
Units for work and energy: joules (j)
Kinetic Energy, and the WorkEnergy Principle
The amount of work that can be done on an
object, is equal to the amount of kinetic energy
or potential energy it gains or loses:
Work = change in energy
• If the net work is positive, energy increases.
• If the net work is negative, energy decreases.
Kinetic Energy, and the Work-Energy
Principle
Because work and kinetic energy can be
equated, they must have the same units:
kinetic energy is measured in joules.
Mechanical Energy and Its
Conservation
Energy cannot be created or destroyed. It can
only transfer from one form to another or be
passed from one object into another.
This is the Law of conservation of
energy.
bowling ball pendulum
Bowling Ball Pendulum – You Tube
Pendulum Analysis Questions
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Where does the bowling ball in the
pendulum have the highest velocity?
How does the original height of the
pendulum compare to its final height
before it begins to come back?
Is energy conserved in the
pendulum? Why or why not?
Conservation of Energy in a
Pendulum
Energy Conservation Example 2
Roller Coasters!!!
Another Example:
http://surendranath.tripod.com/Applets
/Dynamics/Coaster/CoasterApplet.html
Energy Conservation Example 3
Other Forms of Energy; Energy
Transformations and the
Conservation of Energy
•Work is done whenever energy is transferred
from one object to another.
•Work is also done whenever energy changes
forms.
•Accounting for all forms of energy, we find that
the total energy neither increases nor
decreases. Energy as a whole is conserved.
Power
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Power is the rate at which work is
done.
Power = Work*/Time
*(force

x distance)
The unit of power is the watt.
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