Transcript Work and Energy

Work and Energy
Work by a Constant Force
Work by a Varying Force
Kinetic Energy and Work
Chapter 6
Work and Energy
6.1 Work Done by a Constant Force
When something has energy, it has the ability to do work. We
start our discussion of energy with the concept of work.
I am going to ask for several daring volunteers to do some
work for me (hold, push, and lift).
While these volunteers are busy working away, here’s a
question for the rest of the class:
Define “work.” Do your best to come up with a “scientific”
definition. Only one sentence, please. No peeking at later
slides!
You working volunteers, also think of a definition. I might let
you tell it to us after I … finish … with you.
Work is done on an object by a force when the force
moves the object through some distance.
Suppose I lift a 100 pound chunk of lead, put it on your
shoulder, and make you stand there for 10 minutes. Do you
do any work in the process?
You don’t! Not “physics” work.
What a stupid way to define work!
No, the physics definition of work is consistent with the known
laws of physics.
You'll probably object that your muscles say they did work,
and you are correct.
In order to maintain a steady position, your muscles must
continually contract and expand, and in the process they
certainly do work on the tissues of your body.
Go to howstuffworks
to see this in action.
So I lift a 100 pound chunk of lead, put it on your shoulder,
and make you stand there for 10 minutes. I claim you do no
work.
Strictly speaking, I should say you did no work on the chunk
of lead (in this simple example where you did not move the
chunk of lead).
Did anybody do work in this example?
When I lifted the chunk of lead from the floor to your shoulder,
I had to exert a force. The force moved the mass of lead
some distance; therefore I did work.
If you tire and let the chunk of lead fall, do you do any work?
No.
As the lead falls, does it do any work?
No (if you ignore the air it pushes aside, and assuming the
earth does not move “up” to meet the lead).
However, the force of gravity causes the lead to fall, so gravity
does work on the lead as it falls.
Work is done when a force moves a mass through some
distance. Work is proportional to FD, where F is the force that
moves the mass and D the distance. In the SI system of units
that scientists prefer, the units of work are joules, where 1
joule equals 1 newton-meter.
Holding a book: no work done.
Lifting a book: work done.
Here’s an equation: WF = FD = FD cos 
D

Wgrav = mg cos 
mg

OSE:
[WF]if = FD cos .
Use the correct angle, not just any old !
mg
6.2 Work Done by a Varying Force
have to to
skip
thisthis
section.
IfSorry,
you promise
read
section, I won’t test you on it.
6.3 Kinetic Energy and the
Work-Energy Principle
What is kinetic energy?
Your text shows that the work to change the speed of a mass
m from v1 to v2 is
W = ½mv22 – ½mv12
so we define kinetic energy as
OSE:
K = ½mv2 .
With the above definition of K, we have
OSE:
[Wnet]if = K .
This is often called the work-energy theorem. It is one of the
BIG DEALS of physics.*
Note that (stuff) always means (stuff)final - (stuff)initial.
So K means Kfinal - Kinitial.
*Calling this a BIG DEAL will cause you to want to apply it to every problem in this
chapter. There are even BIGGER DEALS later that are usually better starting points
for problems.
Example 6-4. What is the KE of a baseball of mass m thrown
with a velocity of magnitude V? If the ball started from rest,
how much energy did it take to make it reach this speed?
Giancoli does the second part the “physicist way.” Let’s do it
the “official way” here.