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An object rest remains at rest, and an object
in motion remains in motion at constant speed and in a straight line unless
acted on by an unbalanced force. Which law is this?
Today's Focus:
Introduction to Machines via Activity (C'mon Lever A Little)
Power Point Notes on Work and Power:
Copy Only Bold Print Information into ISN:
Work
is done when a net force acts on an object and the object moves in the
direction of the net force.
Work
is the product of the force on an object and the distance through which the
object is moved: the quantity force × distance
If
the force is constant and the motion takes place in a straight line in the
direction of the force, the work done on an object by a net force is the
product of the force and the distance through which the object is moved.
work
= net force × distance
W = Fd
BARBELL EXAMPLE:
Work
is done in lifting the barbell. If the barbell could be lifted twice as high,
the weight lifter would have to do twice as much work.
While
the weight lifter is holding a barbell over his head, he may get really tired,
but he does no work on the barbell.
Work
may be done on the muscles by stretching and squeezing them, but this work is
not done on the barbell.
When the weight lifter raises the
barbell, he is doing work on it
Work and Force:
Some
work is done against another force.
•An archer stretches her bowstring,
doing work against the elastic forces of the bow.
•When the ram of a pile driver is
raised, work is required to raise the ram against the force of gravity.
•When you do push-ups, you do work
against your own weight.
Work and Speed:
Some
work is done to change the speed of an object.
•Bringing an automobile up to speed
or in slowing it down involves work.
In both categories, work involves a transfer of energy
between something and its surroundings
Work Units:
The
unit of measurement for work combines a unit of force, N, with a unit of
distance, m.
•The unit of work is the newton-meter (N•m), also called the joule.
•One
joule (J) of work is done when a force of 1 N is exerted over a distance of 1 m
(lifting an apple over your head).
Larger units are required to
describe greater work.
•Kilojoules (kJ) are thousands of
joules. The weight lifter does work on
the order of kilojoules.
•Megajoules (MJ) are millions of joules. To stop a loaded truck going at 100
km/h takes megajoules of work.
Practice Problem:
think!
Suppose that you apply a 60-N
horizontal force to a 32-kg package, which pushes it 4 meters across a mailroom
floor. How much work do you do on the package?
think!
Suppose
that you apply a 60-N horizontal force to a 32-kg package, which pushes it 4
meters across a mailroom floor. How much work do you do on the package?
Answer:
W = Fd = 60 N × 4 m = 240 J
Apply
the work equation to determine the amount of work done by the applied force in
each of the three situations described below: DIAGRAM A ONLY
Diagram A Answer:
W
= (100 N) * (5 m) = 500 J
Diagram B Answer:
W
= cos(30
degrees) *(100 N) * (5 m) = 433 J
Diagram C Answer:
W = (147 N) * (5 m) = 735
J
Power:
Power
equals the amount of work done divided by the time interval during which the
work is done.
When
carrying a load up some stairs, you do the same amount of work whether you walk
or run up the stairs.
Power
is the rate at which work is done.
Formula:
Power = work done
time interval
High Power Engine:
A
high-power engine does work rapidly.
•An engine that delivers twice the
power of another engine does not necessarily produce twice as much work or go
twice as fast.
•Twice the power means the engine
can do twice the work in the same amount of time or the same amount of work in
half the time.
•A powerful engine can get an
automobile up to a given speed in less time than a less powerful engine can.
Power Units:
The
unit of power is the joule per second, also known as the watt.
•One watt (W) of power is expended
when one joule of work is done in one second.
•One kilowatt (kW) equals 1000
watts.
•One megawatt (MW) equals one
million watts.
•
In
the United States, we customarily rate engines in units of horsepower and
electricity in kilowatts, but either may be used.
In
the metric system of units, automobiles are rated in kilowatts. One horsepower (hp) is the same as
0.75 kW, so an engine rated at 134 hp is a 100-kW engine.
think!
If
a forklift is replaced with a new forklift that has twice the power, how much
greater a load can it lift in the same amount of time? If it lifts the same
load, how much faster can it operate?
Answer:
The
forklift that delivers twice the power will lift twice the load in the same
time, or the same load in half the time.
Tuesday Warm Up:
The tendency of an object to resist being moved or, if the object is moving, to resist change in speed, or direction until an outside force acts on the object is:
Give an example.
Today's Focus:
Motion Gallery Walk Review (Distance Time Graphs)
Completion of Notes on Work and Power.
Homework: Work and Power Reading Handout
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