Transcript Work, Power and Simple Machines

Work has a special meaning in
physics.
Unlike your everyday
experience, where you go to an
after school job and earn
money, work in physics does
not pay for your car insurance.
Work is done on an object
when a force is applied to
an
object
and
Work is
only
done if
the the object is
object displaced.
moves a
Do not confuse
distance.
applying a force
Pushing or
with doing work.
pulling on an
Work = Force x displacement (cosine of angle for
object may
the portion of the force parallel to the distance
make you
traveled)Only the force parallel to the direction
sweat but
you Newton
of the displacement
does work.
Units:
x Meter (Nm)
do no work
unless it Work is a scalar quantity
moves.
W  F d co s 
units: Nm  J  Joule
Read the following five statements and determine whether or not they represent
examples of work.
A teacher applies a force to a wall and becomes
exhausted.
NO
A book falls off a table and free falls to the ground.
YES
A waiter carries a tray full of meals above his head by
one arm across the room. (Careful! This is a tricky
question.)
NO
A rocket accelerates through space.
YES
Power
In Physics Power is the Rate of
Doing Work
P ow er 
W ork
tim e
P 
units:
W

t
Nm
s
F d cos 
t
 W  W att
A watt is a small unit
If you lift a glass of water (~2N) to
your mouth (~0.5 m) in 1 second
you do one joule of work and you
are doing work at rate of one watt
Since a watt is relatively small,
power is often measured in
kilowatts (kW)
Simple
Machines
Simple Machines
A simple machine is an object with few moving
parts that redirects an applied force to perform
work.
Usually a simple machine is used to reduce the force
needed to perform a task.
Simple Machines
and Rube
Goldberg
Work and Machines
• Force of Effort - Fe - The amount of force
that is applied to the machine.
• Force of Resistance - Fr - The amount of
force that the machine applies.
• Distance of Effort - de - The distance the
force of effort is applied.
• Distance of Resistance - dr - The distance
the machine applies the force.
Force of Effort
Force of Resistance
Force of Resistance
Distance of Resistance
Force of Resistance
L
e
v
e
r
Lever
F1 = Fr = Force of Resistance
F2 = Fe = Force of Effort
D1 = dr = Distance of Resistance
D2 = de = Distance of Effort
Fulcrum = Pivot Point
Lever Classes
First Class Lever
Effort - Fulcrum - Resistance
Second Class Lever
Fulcrum - Resistance - Effort
Third Class Lever
Fulcrum - Effort - Resistance
First Class Lever
Pulleys redirect the force of effort.
The number of pulleys and their arrangement
can reduce the amount of effort force
needed to raise a particular load.
The number of lifting strands will determine
the advantage to using a pulley system
Pulleys
1 Lifting Strand
How many lifting
strands for each
pulley system?
3 Lifting Strands
2 Lifting Strands
2 Lifting Strands
3 Lifting Strands
Wi = Work Input – The Amount of Work applied to the
machine
Wi = Fede
Fe=Force of Effort
de=Distance of Effort
Wo = Work Output – the Amount of Work transferred by
the machine
Wo = Frdr
Fr=Force of Resistance
dr=Distance of Resistance
M echanical A dvantage = M A 
Fr
Fe
Ideal M echanical A dvantage = IM A 
de
dr
E fficiency 
Wo
Wi
E fficiency 

Fr d r
Fe d e
MA
IM A
x 100 0 0
x 100 0 0