Transcript PowerPoint Presentation - Work, Power, & Simple Machines

Work, Power,
& Machines
What is work ?
The product of the force applied to an
object and the distance through which
that force is applied.
 Work = Force (N) x Distance (m)
 1 N*m = Joule (J)
Is work being done or not?
 Mowing the lawn
 Weight-lifting
 Moving furniture up a
flight of stairs
 Pushing against a
locked door
 Swinging a golf club
 YES
 YES
 YES
 NO
 YES
Do you do more work
when you finish a job
quickly?
 Work does NOT involve time, only force
and distance.
 No work is done when you stand in place
holding an object.
 Would you do more work if you ran up
the stairs and skipped 2 at a time?
The Joule
 1 newton-meter
is a quantity
known as a
joule (J).
 Named after
British physicist
James Prescott
Joule.
 How quickly work is done.
 Amount of work done per unit time.
 If two people mow two lawns of equal
size and one does the job in half the
time, who did more work?
 Same work. Different power exerted.
 POWER = WORK / TIME
The watt
 A unit named after
Scottish inventor
James Watt.
 Invented the steam
engine.
 P = W/t
 Joules/second
 1 watt = 1 J/s
watts
 Used to measure
power of light
bulbs and small
appliances
 An electric bill is
measured in
kW/hrs.
 1 kilowatt = 1000 W
Horsepower (hp) = 745.5
watts
 Traditionally associated with engines.
(car,motorcycle,lawn-mower)
 The term horsepower was developed to
quantify power. A strong horse could
move a 750 N object one meter in one
second.
750 N
Machines
 A device that makes work easier.
 A machine can change the size, the
direction, or the distance over which a
force acts.
Forces involved:
 Input Force  Output Force
FO
FI
Force
Force
applied
by
applied to
a machine
a machine
Two forces, thus two
types of work
 Work Input
 work done on a
machine
=Input force x the
distance through
which that force acts
(input distance)
 Work Output
 Work done by a
machine
=Output force x the
distance through
which the resistance
moves (output
distance)
Can you get more work
out than you put in?
Work output can never be greater than
work input.
Mechanical Advantage (MA) –
expressed in a ratio WITH
NO UNITS!!
 The number of times a machine
multiplies the input force.
2 types of mechanical
advantage
 IDEAL
 Involves no
friction.
 Is calculated
differently for
different machines
 Usually input
distance/output
distance
 ACTUAL
 Involves friction.
 Calculated the
same for all
machines
Different mechanical
advantages:
 MA equal to one.
(output force = input
force)
 Change the direction
of the applied force
only.
 Mechanical
advantage less than
one
 An increase in the
distance an object is
moved (do)
Efficiency
 Efficiency can never be greater than 100
%. Why?
 Some work is always needed to
overcome friction.
 A percentage comparison of work output
to work input.
 work output (WO) / work input (WI)
Simple Machines
 6 different types of simple machines:
1.) the lever
2.) the wheel and axle
3.) the inclined plane
4.) the wedge
5.) the screw
6.) the pulley
1. The Lever
 A bar that is free to pivot, or move about
a fixed point when an input force is
applied.
 Fulcrum = the pivot point of a lever.
 There are three classes of levers based
on the positioning of the effort force,
resistance force, and fulcrum.
INPUT
FORCE
(EFFORT
FORCE)
OUTPUT
FORCE
(Resistance
Force)
INPUT ARM
(EFFORT
DISTANCE)
OUTPUT ARM
(RESISTANCE
DISTANCE)
FULCRUM
First-Class Lever
• The position of the fulcrum
identifies a first-class lever.
- The fulcrum of a first-class lever is
ALWAYS located between the input
force (effort) and the output force
(resistance)
• Depending on position of fulcrum, MA
of first-class lever can be greater than
1, equal to 1, or less than 1
Which ones are first-class
levers?
Second-Class Levers
• Output force is located
BETWEEN the input force and
the fulcrum
Example of second-class
lever
When you lift handles of
wheelbarrow, it rotates
around its fulcrum
Input distance greater than
output distance
Increased input distance
means it takes LESS force
from you to lift the load
MA of a second-class lever is
always greater than 1
Third-Class Levers
Input force is located between the
fulcrum and the output force
• The output distance over which the lever exerts its force is always
larger than the input distance you move the lever through. Therefore
MA for third-class levers is always less than 1
Wheel & Axle
 Consists of 2 discs or cylinders, each one
with a different radius.
wheel
axle
Can have MA greater than or less
than 1
Would the MA for the steering wheel be greater than or less than 1?
Greater than 1: input distance larger than output distance
Inclined Planes
 Imagine how hard it would be to walk up the side of a
steep hill.
 It would be MUCH easier to follow a gentle slope of a
winding trail….why is this?
What is happening to input distance if
you decide to take the curvy trail
instead of going straight up the hill?
Because input distance is greater
than output distance, in this case, the
input force is decreased…..so it’s
easier for you!
Inclined Planes
 Inclined plane: a slanted surface along
which a force moves an object to a
different elevation
Input distance
Output distance
Inclined Planes
 What is the MA of the following inclined
plane?
6m
1m
Mechanical Advantage = 6
MA for inclined plane will NEVER be less than 1
Wedges & Screws
 Similar to inclined planes BUT sloping
surfaces can move.
 Wedge: a V-shaped object whose sides
are two inclined planes sloped toward
each other.
A thin wedge of a given
length has a greater MA than
thick wedge of the same
length. Why is this so?
Examples: knife, axe, razor
blade
Length of wedge is divided by
width of the opening. Smaller
the opening (thickness), the
larger your MA will be
Wedge
Why did Samurai’s feel it was so
important to keep their blade
sharp?
They wanted to have the highest
MA possible!
Sharpening the sword increases
MA (just like having a thinner
wedge in the wood worked better
than a thicker one)
Screws
 Screw: an inclined plane wrapped around
a cylinder
threads
Which screw would be easier to put into a
piece of wood?
- A screw with the threads very close
together or a screw with the threads
somewhat far apart
- The screw with threads CLOSE together!
- Though it would take you less turns for
the screw with fewer threads, it would
require you to exert a greater force
- Screws with threads that are closer
together have a greater MA
4. The Wedge
 An inclined plane
that moves.
 Examples: knife, axe,
razor blade
 Mechanical
advantage is
increased by
sharpening it.
5. The Screw
 An inclined plane
wrapped around a
cylinder.
 The closer the
threads, the greater
the mechanical
advantage
 Examples: bolts,
augers, drill bits
6. The Pulley
 A chain, belt , or rope
wrapped around a
wheel.
 Can either change
the direction or the
amount of effort force
 Ex. Flag pole, blinds,
stage curtain
Pulley types
 FIXED
 Can only change
the direction of a
force.
 MA = 1
 MOVABLE
 Can multiply an
effort force, but
cannot change
direction.
 MA > 1
MA = Count # of ropes that
apply an upward force (note
the block and tackle!)
Fe
 A combination of two or more simple
machines.
 Cannot get more work out of a compound
machine than is put in.