Transcript Chapter 8 - USD305.com

Chapter 8
Work and Machines
Lesson 1: Work
Work
 when something
moves because of a force
being applied
Work = force x distance
Label is newton-meter or
joule
Work formula
w
f
d
Lesson 2: Energy
Energy
ability to do work
There are 2 kinds of energy
◦Kinetic energy in motion
◦KE = ½ mv²
◦Potential stored energy
◦PE = mgh
gravity = 9.8 m/s²
6 Forms of Energy
Chemical
energy  stored
in the bonds btw. atoms
Heat energy  moving
particles in matter
Mechanical energy  in
moving objects
6 Forms of Energy cont.
Nuclear
energy  stored in
the nucleus of atoms
Radiant energy  light
energy
Electrical energy  causes
electrons to move
Energy
can be converted
from one form to another
 Generator
device used to
convert mechanical energy to
electrical energy
Law
of conservation of
energy energy cannot be
created nor destroyed
Lesson 3: Levers
Simple
machines tool that
makes it easier or possible to
do work
Lever bar that is free to
turn around a fixed point
Fulcrum fixed point around
a lever turns
Levers cont.
Effort
force (FE force
applied to a machine by the
user
Resistance force (FR) force
applied to the machine by
the object to be moved
3 Classes of Levers
Based on the position of the
resistance, fulcrum, & effort
First-class lever fulcrum is
btw effort & resistance
 Changes
direction of force &
can increase force
 Examples
of class 1 levers
include:
 Teeter-totter
 Scissors
 Pair of pliers
Second-class
lever
resistance is btw effort and
fulcrum
Always increases force
Do not change direction
 Examples
of class 2 levers
include:
 Wheelbarrow
 Crowbar
 Nut cracker
Third-class
levers effort
is btw resistance & fulcrum
Increases distance which
cause resistance to move
further or faster
Examples of class 3 levers include:
 Tweezers
 Mousetrap
 Stapler
 Broom
 Hockey
stick
Efficiency
A
simple machine cannot do
more work than the person
using it
Machines increase or change
the direction of force
If less effort is needed, more
distance is needed also
Effort distance & resistance distance
Effort
distance (dE) the
distance the effort moves
Resistance distance (dR)
the distance the resistance
moves
Effort distance is greater than
resistance distance
Work Input & Work Output
Work
input work put
into a machine by the user
◦Work input = FE x dE
Work
output work done
by a machine against the
resistance
◦Work output = FR x dR
Efficiency
Efficiency
=
work output
work input
FR x dR
FE x dE x 100
x 100
Lesson 4: Mechanical advantage
Mechanical advantage
number of times a machine
multiplies its effort force
◦Mechanical advantage =
resistance force
effort force
◦MA = FR
FE
Effort Arm & Resistance Arm
Effort
arm distance btw
the fulcrum & effort force
Resistance arm distance
btw the fulcrum &
resistance force
◦MA =
effort arm
resistance arm
Lesson 5: Other simple machines
Pulley
wheel w/ a rope,
chain, or belt around it
A single pulley changes
direction, not force; ma = 1
Fixed pulley attached at top
Movable pulley entire pulley
& object attached will rise
Pulley cont.
MA of a pulley = number of
ropes that pull upward
The easier to lift an object,
the more distance you pull
on the rope
Inclined Plane
Inclined plane made of a
ramp used to lift an object
MA = length of ramp
height of ramp
◦Gradual slant = greater MA, but
greater distance
◦Steeper slant = less MA, but
shorter distance
Screw
Screw
inclined plane
wrapped around a nail
MA depends on distance btw
threads
◦Smaller distance = more MA
Wedge
Wedge
2 inclined planes
placed back to back; inclined
plane that moves when used
Thick at one end, thinner at
the other
◦Thinner, more gradual wedge
= greater MA
Wheel & Axle
Wheel
& axle wheel
attached to a shaft
Increases the force you apply
to the wheel
MA depends on the size of
the wheel & thickness of axle