machines – unit 5. definition of machine any device that helps people do work. it does not change...
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DEFINITION OF MACHINE
• ANY DEVICE THAT HELPS
PEOPLE DO WORK.
• IT DOES NOT CHANGE THE
AMOUNT OF WORK DONE
WORK CONSERVATION
• MACHINES DO NOT INCREASE THE
WORK YOU PUT INTO THEM. THE
WORK THAT COMES OUT OF A MACHINE CAN
NEVER BE GREATER THAN THE
WORK THAT GOES INTO THE MACHINE.
• MACHINES ALSO DON’T CHANGE THE
AMOUNT OF WORK NEEDED, SO…
HOW DO MACHINES MAKE WORK EASIER?
• BY CHANGING SIZE OF THE FORCE NEEDED
TO DO THE WORK AND THE DISTANCE OVER
WHICH THE FORCE IS APPLIED
• BY CHANGING THE DIRECTION
IN WHICH THE FORCE IS APPLIED
SO, WHY DO WE SAY THAT MACHINES MAKE
WORK “EASIER”?
THERE ARE 3
WAYS THAT A
MACHINE CAN MAKE
A TASK “EASIER”.
1. IT CAN MULTIPLY THE SIZE OF
THE INPUT FORCE, BUT DECREASE
THE DISTANCE OVER WHICH
THE FORCE MOVES
OUTPUT DISTANCE IS LESS
THAN INPUT DISTANCE
2. IT CAN MULTIPLY THE INPUT
DISTANCE, BUT DECREASE
THE SIZE OF THE FORCE.
OUTPUT DISTANCE IS MORE
THAN INPUT DISTANCE
3. IT CAN LEAVE BOTH
FORCE AND DISTANCE
UNCHANGED, BUT
CHANGE THE DIRECTION
IN WHICH THE FORCE MOVES.
IN OTHER WORDS…MOST MACHINES
MAKE WORK EASIER BY MULTIPLYING
EITHER FORCE OR DISTANCE - BUT
NEVER BOTH. NO MACHINE CAN
MULTIPLY BOTH FORCE & DISTANCE!!!
(THIS IS WORK CONSERVATION)
MECHANICAL ADVANTAGE
• THE NUMBER OF TIMES A
MACHINE MULTIPLIES
THE EFFORT FORCE• Formula = OUTPUT FORCE
___________________________________________________________
ie. pulleys INPUT FORCE
• FOR INCLINED PLANES:
LENGTH OF INCLINE / HEIGHT OF
INCLINE
OR… INPUT DISTANCE / OUTPUT DISTANCE
• FOR WHEEL AND AXLES:
WHEEL RADIUS / AXLE RADIUS
OR… INPUT DISTANCE / OUTPUT DISTANCE
WHAT ABOUT OTHER MACHINES
WORK TRANSFERS ENERGY
• MACHINES TRANSFER ENERGY TO OBJECTS ON
WHICH THEY DO WORK (INCLUDING YOURSELF)
• WHAT’S THE DIFFERENCE IN WORK BETWEEN
CLIMBING A MOUNTAIN STRAIGHT
UP AND HIKING UP THE GENTLE
SLOPE ON THE SIDE?
TWO TYPES OF WORK INVOLVED IN USING A MACHINE
• WORK INPUT = THE WORK THAT
GOES INTO A MACHINE; EFFORT
FORCE EXERTED OVER DISTANCE
KEY WORDS
FORCE DISTANCE WORK
INPUT FORCE INPUT DISTANCE WORK INPUT
OUTPUT FORCE OUTPUT DISTANCE WORK OUTPUT
WORK EFFICIENCY (%)
• COMPARISON OF WORK OUTPUT TO WORK INPUT
• THE CLOSER WORK OUTPUT IS TO WORK INPUT, THE
MORE EFFICIENT THE MACHINE.
• NO MACHINE IS EVER 100% EFFICIENT…WHY?
FORMULA = WORK OUTPUT -------------------------------------------------------------------------------------------------- x 100%
WORK INPUT
MACHINE EFFICIENCY PROBLEMS
• YOU DO 4000J OF WORK USING A SLEDGE HAMMER. THE SLEDGE HAMMER DOES 3000J OF WORK ON THE SPIKE. WHAT IS THE EFFICIENCY OF THE SLEDGE HAMMER?
WORK OUTPUT / WORK INPUT X 100
3000J / 4000J = .75 X 100% = 75%
• YOU DO 250J OF WORK USING AN INCLINED PLANE. THE INCLINED PLANE DOES 100J OF
WORK ON THE OBJECT. WHAT IS THE EFFICIENCY?
100J / 250J = .4 X 100 = 40%
REMEMBER…MACHINES CAN
MULTIPLY FORCE, BUT NOT WORK.
YOU CAN’T GET MORE WORK OUT OF
A MACHINE THAN YOU PUT INTO IT
EVEN IF YOU DO GET MORE FORCE
INCREASING EFFICENCY
• ONE WAY TO INCREASE EFFICIENCY
OF A MACHINE IS TO REDUCE FRICTION
• HOW CAN YOU INCREASE THE
EFFICENCY OF A BIKE?
RESISTANCE
• THE FORCE THAT YOU AND THE MACHINE
ARE WORKING AGAINST.
• ALSO REFERRED TO AS THE LOAD.
• FOR EXAMPLE, WHEN USING A WHEELBARROW
TO MOVE DIRT, THE WEIGHT OF THE DIRT IS
THE RESISTANCE.
LEVER
• A RIGID BAR THAT’S FREE TO MOVE ABOUT A
FIXED POINT CALLED A FULCRUM
Examples: Shovel, nutcracker, seesaw, crowbar, tweezers, fishing pole, door,
etc
THE PARTS OF THE LEVER
• WHERE IS THE FULCRUM?
• WHERE IS THE EFFORT OR FORCE?
• WHERE IS THE RESISTANCE?
THREE CLASSES OF LEVERS
• THERE ARE THREE CLASSES, OR TYPES, OF LEVERS
FIRST-CLASS
SECOND-CLASS
THIRD-CLASS
• A LEVER’S CLASS IS DETERMINED BY WHAT IS IN
THE MIDDLE: THE FULCRUM, THE RESISTANCE,
OR THE EFFORT.
PLEASE TURN TO PAGE 155 IN YOUR TEXTBOOK
• FIRST-CLASS LEVER: FULCRUM IS IN THE MIDDLE.
• SECOND-CLASS LEVER: RESISTANCE IS IN THE MIDDLE.
• THIRD-CLASS LEVER: EFFORT IS IN THE MIDDLE.
How can you affect the mechanical advantage of a lever?If you move the position of the fulcrum & push down on the
lever, the box becomes easier to lift. But in order for it to become easier you have to push the lever down a great
distance of 1.5 meters to lift the box up a short distance of .5 meters at the other end of the lever. The task becomes
easier because it’s a small force but over a large distance which is converted into a large force over a short distance.
WHEEL & AXLE
• MADE UP OF TWO CIRCULAR OBJECTS
OF DIFFERENT SIZES WITH THE WHEEL
AS THE LARGER OBJECT
WHEEL & AXLE
• WHEEL IS LARGER SO IT TRAVELS A GREATER
DISTANCE. THE FORCE APPLIED TO THE WHEEL
IS MULTIPLIED WHEN IT’S TRANSFERRED TO THE
AXLE, WHICH TRAVELS A SHORTER DISTANCE.
• WHAT HAPPENS IF THE FORCE IS
APPLIED TO THE AXLE?
PULLEY
• FIXED PULLEYS ONLY CHANGE THE
DIRECTION OF A FORCE
• MOVABLE PULLEYS CHANGE THE
AMOUNT OF THE FORCE
• PULLEY SYSTEMS MAY CHANGE THE DIRECTION
OF A FORCE AND THE AMOUNT OF THE FORCE
INCLINED PLANE
• MULTIPLIES THE INPUT FORCE,
AND DECREASES THE DISTANCE
OVER WHICH OUR FORCE IS
EXERTED.
SCREW
• IT ROTATES & WITH EACH TURN MOVES
A CERTAIN DISTANCE.
• IT MULTIPLIES THE EFFORT FORCE BY ACTING
THROUGH A LONG DISTANCE.
MECHANICAL ADVANTAGE OF SIMPLE MACHINES
Formula = OUTPUT FORCE ___________________________________________________________
INPUT FORCE
IF A MACHINE DECREASES THE FORCE YOU USE YOU
USE TO DO WORK, YOU CAN FIND THE “IDEAL”
MECHANICAL ADVANTAGE WITHOUT KNOWING THE
INPUT AND OUTPUT FORCES. BUT, YOU MUST
ASSUME THE SIMPLE MACHINE IS 100% EFFICIENT
WH
•TO FIND THE IDEAL MECHANICAL ADVANTAGE YOU DIVIDE:
LENGTH OF THE INCLINE--------------------------------------
HEIGHT OF THE INCLINE
l IMA = ------ h
IMA - INCLINED PLANE
WH
•TO FIND THE IDEAL MECHANICAL ADVANTAGE YOU DIVIDE:
RADIUS OF THE INPUT--------------------------------------
RADIUS OF THE 0UTPUT
R (in)
IMA = ---------- R (out)
IMA – WHEEL & AXLE
WH
•TO FIND THE IDEAL MECHANICAL ADVANTAGE YOU DIVIDE:
DISTANCE FROM THE INPUT FORCE TO THE FULCRUM-----------------------------------------------------------------------------DISTANCE FROM THE OUTPUT FORCE TO THE FULCRUM
d (in)
IMA = ---------- d (out)
IMA – LEVERS
GEARS
• TWO GEARS THE SAME SIZE AND NUMBER OF
TEETH WILL TURN THE SAME SPEED,
BUT OPPOSITE DIRECTIONS
• A THIRD GEAR CAN BE ADDED TO MAKE THEM
MOVE IN THE SAME DIRECTION
GEARS
• THE GEAR THAT TURNS ANOTHER GEAR
EXERTS THE INPUT FORCE
• THE GEAR THAT IS TURNED EXERTS
THE OUTPUT FORCE
GEARS
• A DIFFERENCE IN SPEED BETWEEN
TWO GEARS (CAUSED BY A
DIFFERENCE IN SIZE AND DISTANCE
EACH TURNS THROUGH)
PRODUCES A CHANGE IN FORCE
MECHANICAL ADVANTAGE OF COMPOUND MACHINES
• THE M.A. OF A COMPOUND MACHINE IS
EQUAL TO THE
SUM OF THE M.A. OF ALL
THE SIMPLE MACHINES THAT
MAKE UP THE COMPUND MACHINE