Transcript Simple Machines - 8th Grade Science

Simple Machines
Unit 2
Simple Machines
S8P3. Students will investigate the relationship
between force, mass, and the motion of
objects.
c. Demonstrate the effect of simple machines
(lever, inclined plane, pulley, wedge, screw, and
wheel and axle) on work.
Work

In science, the word work has a different
meaning than you may be familiar with in your
everyday life.
 The scientific definition of work is:
• The transfer of energy when a force
moves an object over a distance in the
same direction of the force.
 Energy: the ability to do work
• If no movement happens, no work is
done.
• Work = force x distance (W = Fd)
• Measured in newton-meters or joules (J)
• Examples: pushing a shopping cart,
turning a door knob, kicking a soccer ball,
lifting a box
Work or Not Work

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
A scientist delivers a speech to an audience
of his peers.
 No
A bodybuilder lifts a dumbbell above his
head.
 Yes
A student pushes against a wall that does
not move.
 No
A father pushes a baby in a carriage.
 Yes
A woman carries a grocery bag to her car.
 No
Simple Machines


Simple Machine
 A device that makes work
easier by changing the size
and/or the direction of the
force used to do the work.
A simple machine does not
help you to do less work.

Work with a simple machine = Work
without a simple machine

No machine can increase both
the magnitude of the force and
the distance an object travels
at the same time.
 Therefore, there is a tradeoff between force and
distance.
Work
 Work
Input (Win)
 work done on a machine
 Work Output (Wout)

work done by a machine
Work



Law of Conservation of Energy
 Energy can never be created or destroyed.
 Energy can be transformed from one form to another.
 You can never get more work out than what you put in.
In an ideal machine...
Win = Wout
In a real machine...
 some energy (output force) is given off (“lost”) as friction.
Win > Wout
Mechanical Advantage (MA): number of times a machine multiplies the
effort force
6 Kinds of Simple
Machines
 Inclined
Plane Family
 Inclined Plane
 Wedge
 Screw
 Lever
Family
 Lever
 Pulley
 Wheel and Axle
Inclined Plane
W  Fd

Inclined Plane
 A straight, slanted surface used to raise
objects because it is higher on one end
 Example: Ramps, stairs, ladders
How does the Simple
The Effects on Work
Machine Make Work Easier
for You?
Inclined planes: You use
The longer the inclined
less effort force over a
plane = smaller effort force
longer distance.
needed = easier work
l
Mechanical Advantage
MA greater than 1 (MA > 1)
h
Wedge
W  Fd

Wedge
 A moving inclined plane with 1 or 2 sloping
sides
 Examples: knives, hatchets, ax blade, blades of
scissors, nails, teeth, plow, and chisel
 A wedge transfers force in one direction into
force in two directions.
 Wedges are used to split or cut things
apart.

W  Fd
Screw
Screw
 A screw is an inclined plane
wrapped around a shaft or
cylinder.
 Examples: a fastener (screw),
jar lid, top of jar, drill bit, light
bulb, vise
 The inclined plane allows the
screw to move itself when
rotated.
How does the Simple
The Effects on Work
Mechanical
Machine Make Work
Advantage
Easier for You?
Screws: You use less
The closer the threads on
MA greater than 1
effort force over a longer the screw = longer the
(MA > 1)
distance.
inclined plane = smaller
effort force needed = easier
work
Lever
W  Fd
 Lever
 a bar that pivots at a fixed
point called a fulcrum
Resistance
arm
Effort arm
Fulcrum
Engraving from Mechanics Magazine, London, 1824
“Give me a place to stand and I will move the Earth.”
– Archimedes
The 3 Classes of Levers
The class of a lever is determined by the
location of the effort force, the load, and
the fulcrum.
 Effort force (input force): the force applied
to the lever
 Load (output/resistance force): the object
being moved

Lever
W  Fd
 Mechanical
Advantage (MA)
Le
MA 
Lr
 Le
Effort arm length
(input force)
Resistance
arm length
(output force)
must be greater than Lr in
order to multiply the force.
Le
MA 
Lr
Lever

W  Fd
First Class Lever
 fulcrum is located between the effort force and resistance
force (load)
 can increase force, distance, or neither
 always changes the direction of force (i.e. a downward
effort force on the lever results in an upward movement of
the resistance force)
 Examples: crowbars, scissors, pliers, tin snips, shovels,
and seesaws
Le
MA 
Lr
Lever



W  Fd
Second Class Lever
 the load (resistance) is located between the fulcrum and
the effort force
 always increases effort force
 does not change the direction of force
 effort force moves farther than resistance
When the load is located closer to the fulcrum than to the
effort force, an increase in force (mechanical advantage)
results.
Examples: nut crackers, wheel barrows, doors, and bottle
openers
Le
MA 
Lr
Lever

W  Fd
Third Class Lever
 the effort force is applied between the fulcrum and the
resistance force (load)
 always increases the distance that the effort force travels
 does not change the direction of force
 always produce a gain in speed and distance and a
corresponding decrease in force
 Examples: arm, tweezers, hammers, baseball bats, brooms,
and rakes
Pulley
W  Fd

Pulley
 grooved wheel with a
rope or chain running
along the groove
 a “flexible first-class
lever”
 a load is attached to
one end of the rope and
a force is applied to the
other end
How does the Simple Machine
Make Work Easier for You?
Pulleys: Your effort force
changes direction and/or you
use less effort force over a
longer distance.
The Effects on Work
Your effort force changes
direction = easier work
AND/OR
More pulleys = smaller effort
force needed = easier work
Le
F
Lr
Mechanical Advantage
MA equal to 1 (MA = 1)
MA greater than 1 (MA > 1)
Pulley
W  Fd

Mechanical Advantage
 equal to the number of supporting
ropes
MA = 0
MA = 1
MA = 2
Pulley
W  Fd
 Fixed
Pulley
 MA =
1
 does
not
increase
force
 changes
direction of
force
Pulley
W  Fd
 Movable
Pulley
MA = 2
 increases force
 does not change direction

Pulley
W  Fd
 Pulley System/Block & Tackle

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
MA = 4
combination of fixed and movable
pulleys
increases force
may or may not change direction
W  Fd
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Wheel and Axle
Wheel and Axle
 two wheels of different sizes that rotate together
 the wheel is always larger than the axle
 a pair of “rotating levers”
Examples: door knob, gears, car axle, pencil sharpener, screw
driver, faucet handles
Wheel
Axle
W  Fd
Wheel and Axle

When effort is
applied to move the
wheel, the axle
turns a shorter
distance, but moves
with more force.

The larger the
wheel is when
compared to the
axle, the larger the
mechanical
advantage.
How does the
Simple Machine
Make Work Easier
for You?
Wheel and Axles:
You use less effort
force over a longer
distance when
turning the wheel.
The Effects on
Work
Mechanical
Advantage
The larger the
MA greater than 1
wheel = smaller
(MA > 1)
effort force needed
= easier work