Post on 12-Jan-2016
Simple Machines
Making Life Easier Through Physics
Work and Energy• Energy is conserved – you
can’t create it or destroy it
• Work is transferring energy from one object to another
• Simple machines can’t increase or decrease the amount of work you do
Calculating Work
• Work doesn’t change when you use a machine• Most simple machines make our lives easier
by DECREASING the amount of force we need to apply to finish a job – If you decrease the force, the distance must
increase
• Some DSECREASE the distance instead– If you decrease the distance, the force must
increase
dFW
Types of Simple Machines
•Lever Family–Lever–Pulley–Wheel and axle
•Inclined Plane Family–Inclined plane–Wedge–screw
Lever• Rigid arm with a turning
point• Turning point is called the
fulcrum• Moves force to a new
location– Resistance force is the force
the lever applies to an object– Effort force is the force a
person applies to the lever• Three classes of levers
1st class levers• Effort force and
resistance force are on opposite sides of the fulcrum
• Seesaws are a first class lever
fulcrum
Effort force Resistance force
2nd class levers• Resistance force is
between the fulcrum and the effort force
• wheelbarrows are a second class lever
fulcrum
Effort forceResistance force
3rd class levers• Effort force is
between the resistance force and the fulcrum
• Your biceps are a third class lever
fulcrum
Effort force Resistance force
Wrap-Up #1• Classify the following levers by
class1. Scissors2. Fishing rod3. Stapler
Wrap-Up #1: Answers• Classify the following levers by class1. Scissors
1st class – handle has effort force, the joint is the fulcrum and the blades apply the resistance force
2. Fishing rod3rd class – fulcrum by your body, handle has effort force and the line has the resistance force
3. Stapler 2nd class – the pivot is the fulcrum, you push in the middle and the staple is the resistance force
Pulleys• The middle point is
like the fulcrum of a lever
• Mechanical advantage is equal to number of ropes
• Three kinds– Fixed pulley– Movable pulley– Block and tackle
(multiple pulleys at the same time)
Wheel and Axle•Pulley (wheel)
connected to a shaft (axle)
•Small force on the large wheel is turned into a bigger force on the smaller axle
•Screwdrivers are also wheel and axles
Inclined Plane• Allow you to apply
a horizontal force to move things up
• You use less force than if you lift straight up
• You apply the force over a longer distance, so the work doesn’t change
Wedge• Changes the direction
of the force and splits it into two– If you push down the
wedge pushes to the left and right
• Useful for splitting things like firewood and your food
• Nails are wedges used to hold things together instead
Screw• An inclined plane
wrapped around a cylinder
• Allows you to twist it into place instead of pushing straight down
• Allows a longer plane than if it was straight (spiral staircase)
Wrap-Up #2• Classify each of these images as a type
of simple machine
#1
Wrap-Up #2• Classify each of these images as a type of simple machine
SCREW
• Classify each of these images as a type of simple machine
#1
Wrap-Up #2• Classify each of these images as a type
of simple machine
#2
Wrap-Up #2• Classify each of these images as a type of simple machine
INCLINED PLANE
• Classify each of these images as a type of simple machine
#2
Wrap-Up #2• Classify each of these images as a type
of simple machine
#3
Wrap-Up #2• Classify each of these images as a type of simple machine
PULLEY
• Classify each of these images as a type of simple machine
#3
Wrap-Up #2• Classify each of these images as a type
of simple machine
#4
Wrap-Up #2• Classify each of these images as a type of simple machine
LEVER
• Classify each of these images as a type of simple machine
#4
Mechanical Advantage• A measure of how
much the machine helps you– MA < 1, you apply a
stronger force over a smaller distance
– MA = 1, the force’s direction is changed
– MA > 1, you apply a lower force over a greater distance R
E
E
R
d
d
F
FIMA
Efficiency• Some energy
(work) is always converted into heat by friction
• The percentage of work transferred by the machine is its efficiency
• Higher efficiency means less friction
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