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CONSTRUCTIVETHOUGHTS
Simple MachinesSimple Machines
7.27.2
Six Types of Simple
Machines
leverlever
pulleypulley
wheel and axlewheel and axle
inclined planeinclined plane
screwscrew
wedgewedge
Lever
• A bar free to pivot about a fixed point
• fulcrum = fixed point• effort arm = part of lever where
Fe is applied
• resistance arm = part of lever where Fr is applied
The Mechanical Advantageof Levers
• MAlever =
length of effort arm (Le)
length of resistance arm (Lr)
• Problem:A worker uses an iron bar to raise a manhole cover weighing 65 N. The effort arm of the lever is 60 cm long, and the resistance arm is 10 cm long. What is the MA of the bar?
• First class: multiply force and/or direction
• Second class: multiply force
• Third class: cannot multiply force; MA < 1 = increases speed
Three Types of Levers
R E
F
R E
F
RE
F
Examples of Class Three Levers
Pulley
• grooved wheel with rope/chain running along groove
• fixed or movable:– fixed: changes direction; MA = 1– movable: multiplies force; MA = 2
»block and tackle: system of fixed and movable pulleys
Wheel and Axle
• 2 wheels of different sizes that rotate together– Fe applied to larger wheel– smaller wheel (axle) exerts Fr
• MA = radius of wheel (rw)
radius of axle (ra)»Problem: A steering wheel
with a 24 cm radius turns the column with a 4 cm radius. What is the MA?
Gears
• modified wheel-and-axle machines
• wheel with teeth along edge– larger gear=effort gear– smaller gear=resistance gear
Inclined Plane
• a sloping surface used to raise objects
• Work is same, but distance is increased, so force is less
• MA = length of slope (l)height of slope (h)
Screw
• An inclined plane that wraps in a spiral around a post
• Threads form a tiny ramp
Wedge
• An inclined plane with one or two sloping sides
• examples: chisels, knives, ax blades, etc.
• difference from ramp: the wedge is what moves through material
Identify as many simple machinesas possible in the following clip:
The End