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