CHAPTER 8MACHINES

•Machines make work easier by changing the force you exert insize or direction, or both.•But it does not change the amount of work done. •A machine is a device that makes work easier.

Types of Machines

• Lever

• Inclined Plane

• Pulley

• Wheel and Axle

• Wedge

• Screw

SIMPLE MACHINES

Do work

with one

movement

COMPOUND MACHINES

Are a combination

of two or more simple machines.

MACHINES

SIMPLE MACHINES

INCLINED PLANETYPE

LEVER TYPE

•Inclined Plane•Screw•Wedge

•Lever•Wheel & Axle•Pulley

COMPOUND MACHINE

AXE

Wedge and

Lever

EFFORT FORCE (Fe) is the force you exert on a machine*

RESISTANCE FORCE (Fr) is the force applied by the machine to overcome resistance.*

EFFORT DISTANCE (de) is the distance moved by the effort force.*

RESISTANCE DISTANCE (dr) is the distance moved by the resistance force.

WORK INPUT (Win) is the work put into the machine

eein dFW

WORK OUTPUT (Wout) is the work that the machine puts out.

rrout dFW

IDEAL MACHINE

rree

outin

dFdF

WW

MECHANICAL ADVANTAGE (MA) is the number of times a machine multiplies the effort force.

e

r

FF

MA

IDEAL MECHANICAL ADVANTAGE (IMA)

r

e

dd

IMA

EFFICIENCY

%100

%100

ee

rr

in

out

dF

dF

W

WEfficiency

An ideal machine has an efficiency of 100%. Machines have efficiency of < 100%

Machines make work easier by changing the force you exert in size and/or direction.

LEVERSA lever is a bar that is free to pivot, or turn, about a fixed point.The fixed point of a lever is called the fulcrum. EFFORT FORCE

FULCRUMRESISTANCE

FORCE

LEVERSThe part of the lever on which the effort force is applied is called the effort arm.The part of the lever that exerts the resistance force is called the resistance arm.

RESISTANCE ARM

EFFORT ARM

LEVERS

RESISTANCE ARM

EFFORT ARM

Arm Resistance ofLength

armeffort ofLength

r

e

d

dIMA

FIRST CLASS LEVER

The fulcrum is always in between the effort force and the resistance force. Examples: seesaw, hammer, hair clip, bottle opener, crow bar and cart

FIRST CLASS LEVER

RESISTANCEARM

EFFORT ARM

EFFORT FORCE

RESISTANCE FORCE

SECOND CLASS LEVERThe resistance is located between the effort force and the fulcrum. They always multiply effort force. Examples: nutcracker, wheelbarrow, hole puncher,bottle opener, lemon squeezer

SECOND CLASS LEVER

RESISTANCE ARM

EFFORT ARM

EFFORT FORCE

RESISTANCE FORCE

THIRD CLASS LEVERThe effort is located between the resistance force and the fulcrum. They do not multiply effort force. Examples: rake, broom,fishing pole, your arm, andhockey stick

THIRD CLASS LEVER

RESISTANCE ARM

EFFORT ARM

EFFORT FORCE

RESISTANCE FORCE

THIRD CLASS LEVER

RESISTANCE ARM

EFFORT ARM

EFFORT FORCE

RESISTANCE FORCE

Effort arm is always shorter than the resistance arm, so it cannot multiply force. It’s MA isalways less than 1.

LEVERS WITH A HUMAN TOUCH

Your neck muscles support the effort force to hold up the resistance weight of your head.

First Class Lever

Tip of your feet-FulcrumWeight -ResistanceMuscles- Effort

Second Class Lever

LEVERS WITH A HUMAN TOUCH

Elbow-FulcrumBiceps-EffortWeight-resistance

Third Class Lever

RF

E

PROBLEM #1

A lever uses a force of 250 N to move a rock that weighs 3000 N. Find the mechanical advantage of using the lever.

12250

3000

N

N

F

FMA

e

r

A simple machine allows a worker to move an effort distance 45 cm in order to move a resistance distance 20 cm. What is the ideal mechanical advantage of the machine?

PROBLEM #3

25.220

45

r

e

d

dIMA

PULLYING WITH PULLEYSA pulley is a grooved wheel with a rope or a chain running along the groove. A pulley works something like a first class lever. Instead of a bar a pulley has a rope. The axle of the

pulley acts like the fulcrum. The two sides of the pulleyare the effort arm and the resistancearm.

SINGLE FIXED PULLEY•It is lifted the same distance as it is pulled at the other end of the rope.•Example: flagpole, blinds.•Force is not multiplied. There is only a change in the direction of the force applied.

W F

SINGLE MOVABLE PULLEYS

•Force is multiplied.IMA = 2, since two wires are supporting the resistance. The effort distance must be twice as large as the resistance distance. Example:construction cranes.

W

F

BLOCK AND TACKLE

•It is combination of fixed and movable pulley.•It can produce large enough force to lift automobiles.

Wheel and axle is a simple machine consistingof two wheels of different sizes that rotate together.

Examples: doorknob, faucets, handles, steering wheel of a car etc.

An effort force is applied to the larger wheel. The smaller wheel, called the axle, exerts the resistance force.

It allows us to use less force to do work.

WHEEL AND AXLE

Axleof Radius Wheelof RadiusIMA

Gear is a modified wheel and axle machine. It is wheel with teeth along its circumference. Effort is exerted on one of the gears and the other gear turns.

WHEEL AND AXLE

Slanting boards and ramps are called inclined planes. They are sloping surfaces used to raise objects. W=weight, H=height, L=length, F=force

F

W

H

L

INCLINED PLANE

F

W

H

L

HLIMA

FWMA

HWW

LFW

out

in

%100

%100

LFHW

WW

Efficiencyin

out

What is the ideal mechanical advantage of a ramp 4 cm long and 2 cm high?

22

4

H

LIMA

PROBLEM #5

What is the efficiency of a machine with work input 1000 J and work output 700 J?

%%xJ

Jefficiency

%xWin

Woutefficiency

701001000

700

100

PROBLEM #8

Screw is another form of inclined plane. The threads form a tiny ramp that runs from its tip to near its top. As you turn the screw, the threads seem to pull the screw into the wood.

SCREWS

When a screw is turned ,it moves forward. At the same time, it presses against the material around it. Many turns are needed to move the screw a short distance. But the screw produces a far greater force than the force needed to turn it.

SCREWS

Wedge is a form of inclined plane. It is a moving inclined plane. The material remains in one place while the wedge moves through it.Examples: blade of an axe, knife blade, razor blade, door stop, chisel.

WEDGE