homework encyclopedia

196 Science and Technology

From bottle openers to cranes, machines make work

easier. Simple machines include. LEVERS,. PULLEYS,

. INCLINED PLANES, and. GEARS. They transfer

force and movement from one place to another, often

magnifying the force or the movement at the same time.

WHAT ARE THE DIFFERENT CLASSES OF LEVER?

The three classes of lever are suited to carrying out

different jobs. In a first-class lever, such as a seesaw,

the effort and load are on opposite sides of the

fulcrum. In a second-class lever, such as a bottle

opener, the fulcrum is at one end, the effort at the

other, and the load in between. In a third-class lever,

such as chopsticks, the fulcrum and the load are at

either end, with the effort in between.

WHAT IS A WHEEL AND AXLE?

The steering wheel of a car works like a circular lever

— it magnifies a turning force. Your hands on the

wheel move through a much larger distance than the

axle (the steering column). The turning effort is

magnified to produce enough force to steer the

car wheels. A crank is a turning handle that works

in a similar way, to raise a heavy bucket of water up

a well with a small effort on the winding handle.

HOW IS A MACHINE’S EFFICIENCY MEASURED?

The efficiency of a machine is the ratio of the work

output to the work input. If the machine is perfect,

then all the work (energy) that you put in (the effort)

is used to move the load. It would be 100 per cent

efficient. In reality, a machine always wastes some

energy as a result of friction between its parts.

WHAT IS MECHANICAL ADVANTAGE?

The mechanical advantage of a machine is the amount

that it magnifies (increases) a force to overcome a

load. Nutcrackers have a mechanical advantage of

about five. The force you use to squeeze the handles is

magnified five times – making it strong enough to

crack even the hardest nut.

A lever is a rod or bar that turns on a

pivot (the fulcrum). The effort applied at

one place moves a load at another place

via the fulcrum. There are three different

types of lever, each with the effort, load,

and fulcrum in different places.

4 THIRD-CLASS LEVER

In chopsticks, the effort is between the fulcrum and

the load. The effort is reduced by this lever, but the

movement is magnified. With small movements of the

hand, you can pick up anything from a grain of rice to

a large nut – but you can’t crack a nut.

INDUSTRIAL DIGGER 3

A complicated machine, such as

this digger, is built from many

simple machines that work

together – the digger arms are

levers, and the teeth on the

bucket are inclined planes.

The driver operates the machine

to move a load far beyond his

own strength.

1 FIRST-CLASS LEVER

These pliers are a pair of first-

class levers. The fulcrum is

between the load and the effort.

The effort is magnified because

the load is closer to the fulcrum.

Machines

LEVERS

1 SECOND-CLASS LEVER

These nutcrackers have the

fulcrum at one end, the effort at

the other, and the load between

• ideal for cracking open a nut.

Wide wheels

are needed to

cross rough

ground

Oil pressure

pushes the

piston along a

hydraulic cylinder

Load

lifted by

the digger

Pistons

apply the effort

to lift the load

Arms raise or

tip the bucket

Effort

Fulcrum

Fulcrum

Load

Effort

Load

Fulcrum

Effort

Effort

Effort

Load

Effort

machines