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FIND OUT MORE. Engines 198–199 • Forces 164 • Motion 165

WHY DOES A SLOPE MAKE IT EASIER

TO RAISE A LOAD?

When you push a load up a slope, the weight of the

load is shared between your effort and the slope.

You do not have to lift the whole weight in one go.

However, although the effort is smaller, the distance

moved is greater. If you climb a zig-zag path up a

mountain, each step is easier, but you take many more

steps to reach the top than if you follow a steeper,

but more direct route to the top.

The first construction machine was

probably the inclined plane. Early builders

raised loads such as blocks of building

stone by pushing them gradually up a

slope. The more gradual the slope, the

easier it is to raise the load, but the farther

you must travel to get to the top.

WHAT IS A WEDGE?

A wedge is a movable inclined plane. As it is pushed

forwards, it pushes a load sideways. The sharper the

wedge, the further it must be pushed to produce the

same sideways movement, but the greater the

sideways force it applies. The blade of an axe is a

wedge that produces a splitting force as it cuts into

hard materials, such as wood, rock, or ice.

WHAT IS A BLOCK AND TACKLE?

A block and tackle is two sets of pulleys linked by a

single rope. Pulling the rope draws the block and

tackle together. This magnifies the effort put in so that

you can lift or lower a heavy load – a car engine, for

example. Block and tackle pulley systems have been

used for centuries to raise sails and shift cargo on

sailing ships.

Gears are toothed wheels that transfer turning motion and forces

from one place to another, for example from a car engine to the

car wheels. The gear teeth mesh (fit together) so that as one gear

turns, it forces its neighbour to turn in the opposite direction.

WHAT IS A GEAR RATIO?

If two gears have the same number of teeth then

they turn at the same rate and with the same force.

If one gear has twice the number of teeth as the

other, the gear with more teeth rotates at half the

speed of the other gear, but with twice the force.

A pulley is a grooved wheel that a rope or

belt runs over. A single pulley changes the

direction of a force, without changing its

size – for example, to raise a flag up a pole

by pulling down on the rope.

4 INCLINED PLANE

This road follows a zig-zag path up a hillside. The distance travelled is

greater than following the direct route to the top, but the slope is

gentler, so less effort is needed.

WATCH GEARS 3

The gears in this watch carry the turning force from the

spring inside the watch to the hands on its face. The gear ratios

make each hand turn at the correct rate to keep time.

GEARS

BLOCK AND TACKLE 3

A block and tackle is a pulley

system designed to lift a large

load with a smaller effort. The

crane on the far right is equipped

with a block and tackle to move

heavy loads, such as steel plates,

into position on a building site.

A single length of rope

runs over four pulleys. The

mechanical advantage of this

pulley system means it can lift

a load that is four times bigger

than the effort applied.

PULLEYS

INCLINED PLANES

Cab where the

driver operates

controls

Upper pulley

block

Lower

pulley

block

Effort

Load