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FIND OUT MORE. Atoms 157 • Forces 164 • Materials 170

From molecules in this sheet of paper, to planets in

orbits, the objects around us are in constant motion.

The simplest motion is in a straight line at constant

speed. When the speed or direction change, scientists

say that motion is accelerated.

HOW IS SPEED MEASURED?

Two measurements are needed to find

speed – the distance moved, and the time taken. Speed

is calculated by dividing the distance by the time. If a

runner covers five metres in one second, his or her

speed is five metres per second. A car that travels

100 kilometres in two hours has

an average speed of 50

kilometres per hour.

HOW CAN FRICTION BE REDUCED?

Friction between parts of machines can damage them

by wear and tear. Friction also wastes energy as heat

instead of movement. Friction can be reduced by

using oil as a lubricant to make a slippery film

between surfaces. Machines built with a streamlined

shape reduce drag. Aeroplanes are designed to let air

flow over them smoothly with the least resistance.

The study of air flow is called aerodynamics.

The force of friction opposes motion when

one surface slides, or tries to slide, over

another. You feel friction as you drag your

hand across a table. Friction is produced by

forces between the molecules in the surfaces.

Drag is the friction between a solid object

and the fluid it is travelling through.

HOW CAN WE USE FRICTION?

Friction is not always a problem – sometimes we use

it to prevent or slow down motion. Without friction

your shoes would not grip the ground and you would

slip over, and a car’s wheels would spin and skid.

Friction is increased by making shoe soles and tyres

from soft, sticky materials, such as rubber.

4 COMPLEX MOTION

The complicated motion of a gymnast’s front handspring

combines movement in a straight line and movement

in a circle.

FRICTION BY DESIGN 3

An athlete needs friction between the soles

of his shoes and the road to run. Without

friction, his feet would slide on the spot.

PENDULUM SWING 1

The child on the swing is moving

like a pendulum. She kicks to

start. Gravity slows her down as

she rises, then speeds her up

again in the opposite direction.

She swings to and fro, until

friction and air resistance

eventually bring her to rest.

CIRCULAR MOTION 3

Whirling in a circle involves

constantly changing

direction. The ice skater’s

arms fly out as she spins.

By using a force to bring

them straight above her

head, she can increase her

turning speed.

Motion

FRICTION

Grooves channel

water away

Soft rubber soles

grip the ground

Pendulum

WHAT IS ACCELERATION?

When you drop a stone, it starts from rest (speed

equals zero), then speeds up as it falls. The

stone is accelerating. A force is always needed

to produce acceleration – in this case, it is the

force of gravity. Acceleration is slowed by

. FRICTION– in this case, air resistance.

motion