Childrens Illustrated Encyclopedia

215

NewtoN’s laws of motioN

in 1687, the english scientist isaac Newton

(1642-1727) published his three laws of

motion. the first law explains that an

object stays at rest or moves at a constant

speed unless a force pushes or pulls it. the

second law explains how force overcomes

inertia and causes acceleration. the third

law explains that when a force (or action)

pushes one way, an equal force (or reaction)

always pushes in the opposite direction.

iNertia

it takes a strong force to start a heavier (more massive) object moving. in the same

way, a strong force is needed to make it slow down and stop. this reluctance to

start or stop moving is called inertia. the heavier the object, the greater its inertia.

acceleratioN

the action of a force produces motion, making an object accelerate

(speed up). for example, the force produced by the engine makes a

ship accelerate. the stronger the force, the greater the acceleration.

water and air resist motion, producing a force called drag.

a small boat accelerates easily and soon reaches its

cruising speed. But drag increases as speed increases.

when drag force balances the driving force

of the engines, speed stays constant.

Friction

when two surfaces rub against each

other, they produce a force called

friction, which opposes motion.

for example, brakes use

friction to slow a wheel

down. friction produces

heat and wastes energy.

Putting a layer of oil

between the moving

parts of a machine

reduces friction and

improves efficiency.

Find out more

atoms and molecules

Bridges

Gravity

magnetism

Physics

actioN aNd reactioN

a rowboat moves by action and reaction. the

force of the oars pushing on the water is the action.

the moving water exerts an equal and opposite reaction

on the oars. this reaction force pushes the boat forward.

chaNGiNG directioN

when you move in a circle, on a fairground

ride for example, a constant force is needed to

change the direction of your motion. this force

acts toward the center of the circle. on the ride

shown above, the force comes from the tension

in the ropes that support the seats.

what is it that makes oBjects move? why does a

boat float? how does a magnet work? left to itself, any

object would remain still, but when it is pushed or pulled,

it begins to move. something that pushes or pulls is

called a force. forces often produce motion, or

movement. for example, an engine produces a force

that pushes a car forward. there are several different

kinds of forces. a magnet produces a magnetic force

which pulls pieces of iron toward it, and a rubber band

produces an elastic force when you stretch it. liquids

produce forces, too. a boat floats because of the force of

water pushing upward on the hull. and a drop of water holds

together because of a force called surface tension, which makes

all liquids seem as though they have an elastic skin around them.

from the smallest particle inside an atom to the largest galaxy, the

whole universe is held together by powerful forces. one of these

forces is gravity, which holds us on to the surface of earth.

In an arch

bridge, the piers

(ends of the

bridge) support the

weight of the arch.

Force and motion

static

forces

when two teams

in tug-of-war pull

equally hard on the

rope, neither team moves.

this is because the forces

produced by the teams balance

exactly. forces that balance and produce no

movement are called static forces. a bridge

stays up because of the balance of static

forces. its weight pushing down is balanced

by parts of the structure pushing up.

US_215_Force_motion.indd 215 21/01/16 4:59 pm