The New Childrens Encyclopedia

FEEL THE FORCE

233

MAGNIFYING FORCES

People use machines to magnify forces.

Machines called levers move around a

fixed point called a fulcrum. Most levers

magnify forces, but they act over a shorter

distance than the force you put in. Simple

levers include chopsticks, a pair of pliers,

and a nutcracker.

BALANCED FORCES

When two or more forces act

on an object, they combine to

produce a single “net force.” In

some cases, the forces combine to

make a larger net force. In other

cases, the forces work against each

other, resulting in a weaker net

force. Sometimes the two forces

cancel each other out completely.

 TUGOFWAR If both

teams pull on a rope with the

same force, the net force is zero

and nobody moves.

O

Aristotle (c. 384–322 BCE) The ancient

Greeks were the first to study forces.

Aristotle came up with theories about how

forces make objects move.

O

Archimedes (c. 287–212 BCE) built war

machines with levers to strengthen forces.

O

Galileo Galilei (1564–1642) The

Italian scientist studied forces by rolling

different balls down ramps and shooting

cannonballs through the air.

O

Sir Isaac Newton (1642–1727) The

English scientist came up with three laws

of motion to explain how forces affect the

motions of objects.

WHO’S WHO?

LOTS OF LEVERS

Disk brakes

create friction

on the brake disk

to slow the car.

FRICTION FORCES Roll a ball along

the ground and eventually it will come to

a stop. Friction acts on the ball to slow it

down. Try to push a heavy box along the

floor. Friction provides grip, making it

hard to get the box moving.

INERTIA When there are no forces acting

on an object, it will either stay still or keep

moving in a straight line at the same speed.

This is called “inertia.” In practice, friction

usually slows down a moving object.

SCIENCE

 RUNAWAY CART!

When you let go of

a shopping cart it

continues moving

under its own inertia.

 PLIERS convert the

weak force of your hand

on one side of the lever

into a stronger gripping

force on the other side

of the lever.

 A NUTCRACKER

crushes a walnut by turning

the weak force of your hand

into a stronger force nearer

the fulcrum.

 YOUR FINGERS

grip the chopsticks at

the pivot point. This

reduces your gripping

force but magnifies

your finger movements.

Fulcrum

Fulcrum

Fulcrum