From the movements of the planets to the energy produced
inside atoms, everything that happens in the Universe is ultimately caused by
forces. A force is a push or pull that can make an object move or TURN around. The bigger the force, the more movement it can produce.
When two or more forces act together on an object, their effects are COMBINED. Sometimes the forces add together to make a larger force, and
sometimes they cancel each other out.
Forces are measured in units called newtons (N), named after English
scientist Sir Isaac Newton. The size of a force can be measured using a device
called a force meter or newtonmeter. As the load pulls on the hook, it
stretches a spring to give a reading on the scale. On Earth, the force of
gravity on 1 kg (2.2 lb) is 9.8 newtons.
If an object is fixed at one point and can rotate around
it, that point is called a pivot. If a force acts on the object, the object
turns around the pivot. The turning force is called a torque and the effect it
produces is called a moment. The bigger the force, the greater the moment. The
moment also increases if the force acts at a greater distance from the
pivot.
It is easier to unscrew a nut with a spanner than with your
fingers, because the spanner’s long handle increases the turning effect
or moment of the force. The size of a moment is equal to the force used times
the distance from the pivot on which it acts. If you use a spanner twice as
long, you double the moment, and the nut is twice as easy to turn.
When forces act in the same direction, they combine to
make a bigger force. When they act in opposite directions, they can cancel one
another out. If the forces acting on an object balance, the object does not
move, but may change shape. If the forces combine to make an overall force in
one direction, the object moves in that direction.
A suspension bridge has to support the weight of its own deck,
plus the weight of the vehicles that go across it. The deck of the bridge hangs
from huge steel cables suspended over giant pillars. The cables and pillars are
arranged so that there is no overall force in any direction. A bridge stays up
because the forces on it are balanced and cancel one another out.
