164 Science and Technology
FIND OUT MORE. Atoms 157 • Earth 16 • Matter 156 • Motion 165 • Planet Earth 36 • Space Travel 30
When you strike a ball with a bat, stretch a
rubber band, or lift a suitcase, you are applying
a force. All forces are pushes and pulls. Some
forces can act over long distances, for example,
the force of. GRAVITY pulls you down when
you jump from a high diving board.
HOW CAN YOU MEASURE A FORCE?
Forces are measured by their effects. Spring balances
are used to measure the effect of forces. The stronger
the force applied to a spring, the more it is stretched.
The amount the spring is stretched from its normal
size is proportional to the force applied to it. The unit
of measurement of force is called a newton. One
newton of force increases the speed of a one kilogram
mass by one metre per second every second.WHY DO THINGS FLOAT OR SINK?
If you place a block of wood in water, it displaces
(pushes aside) some of the water but floats. A force
in the water called upthrust acts upwards on it to
counter the downwards pull of gravity. The upthrust
equals the weight of the displaced water. When you
place a heavy object in water, the weight of the
displaced water, which equals the upthrust, is less
than the weight of the object, so the object sinks.WHAT IS THE DIFFERENCE BETWEEN
MASS AND WEIGHT?
Mass is the amount of matter in an object. The mass
of an object does not change if it is taken from Earth
into space. Weight is a force. The weight of an object
on Earth is the force acting on it because of Earth’s
gravity. On the Moon the object weighs less than on
Earth since the Moon’s force of gravity is less.WHAT CAN FORCES DO?
Forces change motion and shape. The force of a
boot kicking a ball speeds the ball up. The force of
a parachute on a skydiver slows the skydiver down.
The force of a string on a whirling ball constantly
changes the direction of motion, keeping it moving
in a circle. Combinations of forces applied to materials
can stretch, twist, and crush them.Gravity is the force that acts between all
objects with mass. Gravity always attracts,
never repels – it is always a pull and never
a push. The strength of the force of
gravity increases either when mass is
increased or when the distance between
the objects is decreased.FORCE EFFECTS RELATIVE STRENGTH
Gravity Gives objects weight Very weak
Holds moons and planets in their orbits
Electromagnetic Holds electrons in atoms and atoms Strong
in molecules
Gives materials strength and shape
Responsible for electricity, magnetism, light,
and other forms of electromagnetic radiation
Weak nuclear Involved in radioactivity Weak
Strong nuclear Holds protons and neutrons in the Very strong
nuclei of atoms1 SKYDIVERS
The force of gravity pulls these skydivers towards the Earth. Their speed
increases until their weight is exactly balanced by the force from the air
(air resistance), which acts in the opposite direction. They continue to
fall, but at a steady rate. By spreading their arms and legs the skydivers
use air resistance to steer as they fall, linking hands to make a ring.1 IMPACT
Forces always come in pairs.
As you head a ball, the force
on your head is opposite in
direction to the force on the ball.
During the impact both the head
and the ball change shape. Their
motion changes in opposite ways.
The ball goes away from you as
your head goes back.Forces
GRAVITY
FUNDAMENTAL FORCESWHAT ARE BALANCED FORCES?
Two forces are balanced when they are equal
in size and opposite in direction. If the
balanced forces are applied to an object, they
will have no effect on its motion in a straight
line, but may stretch or compress it. If the forces
do not act along the same line, they may cause the
object to rotate.forces