How Do Forces Affect Motion?
Forces
How many times each
day do you push or pull
something to make it
move? Each push or pull
is a force. Forces can be
big or small. The force a
crane uses to lift a truck is
huge. The force of a
feather landing on the
ground is tiny.
Forces Are Important
Forces make objects start moving, stop
moving, speed up, slow down, or change
direction. A large force can cause a large
change in speed or direction. A small force
may produce a smaller change. If you tap
a basketball with your finger, the ball hardly
moves. If you kick the ball, it rolls away
quickly.
How Forces Are Measured
Forces are measured in units
called newtons. This unit is
named after the British
scientist Sir Isaac Newton
(1652-1727). Newton
explained how force and
motion are related. To lift a
mass of one kilogram on
Earth requires 9.8 newtons
of force.
Friction
Think again about the basketball you
kicked to make it roll. Does it roll
forever? No, a force brings it to a stop.
That force is friction. Friction works
against motion. Friction acts between
surfaces of objects that touch. The
surfaces rub against each other, slowing
the object or stopping it from moving.
Friction
How much friction
there is depends on
the objects. You can
slide easily down a
snowy hill because
there is little friction
between the smooth
snow and the plastic
bottom of the slide.
Friction Can Be Helpful
Friction can be a
helpful force. A
gymnast uses
chalk on his
hands to increase
friction and
prevent his hands
from slipping off
the parallel bars.
Inertia
If there were no friction, would a moving
object stop moving? You may be
surprised to learn that the answer is no! A
moving object will continue to move until a
force is applied to it. In a similar way, an
object at rest will not start to move unless
a force acts on it. This property of matter
is called inertia.
friction
• A force between
surfaces that
slows objects
down or stops
them from
moving.
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