Graphics Reference
In-Depth Information
Newton's First Law of Motion
Newton's first law of motion simply reflects what others before him had
already observed: An object at rest will remain at rest until a force acts on it.
That idea would seem to be pretty straightforward and observable in our
everyday experience. If we don't pick up the coffee cup, for example, it is
likely to remain where we placed it. It is for this reason that Newton's first law
of motion is also often referred to as the
law of inertia
. What is a little beyond
our first-hand experience is the assertion that if an object is in motion, it
will remain in motion in the same direction and at the same speed until it
encounters a force to act on it. Under normal circumstances this is not what
we experience. If we kick a ball, it will travel so far and no further. It will also
travel in an arc and not in a straight line.
It was Galileo that established that a force acting on an object resulted in the
acceleration
of the object, not the object's speed. Newton's first law goes on to
state that if there is no force acting on an object, there can be no acceleration
of the object, and as a result the object will maintain its current velocity. If the
velocity of the object is at zero, it follows that the object will remain at rest.
Put very simply:
• An object at rest will remain at rest until a force acts on it.
• An object in motion will not change its velocity until a force acts on it.
It's this second point that seems to imply that an object will move and continue
to move. We kick a ball and the reason we observe it slowing down is in fact
FIG 2.3
An object remains at rest.
a: An object remains a rest if no force
is applied. b: An object will move once
a force is applied. c: The low friction of
a smooth surface allows an object to
move further than one moving along
a rough surface with more friction.
d: A high-friction surface.
Constant velocity. e: The level of force
applied to an object determines how
far that object will travel. f : Greater
forces applied to objects enable
greater distances to be gained.
