Lesson 1: Newton's First Law of
Motion
Newton's First
Law
In a previous unit of
study, the variety of ways by which motion can be
described (words, graphs, diagrams, numbers, etc.)
was discussed. In this unit (Newton's Laws of Motion), the
ways in which motion can be explained will be
discussed. Isaac Newton (a 17th century scientist) put forth
a variety of laws which explain why objects move (or don't
move) as they do. These three laws have become known as
Newton's three laws of motion. The focus of Lesson 1 is
Newton's first law of motion - sometimes referred to as the
"law of inertia."
Newton's first law of
motion is often stated as
An object at rest tends to stay at
rest and an object in motion tends to stay in motion
with the same speed and in the same direction
unless acted upon by an
unbalanced force.
There are two parts to this statement -
one which predicts the behavior of stationary objects and
the other which predicts the behavior of moving objects. The
two parts are summarized in the following diagram.
![diagram]()
The
behavior of all objects can be described by saying that
objects tend to "keep on doing what they're doing"
(unless acted upon by an
unbalanced force). If at rest, they will continue in
this same state of rest. If in motion with an eastward
velocity of 5 m/s, they will continue in this same state of
motion (5 m/s, East). If in motion with a leftward velocity
of 2 m/s, they will continue in this same state of motion (2
m/s, left). The state of motion of an object is maintained
as long as the object is not acted upon by an
unbalanced force. All objects resist changes in their state
of motion - they tend to "keep on doing what they're
doing."
Remember the Pass the
Water lab performed in class? Students participated in a
relay race, carrying a plastic container of water around a
race track. The water had a tendency to spill from the
container during specific locations on the track. In general
the water spilled when:
- the container was at rest and you attempted to move
it
- the container was in motion and you attempted to stop
it
- the container was moving in one direction and you
attempted to change its direction.
The
water was spilled whenever the state
of motion of the container was changed. The water
resisted this change in its own state of motion. The water
tended to "keep on doing what it was doing." The container
was moved from rest to a high speed at the starting line;
the water remained at rest and spilled onto the table. The
container was stopped near the finish line; the water kept
moving and spilled over container's leading edge. The
container was forced to move in a different direction to
make it around a curve; the water kept moving in the same
direction and spilled over its edge. The behavior of the
water during the relay race can be explained by Newton's
first law of motion.
There are many applications of Newton's
first law of motion. Consider some of your experiences in an
automobile. Have you ever observed the behavior of coffee in
a coffee cup filled to the rim while starting a car from
rest or while bringing a car to rest from a state of motion?
Coffee tends to "keep on doing what it is doing." When you
accelerate a car from rest, the road provides an unbalanced
force on the spinning wheels to push the car forward; yet
the coffee (which was at rest) wants to stay at rest. While
the car accelerates forward, the coffee remains in the same
position; subsequently, the car accelerates out from under
the coffee and the coffee spills in your lap. On the other
hand, when braking from a state of motion the coffee
continues forward with the same speed and in the same
direction, ultimately hitting the windshield or the
dash. Coffee in motion tends to stay in motion.
Have
you ever experienced inertia (resisting changes in your
state of motion) in an automobile while it is braking to a
stop? The force of the road on the locked wheels provides
the unbalanced force to change the car's state of motion,
yet there is no unbalanced force to change your own state of
motion. Thus, you continue in motion, sliding along the seat
in forward motion. A person in motion tends to stay in
motion with the same speed and in the same direction ...
unless acted upon by the
unbalanced force of a seat belt. Yes, seat belts are
used to provide safety for passengers whose motion is
governed by Newton's laws. The seat belt provides the
unbalanced force which brings you from a state of motion to
a state of rest. Perhaps you could speculate what would
occur when no seat belt is used.
There
are many more applications of Newton's first law of motion.
Several applications are listed below - it is hoped that you
could provide explanations for each application.
-
blood rushes from your head to your feet while
quickly stopping when riding on a descending
elevator.
-
the head of a hammer can be tightened onto the wooden
handle by banging the bottom of the handle against a hard
surface.
-
a brick is painlessly broken over the hand of a
physics teacher by slamming it with a hammer. (CAUTION:
do not attempt this at home!)
-
to dislodge ketchup from the bottom of a ketchup
bottle, it is often turned upside down and, thrusted
downward at high speeds and then abruptly halted.
-
headrests are placed in cars to prevent whiplash
injuries during rear-end collisions.
-
while riding a skateboard (or wagon or bicycle), you
fly forward off the board when hitting a curb or rock or
other object which abruptly halts the motion of the
skateboard.
And perhaps you remember a few
demonstrations performed in class which further illustrate
applications of Newton's first law.
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