Lesson 1: Newton's First Law of Motion
Newton's First Law
Unit 1 of the Physics Classroom discussed a variety of ways by which
motion can be described – words, graphs, diagrams, numbers, etc.
This unit, Newton's Laws of Motion, will discuss the ways in which
motion can be explained. Isaac Newton (a 17th century scientist) put
forth three laws which explain why objects move (or don't move) as
they do and 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. These two parts are
summarized in the following 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."
There is a Pass the Water
exercise that demonstrates this principle. If students participate
in a relay race carrying a plastic container of water around a race
track, the water will have a tendency to spill from the container at
specific locations on the track. In general the water will spill
when:
- the container is at rest and you attempt to move it
- the container is in motion and you attempt to stop it
- the container is moving in one direction and you attempt to
change its direction.
 The behavior of the water during the relay
race can be explained by Newton's first law of motion. The water
spills whenever the state of motion of the container changes. The water resists this
change in its own state of motion and tends to "keep on doing what
it is doing." If the container is moved from rest to a high speed at
the starting line; the water remains at rest and spills onto the
table. When the container stops near the finish line; the water
keeps moving and spills over the container's leading edge. If the
container is forced to move in a different direction to make it
around a curve; the water keeps moving in the original direction and
spills over its edge.
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 is 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 to move forward with the same
speed and in the same direction, ultimately hitting the windshield
or the dashboard. 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 forward along the seat. 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 – try to
provide explanations for each application.
- blood rushes from your head to your feet when riding on a
descending elevator which suddenly stops.
- 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 the brick with a hammer. (CAUTION: Do not
attempt this at home!)
- to dislodge ketchup from the bottom of a ketchup bottle, the
bottle is often turned upside down, thrust downward at a high
speed 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, a rock or another
object which abruptly halts the motion of the skateboard.
Can you think of a few more examples which
further illustrate applications of Newton's first law?
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