Newton's Laws
Newton's First Law of Motion
In a previous
chapter 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 that 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 thelaw of
inertia.
Newton's first law of motion is often stated as
An
object at rest stays at rest and an object in motion stays in motion with the
same speed and in the same direction unless
acted upon by an unbalanced force.
Two
Clauses and a Condition
There are two clauses or parts to this statement - one that
predicts the behavior of stationary objects and the other that predicts the behavior of
moving objects. The 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 an important condition that must be
met in order for the first law to be applicable to any given motion. The
condition is described by the phrase "... unless acted upon by an
unbalanced force." As the long as the forces are not unbalanced - that is,
as long as the forces are balanced - the first law of motion applies. This
concept of a balanced versus and unbalanced force will be discussed in more
detail later in Lesson 1.
Suppose that you filled a baking dish to the rim with water and walked
around an oval track making an attempt to complete a lap in the least amount of
time. The water would have 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 spills whenever the state
of motion of the container is
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 lap
around the track can be
explained by Newton's first law of motion.
Everyday
Applications of Newton's First Law
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
"keeps 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 (that 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 stays 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 stays 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 that
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. Perhaps you could think about the law of
inertia and 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 that abruptly halts the motion of the
skateboard.
Try This At Home
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