|
Newton's First Law of Motion |
|
|
The First Law
To gain some insight into Newton's first law, think about the game of ice hockey (Figure 4.2). If a player does not hit a stationary puck, it will remain at rest on the ice. After the puck is struck, however, it coasts
on its own across the ice, slowing down only slightly because of friction. Since ice is very slippery, there is only a relatively
small amount of friction to slow down the puck. In fact, if it were possible to remove all friction and wind resistance, and
if the rink were infinitely large, the puck would coast forever in a straight line at a constant speed. Left on its own, the
puck would lose none of the velocity imparted to it at the time it was struck. This is the essence of Newton's first law of
motion:
|
|
|
|
Figure 4.2 |
The game of ice hockey can give some insight into Newton's laws of motion.
|
|
|
|
|
|
|
|
Newton's First Law of Motion |
|
|
|
|
|
|
|
|
|
In the first law the phrase “net force” is crucial. Often, several forces act simultaneously on a body, and the net force is the vector sum of all of them. Individual forces matter only to the extent that they contribute to the total. For instance, if friction and other opposing
forces were absent, a car could travel forever at 30 m/s in a straight line, without using any gas after it had come up to
speed. In reality gas is needed, but only so that the engine can produce the necessary force to cancel opposing forces such
as friction. This cancellation ensures that there is no net force to change the state of motion of the car.
When an object moves at a constant speed in a constant direction, its velocity is constant. Newton's first law indicates that
a state of rest (zero velocity) and a state of constant velocity are completely equivalent, in the sense that neither one
requires the application of a net force to sustain it. The purpose served when a net force acts on an object is not to sustain the object's velocity, but, rather, to change it.
Inertia and Mass
The physics of seat belts. Figure 4.4 shows a useful application of inertia. Automobile seat belts unwind freely when pulled gently, so they can be buckled. But
in an accident, they hold you safely in place. One seat-belt mechanism consists of a ratchet wheel, a locking bar, and a pendulum.
The belt is wound around a spool mounted on the ratchet wheel. While the car is at rest or moving at a constant velocity,
the pendulum hangs straight down, and the locking bar rests horizontally, as the gray part of the drawing shows. Consequently,
nothing prevents the ratchet wheel from turning, and the seat belt can be pulled out easily. When the car suddenly slows down
in an accident, however, the relatively massive lower part of the pendulum keeps moving forward because of its inertia. The
pendulum swings on its pivot into the position shown in color and causes the locking bar to block the rotation of the ratchet
wheel, thus preventing the seat belt from unwinding.
|
|
|
|
Figure 4.4 |
Inertia plays a central role in one seat-belt mechanism. The gray part of the drawing applies when the car is at rest or moving
at a constant velocity. The colored parts show what happens when the car suddenly slows down, as in an accident.
|
|
|
|
|
An Inertial Reference Frame
Newton's first law (and also the second law) can appear to be invalid to certain observers. Suppose, for instance, that you
are a passenger riding in a friend's car. While the car moves at a constant speed along a straight line, you do not feel the
seat pushing against your back to any unusual extent. This experience is consistent with the first law, which indicates that
in the absence of a net force you should move with a constant velocity. Suddenly the driver floors the gas pedal. Immediately
you feel the seat pressing against your back as the car accelerates. Therefore, you sense that a force is being applied to
you. The first law leads you to believe that your motion should change, and, relative to the ground outside, your motion does
change. But
relative to the car, you can see that your motion does
not change, because you remain stationary with respect to the car. Clearly, Newton's first law does not hold for observers who
use the accelerating car as a frame of reference. As a result, such a reference frame is said to be
noninertial. All accelerating reference frames are noninertial. In contrast, observers for whom the law of inertia is valid are said
to be using
inertial reference frames for their observations, as defined below:
|
|
|
Definition of an Inertial Reference Frame |
|
|
|
|
|
|
|
|
|
The acceleration of an inertial reference frame is zero, so it moves with a constant velocity. All of Newton's laws of motion
are valid in inertial reference frames, and when we apply these laws, we will be assuming such a reference frame. In particular,
the earth itself is a good approximation of an inertial reference frame.
|
Copyright © 2010 John Wiley & Sons, Inc. All rights reserved. |