Page 325 - NS-2 Textbook

Motion, Force, and Aerodynamics

TIle riddle of how and why things move has fascinated Newton's third law of motion states that whenever
humankind since ancient times. About 2,300 years ago, one body exerts a force upon a second body, the second
Greek philosophers studied motion. As they observed exerts an equal but opposite force back upon the first.
the world armmd them, it seemed to them that all matter Stated another way, for every action there is an equal but
should be at rest or motionless in its normal state. Things opposite reaction.
that they observed to be in motion always seemed to As an example of Newton's laws, suppose you want
tend to slow down and eventually stop. To their way of to take a trip in your car. When you first get in you are at
thinking, in order to keep moving an object had to have rest (motionless), because no unbalanced forces are act-
some unbalanced force acting on it. In the absence of ing on you. Then when you press on the acceleratOl, the
such a force, a moving object ,,,,auld slow down and car's engine exerts a force on it and everything in it, in-
eventually stop. cluding you. You therefore accelerate forward, along
The famous Greek philosopher Aristotle (384-322 with the rest of the car. Within the cal, your seat pushes
B.C.) concluded from this that the speed of an object de- forward on you, and you in turn push back on the seat
pends entirely on the force being applied to it and the re- with an equal but opposite force for as long as the car is
sistance it meets. Aristotle's law, however, was later accelerating. Evenhlally you reach cruising speed-say,
proven to be inaccurate. 30 miles per hour-and ease up the pressure on the ac-
In the sixteenth centulY the Italian scientist Gallieo celerator. Though the engine is still running, at this cruis-
Galliei (1564-1642) observed that an object in horizontal ing speed the fOlward force of the engine is exactly
motion would continue to move at the same speed with matched by opposing forces such as air friction, thus pro-
no additional force. Later in the same century this state- ducing a state of no net or unbalanced forces acting on
ment ,vas accepted by Sir Isaac Newton, and with some either you or the car. Therefore you stay at a steady speed
elaboration, it became the basis of the first of his now- of 30 miles per hour. Then when you want to slow down
famous three laws of motion. you apply the brakes, thus generating a net force on the
car in a direction opposite to its motion. It begins to slow
down. Meanwhile, you tend to keep moving forward
NEWTON'S LAWS OF MOTION
inside, Imless you are restrained by your seat belt and
Newton based his laws of motion largely on observation friction with the seat. While the seat belt restrains you,
and experimentation. Like all theoretical laws, Newton's you and the seat belt each exert an equal but opposite
Im'vs \!\Tefe originally based upon what Ne\vton smv force on each othet: Eventually you and the car stop,
around him, and then with some brilliant insight, he ex- v\There once again no net forces are acting on either you
panded his results to include new phenomena and pos- or the car.
sibilities. Taken togethel, Newton's three laws describe the re-
Newton's first law of motion states that a body at lationships among force, mass, acceleration, and velocity
rest tends to relnain at rest, and a body in n1otion tends for all bodies in motion at relatively low speeds as com-
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to remain in motion in a straight line, unless an outside pared to the speed of light, 3 x 10 meters per second or
force acts on the body. This law is sometimes called the 186,000 miles per second. Such motion is often called
law of inertia. Newtolliall motioll. Fortunately, most motion on Earth
Newton's second law of motion states that the accel- falls into this category. We can therefore easily use New-
eration of a body is directly proportional to the force act- ton's laws to make all manner of predictions about things
ing on it, and inversely proportional to the mass of the undergoing Newtonian motion. Sometimes, however,
body, and is in the same direction as the applied force. bodies in space can travel much faster. For them, time,
Mathematically this is often expressed by the formula mass, and length become distorted, and different rules
F=ma. devised by Albert Einstein (1879-1955) apply. These rules

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