Isaac Newton and the Universal Laws of Motion
The First Law:“A MOVING OBJECT WILL CONTINUE MOVING IN A STRAIGHT LINE AT A CONSTANT SPEED, AND A STATIONARY OBJECT WILL REMAIN AT REST, UNLESS ACTED ON BY A UNBALANCED FORCE.”
Newton's first law seems to state the obvious: if you leave an object alone, it would not change its state of motion. In order to change it, you have to push it or pull it, thus applying a force. Yet virtually all scientists from the Greeks to Copernicus would have argued that the first law is wrong. They believed that because the circle is the most perfect geometric shape, objects will move in circles unless something's interferes. They believed that heavenly objects would keep turning without any outside force acting (indeed, they had to believe or face question of why the heavens did not just run down).
Newton, basing his argument on observations and the work of his predecessors, turned this notion around. An object left to itself will move in a straight line, and if you want to get it to move in a circle, you would have to apply a force. You know this is true---if you swing something around your head, it will move in a circle only as long as you hold on to it. Let it go, and off it goes in a straight line.
This simple observation led Newton to recognize two different kinds of motion. An object is in uniform motion if it travels in a straight line at a constant speed. All other motions are called acceleration. Acceleration can involve changes in speed, changes in direction, or both.
Newton's first law tells us that when we see an acceleration, something must have acted to produce that change. We define force as something that produces a change in the state of motion of an object. In fact, we will use the first law of motion extensively to tell us how to recognize when a force, particularly a new kind of force, is acting.
The tendency of an object to remain in uniform motion is called inertia. A body at rest tends to stay at rest because of its inertia, while a moving body tends to keep moving because of its inertia. We often use this idea in everyday speech; for example, we may talk about the inertia in a company or government organization that is resistant to change.
The Second Law: “THE ACCELERATION PRODUCED ON A BODY BY A FORCE IS PROPORTIONAL TO THE MAGNITUDE OF THE FORCE AND INVERSELY PROPORTIONAL TO THE MASS OF THE OBJECT”
If Newton's first law of motion tells you when a force is acting, then the second law of motion tells you what the force does when it acts. This law conforms to our everyday experience: it is easier to move a bicycle than a car, easier to lift a child than an adult, easier to push a ballerina than a defensive tackle.
Newton's second law is often expressed as an equation.
In words: The greater the force, the greater the acceleration; but the more massive the object being acted on by a given the smaller the acceleration.
In equation form: force = mass (kg) x acceleration (m/s2)
In symbols: F = m x a
This equation, well known to generations of physics majors, tells us that if we know the forces acting on a system of known mass, we can predict its future motion. The equation conforms to our experience that an object's acceleration is a balance between two factors: force and mass (the amount of matter in an object.)
A force causes the acceleration. The greater the force, the greater the acceleration. The harder you throw a ball the faster it goes. Mass measures the amount of matter in any object. The greater the object's mass, the more “stuff” you have to accelerate, the less effect a given force is going to have. A given force will acceleration a golf ball more than a bowling ball, for example. Newton's second law of motion thus defines the balance force and mass in producing an acceleration.
Newton's first law defines the concept of force as something that causes a mass to accelerate, but the second law goes much further. It tells us the exact magnitude of the force necessary to cause a given mass to achieve a given acceleration. Because forces equals mass times acceleration, the units of force must be the same as mass times acceleration. Mass is measured in kilograms (kg) and acceleration in meters per second(m/s2), so the unit force is the “kilogram-meter-per-second-squared” (kg-m/s2), which is called the Newton. The symbol for the Newton is N.
The second law of motion does not imply that every time a force acts, motion must result. A book placed on a table still feels the force of gravity, and you can push against a wall without moving it. In these situations, the atoms in the table or the wall shift around and exert their own force that balances the one that acts on them. It is only the net, or unbalanced, force that actually dives rise to acceleration.
The Third Law: “FOR EVERY ACTION THERE IS AN EQUAL AND OPPOSITE REACTION.”
Newton's third law of motion tells us that whenever a force is applied to an object, simultaneously exerts an equal and opposite force. When you push on a wall, for example, it instantaneously pushes back on you; you can feel the force on the palm of your hand. In fact, the force the wall exerts on you is equal magnitude (but opposite in direction) to the force you exert on it.
The third law of motion is perhaps the least intuitive of the three. We tend to think of our world in terms of causes and effects, in which big or fast objects exert forces on smaller, slower ones: a car slams into a tree, a batter drives the ball deep into left field, a boxer punches his opponent's eye. However, in terms of Newton's third law it is equally valid to think of these events the “other way around.” The tree stops the car's motion, the baseball alters the swing of the bat, and the opponent's eye blocks the thrust of the boxer's glove, thus exerting a force and changing the direction and speed of the punch.
Forces always act simultaneously in pairs. You can convince yourself of this fact by thinking about your day's myriad activities. As you sit in a chair reading this web site, your weight exerts a force on the chair, but the chair exerts an equal and opposite force (called a contact force) on you, preventing you from falling. The book feels heavy in your hand as it presses down, but your hands hold the book up, exerting an equal and opposite force. You may feel a slight draft from an open window or fan, but as the air exerts that gentle force on you, your skin just as surly exerts an equal and opposite force on the air, causing it to change its path.
Newton's Laws at Work:
Every motion in your life, indeed, every motion in the universe, involves the constant interplay of all three of Newton's laws. The laws of motion never occur in isolation, but rather are interlocking aspects of every object's behavior. The interdependence of Newton's three laws of motion can be envisioned by a simple example. Imagine a boy standing on roller skates holding a stack of baseballs. He throws the balls, one by one. Each time he throws a baseball, the first law tells us he has to exert a force so that the ball accelerates. The third law tells us that the baseball will exert an equal and opposite force on the body. This force acting on the boy will, according to the second law, cause him to recoil backward.
While the example of the boy and the baseballs may seem a bit contrived, it exactly illustrated the principle by which fish swim and rockets fly. As a fish moves its tail, it applies a force against the water. The water, in turn, pushes back on the fish and propels a force against the water. The water, in turn, pushes back on the fish and propels it forward. In a rocket motor, forces are exerted on hot gases, accelerating them out the tail end. The same argument just presented, this means that an equal opposite force must be exerted on the rocket, propelling it forward. Every rocket, from simple fireworks to a space shuttle, works this way.
Isaac Newton's three laws of motion from a comprehensive description of all possible motions, as well as the forces that lead to them. In and of themselves, however, Newton's laws do not to say anything about the nature of those forces. In fact, much of the progress of science Newton's time has been associated with the discovery and elucidation of the forces of nature.
