|
|
|
The following information comes from The World Book CD-ROM Encyclopedia 1995.
In the 1600's, the English scientist and mathematician Sir Isaac Newton proposed three laws of motion. Newton's laws concern the ideal motion of objects and do not take into account air resistance or other friction. However, these laws have enabled scientists to describe a wide variety of motions.
The First Law states that an object moving in a straight line will continue to move in a straight line unless acted upon by an outside force. This law also states that an object at rest will stay at rest unless a force moves it. Newton's first law is known as the principle of inertia. Inertia is the property of matter that makes an object stay in motion if it is moving or remain motionless if it is not moving. Forces that change an object's motion must first overcome the inertia of the object. The greater an object's mass, the harder it is to put the object into motion or change its velocity. Inertia serves as a measurement of how hard it is to get an object moving.
The Second Law describes how an object changes its motion when a force is applied to it. The change of motion depends on the magnitude of the force and the mass of the object. A heavier object will change its motion less under a given applied force than will an object of lighter weight. Therefore, if the same force is applied to two objects, the change of motion of the lighter-weight object will be greater. Newton's second law also states that the effect of a given force is always in the direction of the force. For example, if an object is pushed toward the west, it moves that way and not toward the east. Newton's second law may be written as F = ma, where F is the applied force. Scientists use this formula to describe motion of all kinds of objects.
According to Newton's second law, forces cause changes in the motion of objects. Suppose a person fires a bullet horizontally from the muzzle of a gun. Newton's first law says that the bullet will continue to travel on a straight line forever if no forces act upon it. However, the earth's gravity does act on the bullet. As a result, the bullet will fall toward the ground. This fall occurs because the force of gravity pulls the bullet downward at right angles to the direction of travel.
If the rifle is fired horizontally from a height above the earth of 16 feet (4.9 meters), the bullet will be accelerated by gravity and hit the ground in one second--the same time it would take a freely falling body to drop. Because of gravity, rifles or any other type of gun have a limited range of distance they can shoot. In addition, bullets must be shot slightly upward to increase the range and make up for the drop.
The Third Law of motion states that for each action there is an equal and opposite reaction. For example, when hot gases escape from a rocket engine during take-off, the rocket is propelled upward. The downward motion of the gases from the rocket generates a reaction of the rocket upward. This reaction helps the rocket overcome air resistance and fly into space. There are many other examples of Newton's third law. When a rifle fires a bullet, the firing of the bullet is the action and the recoil of the rifle is the reaction. Both are caused by the expanding gas of the exploding gunpowder. Rotating lawn sprinklers propel a spray of water in one direction while rotating in the other direction.
Sometimes the reaction is such that it cannot be easily seen. When you throw a ball against a wall and the ball bounces back, you do not see the wall moving in the opposite direction. But there is a small motion of the area of the wall that was hit. If the ball bounces from the ground, the earth also draws back, but the mass of the earth is so great that we cannot see its motion.
Click here to see Wing Design and Shapes
Click here to see Science of Flight
|
|
