Motion is a change of position or movement of an object relative
to another object. All motions take place on a definite path, and the nature of these paths
determines the character of the motions.
A simple type of motion occurs when a ball is thrown at an angle
into the air. Because of the earth's gravity, the ball undergoes a constant downward speed that
first slows its original upward speed and then increases its downward speed as it falls back to
earth. Meanwhile the horizontal component of the original speed remains constant (ignoring air
friction), making the ball travel at a constant speed in the horizontal direction until it hits
Circular motion is another simple type of motion. If an object has constant speed but
an acceleration always at right angles to its path, it will travel in a circle. The required
acceleration is directed toward the center of the circle and is called centripetal acceleration.
Constant speed or velocity is a special type of motion where the average velocity is equal to
the velocity at any particular time. In the simplest case, the velocity might be zero; an
object's position would not change during the time interval. In another special type of motion,
acceleration or increase in speed is constant. A heavy object falling freely (uninfluenced by
air friction) near the surface of the earth undergoes constant acceleration.
Isaac Newton determined the general principles of motion and formulated them into the Three Laws of Motion.
According to the First Law of Motion, an object at rest tends to stay at rest. An object in
motion tends to stay in motion, in the same direction and speed. Basically, if nothing is
happening and nothing does happen, you will never go anywhere. If you are going in a specific
direction, unless something happened to you, you will always go in that direction at the same
In the Second Law of Motion, Newton theorized that the acceleration or increase
in speed of an object is directly proportional to the magnitude of the force, the same direction
as the force, and inversely proportional to the mass of the object. Inversely proportional means
if one value goes up, the other value will go down, assuming everything else stays the same.
The Third Law states that for every action, there is an equal and opposite reaction. If you
knock one metal ball into another, the other ball will start to move. Think about how pinball
machines work. Either the ball bounces perfectly off the side, or it runs into something which
transfers energy to another object, for example the bumper. The total force involved in the
situation remains the same.