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 MOTION 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 the earth. 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 speed forever. 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.

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