![]() The genius of Galileo, who first developed the idea for the first law of motion, and Newton, who clarified it, was to ask the fundamental question: “What is the cause?” Thinking in terms of cause and effect is fundamentally different from the typical ancient Greek approach, when questions such as “Why does a tiger have stripes?” would have been answered in Aristotelian fashion, such as “That is the nature of the beast.” The ability to think in terms of cause and effect is the ability to make a connection between an observed behavior and the surrounding world. Identifying these laws is like recognizing patterns in nature from which further patterns can be discovered. The idea of generally applicable or universal laws is important-it is a basic feature of all laws of physics. Experiments have verified that any change in velocity (speed or direction) must be caused by an external force. Newton’s first law is general and can be applied to anything from an object sliding on a table to a satellite in orbit to blood pumped from the heart. Additionally, if we know enough about the friction, we can accurately predict how quickly the object slows down. However, when the air is turned on, it creates a nearly frictionless surface, and the puck glides long distances without slowing down. When the air is turned off, the puck slides only a short distance before friction slows it to a stop. The object would not slow down if friction were eliminated.Ĭonsider an air hockey table ( Figure). Friction is thus the cause of slowing (consistent with Newton’s first law). Extrapolating to a frictionless surface and ignoring air resistance, we can imagine the object sliding in a straight line indefinitely. If we make the surface even smoother by rubbing lubricating oil on it, the object slides farther yet. If we spray the surface with talcum powder to make the surface smoother, the object slides farther. ![]() ![]() For example, consider what happens to an object sliding along a rough horizontal surface. The idea of cause and effect is crucial in accurately describing what happens in various situations. If friction disappears, will the object still slow down? An object sliding across a table or floor slows down due to the net force of friction acting on the object. This cause is a net external force, which we defined earlier in the chapter. Rather than contradicting our experience, Newton’s first law says that there must be a cause for any change in velocity (a change in either magnitude or direction) to occur. ![]()
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