It is obviously more difficult to change the motion of a large boulder than that of a basketball, for example. As we know from experience, some objects have more inertia than others. Newton’s first law is often called the law of inertia. The property of a body to remain at rest or to remain in motion with constant velocity is called inertia. The genius of Galileo, who first developed the idea for the first law, and Newton, who clarified it, was to ask the fundamental question, “What is the cause?” Thinking in terms of cause and effect is a worldview 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, “That is the nature of the beast.” True perhaps, but not a useful insight. 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 not only here-it is a basic feature of all laws of physics. Experiments have thoroughly verified that any change in velocity-speed or direction-must be caused by an external force. Newton’s first law is completely 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 will slow 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 at all if friction were completely eliminated. Friction is thus the cause of the slowing, which is consistent with Newton’s first law. Extrapolating to a frictionless surface, 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 disappeared, would 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. We will define net external force in the next section. Rather than contradicting our experience, Newton’s first law of motion states that there must be a cause which is a net external force, for there to be any change in velocity-either a change in magnitude or direction- in an inertial reference frame. Note that Newton’s laws of motion are only valid for inertial reference frames. Finally, when the car goes around a turn, which is due to an acceleration changing the direction of the velocity vector, it is not an inertial reference frame. A car slowing down for a stoplight, or speeding up after the light turns green, will be accelerating and is not an inertial reference frame. A car traveling at constant velocity is an inertial reference frame. An inertial reference frame is any reference frame that is not itself accelerating. The first law of motion postulates the existence of at least one frame of reference which we call an inertial reference frame, relative to which the motion of an object not subject to forces is a straight line at a constant speed. We can think of this law as preserving the status quo of motion. Note the repeated use of the verb remains. There exists an inertial frame of reference such that a body at rest remains at rest, or, if in motion, remains in motion at a constant velocity unless acted on by a net external force.
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