Why? Because, according to Newton's first law, an object in motion will remain in motion until an outside force acts on it. If the car slams into a wall, the dummy flies forward into the dashboard. Now imagine that a crash test dummy is inside that car, riding in the front seat. Imagine for a moment that a car at a test track is traveling at a speed of 55 mph (80 kph). In fact, seat belts exist in cars specifically to counteract the effects of inertia. You experience inertia in a moving car all the time. Which person in our example would be easier to move? Common sense tells you that the boy would be easier to move, or less resistant to inertia. Remember the object of sumo wrestling is to move your opponent from his position. In practice, it is very difficult to count and identify all of the atoms and molecules in an object, so masses are not often determined in this manner.Let's say the wrestler on the left has a mass of 136 kilograms, and the boy on the right has a mass of 30 kilograms (scientists measure mass in kilograms). The mass of an object is the same on Earth, Unlike weight, mass does not vary with location. The quantity or amount of matter in an object is determined by the numbers of atoms and molecules of various types it contains. Roughly speaking, mass is a measure of the amount of "stuff" The inertia of an object is measured by its mass. 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 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 fundamentallyĭifferent 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". Identifying these laws is like recognizing patterns in nature from which further 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) mustīe 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. 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. 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 (consistent with Newton's first law). Extrapolating to a frictionless surface, we can imagine the object sliding If we make the surface even smoother by rubbing lubricating oil on it, the object slides farther yet. 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 willĭefine 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). We can think of this law as preserving the status quo of motion. Note the repeated use of the verb "remains". 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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