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| Force and Inertia |
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| An interaction which causes an acceleration is called a force. Only when we push a ball does it begin to move. A ball lying on a horizontal floor doesn't start to move all by itself. For that matter, take a parked a car on a horizontal road. Only when we activate the engine and engage the gears does the engine impose some force on the wheels, which in turn, imposes force on the road to begin motion. |
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| You have noticed that when a cube of ice is pushed and left on a very smooth polished surface, like, a polished dining table, it doesn't stop until and unless you hold it with your hands or any other object. |
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| This property of objects to remain at rest or uniform motion unless acted upon by a force is called inertia. |
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| Galileo Galilee (1564-1642), a great Italian scientist raised a number of questions about the action of forces on a body with respect to the motion of the body. He performed various experiments and answered those questions by studying the results of his experiments. |
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| Aristotle, a Greek philosopher had once proposed that for a body to move with uniform velocity, a constant force is required. Galileo questioned the truth of this statement, which was believed to be true for about two thousand years. Galileo set up an experiment with an incline and an object as shown in the figures below. |
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| He observed in the case of planes that sloped downwards, there was already a cause of acceleration present, while on planes sloping upwards there was retardation. Hence, he inferred that if the plane sloped neither upward nor downward, there would neither be any retardation nor acceleration. He concluded that motion along a horizontal plane should be constant. |
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| In another set of experiments, |
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| he noticed that whenever the body was dropped on a slope from a certain height, it always rose to almost the same height on the other slope. As friction decreased on the surfaces, the body rose closer to the same height and when friction was almost zero, it rose to the same height. Hence, he reasoned that motion along a horizontal plane is perpetual. |
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| This property of a body to keep moving with constant velocity in the absence of any force acting on it is its inertia. This is called the law of inertia. |
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| After the death of Galileo, Newton came up with his three laws of motion. |
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| Galileo's law of inertia was reformulated as Newton's first law of motion and stated as Newton's first law. |
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