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| Falling Objects and Acceleration due to Gravity |
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| We know that whenever an object is dropped from a height, it falls towards the Earth. Similarly, when an object is projected vertically upwards, it goes up but after attaining a certain height, it starts falling down. It is the force of gravitation which pulls the object towards the Earth. The object, which is projected vertically upwards, is moving against the force of gravitation. |
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| The objects which move towards the Earth due to force of gravity are called freely falling objects. |
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| According to Newton's Second law of motion, if a force is acting on an object, it will definitely produce acceleration in it. Thus it is clear that the force of gravity like any other external force will produce acceleration in all freely falling objects and also in objects moving against the gravity. |
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| The acceleration produced in an object due to the force of gravity is known as acceleration due to gravity. |
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| Acceleration due to gravity is usually denoted by the letter g. Acceleration due to gravity is a vector quantity and is always directed towards the centre of the Earth or any celestial body. |
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| The acceleration due to gravity on Earth is commonly called acceleration due to gravity. So in this chapter, the term acceleration due to gravity means acceleration due to gravity on Earth. |
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| Aristotle, the great ancient philosopher performed some experiments to study the speed of freely falling objects. He observed that when two bodies of different masses were dropped simultaneously from the same height, the heavier object reached the Earth's surface earlier than the lighter one and he arrived at the conclusion, heavier objects fall faster than lighter ones. |
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| This common belief was disproved by Galileo Galilei. There is a legendary story that Galileo dropped simultaneously two objects of different masses from the leaning tower of Pisa and he observed that both the objects hit the ground at approximately the same time. He explained that if at all there is a difference in the rate of fall or acceleration produced in it, then it is due to the resistance offered by air. |
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| Leaning Tower of Pisa |
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| To understand how the resistance offered by air affects the rate of fall of freely falling objects, let us consider the following experiment. |
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| Drop a pebble and a piece of paper simultaneously from the same height. The pebble will hit the ground first. Now repeat the above experiment by crumpling the paper into a small paper ball. You will find that the pebble and the paper ball would reach the ground almost at the same time. In other words, the mass of the paper is the same whether it is flat or squeezed. But the paper ball moves faster because the resistance offered by air is less because of its shape. |
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| Galileo concluded from a series of experiments that all objects irrespective of their masses, dropped simultaneously from the same height, would hit the ground at the same time. He was not able to show this experimentally. But later on Robert Boyle proved this experimentally. He took a glass tube which was sealed at one end and placed a feather and a guinea in the tube. The open end of the tube was also sealed with a cork having a brass tube. On suddenly inverting the glass tube, he observed that guinea fell down faster than the feather. He repeated the experiment by removing air with the help of an air pump which was connected to the brass tube. He found that both the guinea and the feather fell down at the same time, when the glass tube was suddenly inverted. Thus we can conclude that in the absence of air resistance, all bodies irrespective of their mass fall at the same rate. That is, the acceleration due to gravity experienced by a feather and a coin is the same. |
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| Robert Boyle's Experimental Set - up |
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| Now let us derive an expression for acceleration due to gravity. |
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