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| Newton's First Law of Motion |
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| Every body continues in its state of rest or of uniform motion in a straight line unless compelled by some external force to act otherwise. |
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| It is a matter of common experience that objects at rest do not begin to move of their own accord. If we place an object in a certain place we expect it to remain there unless a force is applied to it. Some simple parlour tricks are based on this principle. For example, if a pile of draughts or pennies is placed on a table the bottom one can be removed without disturbing the remainder, simply by flicking it sharply with a piece of thin wood or metal. In this case the force of friction between the bottom disc and the one in contact with it acts for too short a time to cause any appreciable movement of the discs above. Incidentally, the use of a ruler is not recommended for this experiment, as the blows it receives will not improve its straightedge. |
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| (a) The bottle and coin trick is equally effective in demonstrating the same effect. A coin is put on a card and placed over the mouth of a bottle. When the card is flicked away with the finger the coin drops neatly into the bottle. For success in performing this trick the finger should move in horizontal plane so that the card is not tilted. |
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| Place a coin on a smooth card over a water glass. Give the edge of the card a sharp blow with the flat side of a ruler. The card will be knocked off the glass, but the coin will fall into it. |
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| (b) Pile up smooth blocks and give the bottom one a sharp rap with the edge of a ruler. The bottom block will sail out from under, and the rest of the stack will not be toppled but will drop down still in a pile. |
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| It is not immediately obvious that a body moving with uniform velocity in a straight line tends to go on moving forever without coming to rest. The fact is that no one has yet found a means of eliminating all the various outside forces, which can retard a moving body. |
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| A person riding a bicycle along a level road does not come to rest immediately as he stops pedaling. The bicycle continues to move forward, but eventually it comes to rest as a result of the retarding action of air resistance and friction. In a collision between two motor vehicles the passengers are frequently injured when they hit the windscreen. An external force stops the vehicle, but not the passengers who simply continue their straight line motion in accordance with Newton's first law; hence the advantage of a safety harness. |
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| When a bullet is fired from a gun its motion is opposed both by air resistance and the pull of the earth. Sooner or later it returns to the earth, but it would be reasonable to suppose that, if air resistance and gravitation could be eliminated, the bullet would go on moving in a straight line for ever. |
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| When a train suddenly starts, the passengers tend to fall backwards. This is because the lower part of the body which is in contact with the train begins to move while the upper part of the body tends to maintain its position of rest. As result, the upper part tends to fall backwards |
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| The first law of motion and inertia also applies to objects in motion. For example, |
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 When you pedal a bicycle, you accelerate to make the
bicycle move forward. |
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| Because the forces acting on the bicycle are balanced, the
bicycle and the rider keep moving at the same speed and in the same direction. The
bicycle has inertia. |
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| Inertia can be overcome only if the rider applies some type of force. The outside force could result from pushing the pedals, turning the handle bars, or using the brake. |
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You must have observed that when people alight from a moving train, they continue to run alongside the train. If they were to stop at once, the feet would come to rest suddenly but the upper part of the body would still be in motion and they would tend to fall forward. |
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When food packets are dropped from an aircraft, they do not hit the ground vertically below but fall on the ground further ahead. When the food packets leave the aircraft, they are moving at the speed of the aircraft. Due to inertia of motion they continue to move forward while falling. Hence, they touch the ground further ahead. |
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| If you want to start a car by pushing it, you find that it takes a very large force to overcome its inertia. On the other hand, only a small force is needed to start a child's express wagon. The difference between the car and the express wagon is the difference in mass. The car has a large mass, whereas the wagon has a small one. |
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| The inertia of an object provides a way of defining mass. The greater the mass of an object, the greater its tendency to keep at a constant velocity. Therefore mass is often defined as the quantitative measure of the inertia of an object. Like the definition for force, this is an operational definition. It tells you that you can decide which of the two masses is greater by observing which one speeds up less when you apply the same force to each for the same length of time. |
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