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Some of the most important ideas in physics are the laws of motion and of universal gravitation. These laws were discovered during the 1660s by Isaac Newton, an English mathematician and physicist. The three laws of motion and the law of universal gravitation provided a crucial starting point for later discoveries in theoretical physics.
Let us first consider Newton’s three laws of motion. The first law of motion states that every object in a uniform state of motion tends to remain in that state of motion unless an external force is applied to it. This law is also known as the law of inertia. Newton’s first law means that objects tend to maintain their current velocity: objects at rest tend to stay at rest, and objects in motion tend to stay in motion. This law seems to contradict common sense, because the objects we see tend to slow down unless a force is applied to them; however, this deceleration is due to the force of friction that is caused by the air, the water, or the ground. If we could study objects moving in a vacuum, those objects would not slow down at all. The second law of motion explains the relation between the force applied to an object, the mass of an object, and the acceleration of an object.
According to Newton’s second law, the force required to cause a given amount of acceleration of an object of a given mass can be calculated by multiplying the acceleration of the object by the mass of the object. In other words, the force, F, equals the mass, m, times the acceleration, a. It is easy to see how this law works. If you want to cause a heavy car to increase or decrease its speed very suddenly, you must apply a very strong force. If you want to cause a lighter car to increase or decrease its speed quite gradually, you need to apply much less force. The third law of motion states that for every action, there is an equal and opposite reaction. One of the important applications of this idea can be seen in the flight of rockets. When a rocket expels hot, expanding gases from its tail, the backward force of those gases propels the rocket forward. Newton realized that the second law of motion could help to explain why objects, such as apples in an apple tree, tend to fall to the earth, and why the moon is held in orbit around the earth.
Newton discovered the law of universal gravitation, which can be summarized as follows: every object in the universe attracts every other object in the universe with a force that is proportional to the product of their masses and inversely proportional to the distance between them. This law has many important implications for our understanding of the world around us. One of these involves the tides: we can understand the rising and falling of ocean levels by considering the gravitational forces applied by the moon and the sun. Newton’s laws have served as the foundation for physics and engineering since the 17th century. In the next article, we will discuss the revolutionary discoveries made by physicists during the 20th century.